// SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
/*
 * BPF static linker
 *
 * Copyright (c) 2021 Facebook
 */
#include <stdbool.h>
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <linux/err.h>
#include <linux/btf.h>
#include <elf.h>
#include <libelf.h>
#include <fcntl.h>
#include "libbpf.h"
#include "btf.h"
#include "libbpf_internal.h"
#include "strset.h"

#define BTF_EXTERN_SEC ".extern"

struct src_sec {
        const char *sec_name;
        /* positional (not necessarily ELF) index in an array of sections */
        int id;
        /* positional (not necessarily ELF) index of a matching section in a final object file */
        int dst_id;
        /* section data offset in a matching output section */
        int dst_off;
        /* whether section is omitted from the final ELF file */
        bool skipped;
        /* whether section is an ephemeral section, not mapped to an ELF section */
        bool ephemeral;

        /* ELF info */
        size_t sec_idx;
        Elf_Scn *scn;
        Elf64_Shdr *shdr;
        Elf_Data *data;

        /* corresponding BTF DATASEC type ID */
        int sec_type_id;
};

struct src_obj {
        const char *filename;
        int fd;
        Elf *elf;
        /* Section header strings section index */
        size_t shstrs_sec_idx;
        /* SYMTAB section index */
        size_t symtab_sec_idx;

        struct btf *btf;
        struct btf_ext *btf_ext;

        /* List of sections (including ephemeral). Slot zero is unused. */
        struct src_sec *secs;
        int sec_cnt;

        /* mapping of symbol indices from src to dst ELF */
        int *sym_map;
        /* mapping from the src BTF type IDs to dst ones */
        int *btf_type_map;
};

/* single .BTF.ext data section */
struct btf_ext_sec_data {
        size_t rec_cnt;
        __u32 rec_sz;
        void *recs;
};

struct glob_sym {
        /* ELF symbol index */
        int sym_idx;
        /* associated section id for .ksyms, .kconfig, etc, but not .extern */
        int sec_id;
        /* extern name offset in STRTAB */
        int name_off;
        /* optional associated BTF type ID */
        int btf_id;
        /* BTF type ID to which VAR/FUNC type is pointing to; used for
         * rewriting types when extern VAR/FUNC is resolved to a concrete
         * definition
         */
        int underlying_btf_id;
        /* sec_var index in the corresponding dst_sec, if exists */
        int var_idx;

        /* extern or resolved/global symbol */
        bool is_extern;
        /* weak or strong symbol, never goes back from strong to weak */
        bool is_weak;
};

struct dst_sec {
        char *sec_name;
        /* positional (not necessarily ELF) index in an array of sections */
        int id;

        bool ephemeral;

        /* ELF info */
        size_t sec_idx;
        Elf_Scn *scn;
        Elf64_Shdr *shdr;
        Elf_Data *data;

        /* final output section size */
        int sec_sz;
        /* final output contents of the section */
        void *raw_data;

        /* corresponding STT_SECTION symbol index in SYMTAB */
        int sec_sym_idx;

        /* section's DATASEC variable info, emitted on BTF finalization */
        bool has_btf;
        int sec_var_cnt;
        struct btf_var_secinfo *sec_vars;

        /* section's .BTF.ext data */
        struct btf_ext_sec_data func_info;
        struct btf_ext_sec_data line_info;
        struct btf_ext_sec_data core_relo_info;
};

struct bpf_linker {
        char *filename;
        int fd;
        Elf *elf;
        Elf64_Ehdr *elf_hdr;

        /* Output sections metadata */
        struct dst_sec *secs;
        int sec_cnt;

        struct strset *strtab_strs; /* STRTAB unique strings */
        size_t strtab_sec_idx; /* STRTAB section index */
        size_t symtab_sec_idx; /* SYMTAB section index */

        struct btf *btf;
        struct btf_ext *btf_ext;

        /* global (including extern) ELF symbols */
        int glob_sym_cnt;
        struct glob_sym *glob_syms;
};

#define pr_warn_elf(fmt, ...)                                                                   \
        libbpf_print(LIBBPF_WARN, "libbpf: " fmt ": %s\n", ##__VA_ARGS__, elf_errmsg(-1))

static int init_output_elf(struct bpf_linker *linker, const char *file);

static int linker_load_obj_file(struct bpf_linker *linker, const char *filename,
                                const struct bpf_linker_file_opts *opts,
                                struct src_obj *obj);
static int linker_sanity_check_elf(struct src_obj *obj);
static int linker_sanity_check_elf_symtab(struct src_obj *obj, struct src_sec *sec);
static int linker_sanity_check_elf_relos(struct src_obj *obj, struct src_sec *sec);
static int linker_sanity_check_btf(struct src_obj *obj);
static int linker_sanity_check_btf_ext(struct src_obj *obj);
static int linker_fixup_btf(struct src_obj *obj);
static int linker_append_sec_data(struct bpf_linker *linker, struct src_obj *obj);
static int linker_append_elf_syms(struct bpf_linker *linker, struct src_obj *obj);
static int linker_append_elf_sym(struct bpf_linker *linker, struct src_obj *obj,
                                 Elf64_Sym *sym, const char *sym_name, int src_sym_idx);
static int linker_append_elf_relos(struct bpf_linker *linker, struct src_obj *obj);
static int linker_append_btf(struct bpf_linker *linker, struct src_obj *obj);
static int linker_append_btf_ext(struct bpf_linker *linker, struct src_obj *obj);

static int finalize_btf(struct bpf_linker *linker);
static int finalize_btf_ext(struct bpf_linker *linker);

void bpf_linker__free(struct bpf_linker *linker)
{
        int i;

        if (!linker)
                return;

        free(linker->filename);

        if (linker->elf)
                elf_end(linker->elf);

        if (linker->fd >= 0)
                close(linker->fd);

        strset__free(linker->strtab_strs);

        btf__free(linker->btf);
        btf_ext__free(linker->btf_ext);

        for (i = 1; i < linker->sec_cnt; i++) {
                struct dst_sec *sec = &linker->secs[i];

                free(sec->sec_name);
                free(sec->raw_data);
                free(sec->sec_vars);

                free(sec->func_info.recs);
                free(sec->line_info.recs);
                free(sec->core_relo_info.recs);
        }
        free(linker->secs);

        free(linker);
}

struct bpf_linker *bpf_linker__new(const char *filename, struct bpf_linker_opts *opts)
{
        struct bpf_linker *linker;
        int err;

        if (!OPTS_VALID(opts, bpf_linker_opts))
                return errno = EINVAL, NULL;

        if (elf_version(EV_CURRENT) == EV_NONE) {
                pr_warn_elf("libelf initialization failed");
                return errno = EINVAL, NULL;
        }

        linker = calloc(1, sizeof(*linker));
        if (!linker)
                return errno = ENOMEM, NULL;

        linker->fd = -1;

        err = init_output_elf(linker, filename);
        if (err)
                goto err_out;

        return linker;

err_out:
        bpf_linker__free(linker);
        return errno = -err, NULL;
}

static struct dst_sec *add_dst_sec(struct bpf_linker *linker, const char *sec_name)
{
        struct dst_sec *secs = linker->secs, *sec;
        size_t new_cnt = linker->sec_cnt ? linker->sec_cnt + 1 : 2;

        secs = libbpf_reallocarray(secs, new_cnt, sizeof(*secs));
        if (!secs)
                return NULL;

        /* zero out newly allocated memory */
        memset(secs + linker->sec_cnt, 0, (new_cnt - linker->sec_cnt) * sizeof(*secs));

        linker->secs = secs;
        linker->sec_cnt = new_cnt;

        sec = &linker->secs[new_cnt - 1];
        sec->id = new_cnt - 1;
        sec->sec_name = strdup(sec_name);
        if (!sec->sec_name)
                return NULL;

        return sec;
}

static Elf64_Sym *add_new_sym(struct bpf_linker *linker, size_t *sym_idx)
{
        struct dst_sec *symtab = &linker->secs[linker->symtab_sec_idx];
        Elf64_Sym *syms, *sym;
        size_t sym_cnt = symtab->sec_sz / sizeof(*sym);

        syms = libbpf_reallocarray(symtab->raw_data, sym_cnt + 1, sizeof(*sym));
        if (!syms)
                return NULL;

        sym = &syms[sym_cnt];
        memset(sym, 0, sizeof(*sym));

        symtab->raw_data = syms;
        symtab->sec_sz += sizeof(*sym);
        symtab->shdr->sh_size += sizeof(*sym);
        symtab->data->d_size += sizeof(*sym);

        if (sym_idx)
                *sym_idx = sym_cnt;

        return sym;
}

static int init_output_elf(struct bpf_linker *linker, const char *file)
{
        int err, str_off;
        Elf64_Sym *init_sym;
        struct dst_sec *sec;

        linker->filename = strdup(file);
        if (!linker->filename)
                return -ENOMEM;

        linker->fd = open(file, O_WRONLY | O_CREAT | O_TRUNC | O_CLOEXEC, 0644);
        if (linker->fd < 0) {
                err = -errno;
                pr_warn("failed to create '%s': %d\n", file, err);
                return err;
        }

        linker->elf = elf_begin(linker->fd, ELF_C_WRITE, NULL);
        if (!linker->elf) {
                pr_warn_elf("failed to create ELF object");
                return -EINVAL;
        }

        /* ELF header */
        linker->elf_hdr = elf64_newehdr(linker->elf);
        if (!linker->elf_hdr) {
                pr_warn_elf("failed to create ELF header");
                return -EINVAL;
        }

        linker->elf_hdr->e_machine = EM_BPF;
        linker->elf_hdr->e_type = ET_REL;
#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
        linker->elf_hdr->e_ident[EI_DATA] = ELFDATA2LSB;
#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
        linker->elf_hdr->e_ident[EI_DATA] = ELFDATA2MSB;
#else
#error "Unknown __BYTE_ORDER__"
#endif

        /* STRTAB */
        /* initialize strset with an empty string to conform to ELF */
        linker->strtab_strs = strset__new(INT_MAX, "", sizeof(""));
        if (libbpf_get_error(linker->strtab_strs))
                return libbpf_get_error(linker->strtab_strs);

        sec = add_dst_sec(linker, ".strtab");
        if (!sec)
                return -ENOMEM;

        sec->scn = elf_newscn(linker->elf);
        if (!sec->scn) {
                pr_warn_elf("failed to create STRTAB section");
                return -EINVAL;
        }

        sec->shdr = elf64_getshdr(sec->scn);
        if (!sec->shdr)
                return -EINVAL;

        sec->data = elf_newdata(sec->scn);
        if (!sec->data) {
                pr_warn_elf("failed to create STRTAB data");
                return -EINVAL;
        }

        str_off = strset__add_str(linker->strtab_strs, sec->sec_name);
        if (str_off < 0)
                return str_off;

        sec->sec_idx = elf_ndxscn(sec->scn);
        linker->elf_hdr->e_shstrndx = sec->sec_idx;
        linker->strtab_sec_idx = sec->sec_idx;

        sec->shdr->sh_name = str_off;
        sec->shdr->sh_type = SHT_STRTAB;
        sec->shdr->sh_flags = SHF_STRINGS;
        sec->shdr->sh_offset = 0;
        sec->shdr->sh_link = 0;
        sec->shdr->sh_info = 0;
        sec->shdr->sh_addralign = 1;
        sec->shdr->sh_size = sec->sec_sz = 0;
        sec->shdr->sh_entsize = 0;

        /* SYMTAB */
        sec = add_dst_sec(linker, ".symtab");
        if (!sec)
                return -ENOMEM;

        sec->scn = elf_newscn(linker->elf);
        if (!sec->scn) {
                pr_warn_elf("failed to create SYMTAB section");
                return -EINVAL;
        }

        sec->shdr = elf64_getshdr(sec->scn);
        if (!sec->shdr)
                return -EINVAL;

        sec->data = elf_newdata(sec->scn);
        if (!sec->data) {
                pr_warn_elf("failed to create SYMTAB data");
                return -EINVAL;
        }

        str_off = strset__add_str(linker->strtab_strs, sec->sec_name);
        if (str_off < 0)
                return str_off;

        sec->sec_idx = elf_ndxscn(sec->scn);
        linker->symtab_sec_idx = sec->sec_idx;

        sec->shdr->sh_name = str_off;
        sec->shdr->sh_type = SHT_SYMTAB;
        sec->shdr->sh_flags = 0;
        sec->shdr->sh_offset = 0;
        sec->shdr->sh_link = linker->strtab_sec_idx;
        /* sh_info should be one greater than the index of the last local
         * symbol (i.e., binding is STB_LOCAL). But why and who cares?
         */
        sec->shdr->sh_info = 0;
        sec->shdr->sh_addralign = 8;
        sec->shdr->sh_entsize = sizeof(Elf64_Sym);

        /* .BTF */
        linker->btf = btf__new_empty();
        err = libbpf_get_error(linker->btf);
        if (err)
                return err;

        /* add the special all-zero symbol */
        init_sym = add_new_sym(linker, NULL);
        if (!init_sym)
                return -EINVAL;

        init_sym->st_name = 0;
        init_sym->st_info = 0;
        init_sym->st_other = 0;
        init_sym->st_shndx = SHN_UNDEF;
        init_sym->st_value = 0;
        init_sym->st_size = 0;

        return 0;
}

int bpf_linker__add_file(struct bpf_linker *linker, const char *filename,
                         const struct bpf_linker_file_opts *opts)
{
        struct src_obj obj = {};
        int err = 0;

        if (!OPTS_VALID(opts, bpf_linker_file_opts))
                return libbpf_err(-EINVAL);

        if (!linker->elf)
                return libbpf_err(-EINVAL);

        err = err ?: linker_load_obj_file(linker, filename, opts, &obj);
        err = err ?: linker_append_sec_data(linker, &obj);
        err = err ?: linker_append_elf_syms(linker, &obj);
        err = err ?: linker_append_elf_relos(linker, &obj);
        err = err ?: linker_append_btf(linker, &obj);
        err = err ?: linker_append_btf_ext(linker, &obj);

        /* free up src_obj resources */
        free(obj.btf_type_map);
        btf__free(obj.btf);
        btf_ext__free(obj.btf_ext);
        free(obj.secs);
        free(obj.sym_map);
        if (obj.elf)
                elf_end(obj.elf);
        if (obj.fd >= 0)
                close(obj.fd);

        return libbpf_err(err);
}

static bool is_dwarf_sec_name(const char *name)
{
        /* approximation, but the actual list is too long */
        return strncmp(name, ".debug_", sizeof(".debug_") - 1) == 0;
}

static bool is_ignored_sec(struct src_sec *sec)
{
        Elf64_Shdr *shdr = sec->shdr;
        const char *name = sec->sec_name;

        /* no special handling of .strtab */
        if (shdr->sh_type == SHT_STRTAB)
                return true;

        /* ignore .llvm_addrsig section as well */
        if (shdr->sh_type == SHT_LLVM_ADDRSIG)
                return true;

        /* no subprograms will lead to an empty .text section, ignore it */
        if (shdr->sh_type == SHT_PROGBITS && shdr->sh_size == 0 &&
            strcmp(sec->sec_name, ".text") == 0)
                return true;

        /* DWARF sections */
        if (is_dwarf_sec_name(sec->sec_name))
                return true;

        if (strncmp(name, ".rel", sizeof(".rel") - 1) == 0) {
                name += sizeof(".rel") - 1;
                /* DWARF section relocations */
                if (is_dwarf_sec_name(name))
                        return true;

                /* .BTF and .BTF.ext don't need relocations */
                if (strcmp(name, BTF_ELF_SEC) == 0 ||
                    strcmp(name, BTF_EXT_ELF_SEC) == 0)
                        return true;
        }

        return false;
}

static struct src_sec *add_src_sec(struct src_obj *obj, const char *sec_name)
{
        struct src_sec *secs = obj->secs, *sec;
        size_t new_cnt = obj->sec_cnt ? obj->sec_cnt + 1 : 2;

        secs = libbpf_reallocarray(secs, new_cnt, sizeof(*secs));
        if (!secs)
                return NULL;

        /* zero out newly allocated memory */
        memset(secs + obj->sec_cnt, 0, (new_cnt - obj->sec_cnt) * sizeof(*secs));

        obj->secs = secs;
        obj->sec_cnt = new_cnt;

        sec = &obj->secs[new_cnt - 1];
        sec->id = new_cnt - 1;
        sec->sec_name = sec_name;

        return sec;
}

static int linker_load_obj_file(struct bpf_linker *linker, const char *filename,
                                const struct bpf_linker_file_opts *opts,
                                struct src_obj *obj)
{
#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
        const int host_endianness = ELFDATA2LSB;
#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
        const int host_endianness = ELFDATA2MSB;
#else
#error "Unknown __BYTE_ORDER__"
#endif
        int err = 0;
        Elf_Scn *scn;
        Elf_Data *data;
        Elf64_Ehdr *ehdr;
        Elf64_Shdr *shdr;
        struct src_sec *sec;

        pr_debug("linker: adding object file '%s'...\n", filename);

        obj->filename = filename;

        obj->fd = open(filename, O_RDONLY | O_CLOEXEC);
        if (obj->fd < 0) {
                err = -errno;
                pr_warn("failed to open file '%s': %d\n", filename, err);
                return err;
        }
        obj->elf = elf_begin(obj->fd, ELF_C_READ_MMAP, NULL);
        if (!obj->elf) {
                err = -errno;
                pr_warn_elf("failed to parse ELF file '%s'", filename);
                return err;
        }

        /* Sanity check ELF file high-level properties */
        ehdr = elf64_getehdr(obj->elf);
        if (!ehdr) {
                err = -errno;
                pr_warn_elf("failed to get ELF header for %s", filename);
                return err;
        }
        if (ehdr->e_ident[EI_DATA] != host_endianness) {
                err = -EOPNOTSUPP;
                pr_warn_elf("unsupported byte order of ELF file %s", filename);
                return err;
        }
        if (ehdr->e_type != ET_REL
            || ehdr->e_machine != EM_BPF
            || ehdr->e_ident[EI_CLASS] != ELFCLASS64) {
                err = -EOPNOTSUPP;
                pr_warn_elf("unsupported kind of ELF file %s", filename);
                return err;
        }

        if (elf_getshdrstrndx(obj->elf, &obj->shstrs_sec_idx)) {
                err = -errno;
                pr_warn_elf("failed to get SHSTRTAB section index for %s", filename);
                return err;
        }

        scn = NULL;
        while ((scn = elf_nextscn(obj->elf, scn)) != NULL) {
                size_t sec_idx = elf_ndxscn(scn);
                const char *sec_name;

                shdr = elf64_getshdr(scn);
                if (!shdr) {
                        err = -errno;
                        pr_warn_elf("failed to get section #%zu header for %s",
                                    sec_idx, filename);
                        return err;
                }

                sec_name = elf_strptr(obj->elf, obj->shstrs_sec_idx, shdr->sh_name);
                if (!sec_name) {
                        err = -errno;
                        pr_warn_elf("failed to get section #%zu name for %s",
                                    sec_idx, filename);
                        return err;
                }

                data = elf_getdata(scn, 0);
                if (!data) {
                        err = -errno;
                        pr_warn_elf("failed to get section #%zu (%s) data from %s",
                                    sec_idx, sec_name, filename);
                        return err;
                }

                sec = add_src_sec(obj, sec_name);
                if (!sec)
                        return -ENOMEM;

                sec->scn = scn;
                sec->shdr = shdr;
                sec->data = data;
                sec->sec_idx = elf_ndxscn(scn);

                if (is_ignored_sec(sec)) {
                        sec->skipped = true;
                        continue;
                }

                switch (shdr->sh_type) {
                case SHT_SYMTAB:
                        if (obj->symtab_sec_idx) {
                                err = -EOPNOTSUPP;
                                pr_warn("multiple SYMTAB sections found, not supported\n");
                                return err;
                        }
                        obj->symtab_sec_idx = sec_idx;
                        break;
                case SHT_STRTAB:
                        /* we'll construct our own string table */
                        break;
                case SHT_PROGBITS:
                        if (strcmp(sec_name, BTF_ELF_SEC) == 0) {
                                obj->btf = btf__new(data->d_buf, shdr->sh_size);
                                err = libbpf_get_error(obj->btf);
                                if (err) {
                                        pr_warn("failed to parse .BTF from %s: %d\n", filename, err);
                                        return err;
                                }
                                sec->skipped = true;
                                continue;
                        }
                        if (strcmp(sec_name, BTF_EXT_ELF_SEC) == 0) {
                                obj->btf_ext = btf_ext__new(data->d_buf, shdr->sh_size);
                                err = libbpf_get_error(obj->btf_ext);
                                if (err) {
                                        pr_warn("failed to parse .BTF.ext from '%s': %d\n", filename, err);
                                        return err;
                                }
                                sec->skipped = true;
                                continue;
                        }

                        /* data & code */
                        break;
                case SHT_NOBITS:
                        /* BSS */
                        break;
                case SHT_REL:
                        /* relocations */
                        break;
                default:
                        pr_warn("unrecognized section #%zu (%s) in %s\n",
                                sec_idx, sec_name, filename);
                        err = -EINVAL;
                        return err;
                }
        }

        err = err ?: linker_sanity_check_elf(obj);
        err = err ?: linker_sanity_check_btf(obj);
        err = err ?: linker_sanity_check_btf_ext(obj);
        err = err ?: linker_fixup_btf(obj);

        return err;
}

static bool is_pow_of_2(size_t x)
{
        return x && (x & (x - 1)) == 0;
}

static int linker_sanity_check_elf(struct src_obj *obj)
{
        struct src_sec *sec;
        int i, err;

        if (!obj->symtab_sec_idx) {
                pr_warn("ELF is missing SYMTAB section in %s\n", obj->filename);
                return -EINVAL;
        }
        if (!obj->shstrs_sec_idx) {
                pr_warn("ELF is missing section headers STRTAB section in %s\n", obj->filename);
                return -EINVAL;
        }

        for (i = 1; i < obj->sec_cnt; i++) {
                sec = &obj->secs[i];

                if (sec->sec_name[0] == '\0') {
                        pr_warn("ELF section #%zu has empty name in %s\n", sec->sec_idx, obj->filename);
                        return -EINVAL;
                }

                if (sec->shdr->sh_addralign && !is_pow_of_2(sec->shdr->sh_addralign))
                        return -EINVAL;
                if (sec->shdr->sh_addralign != sec->data->d_align)
                        return -EINVAL;

                if (sec->shdr->sh_size != sec->data->d_size)
                        return -EINVAL;

                switch (sec->shdr->sh_type) {
                case SHT_SYMTAB:
                        err = linker_sanity_check_elf_symtab(obj, sec);
                        if (err)
                                return err;
                        break;
                case SHT_STRTAB:
                        break;
                case SHT_PROGBITS:
                        if (sec->shdr->sh_flags & SHF_EXECINSTR) {
                                if (sec->shdr->sh_size % sizeof(struct bpf_insn) != 0)
                                        return -EINVAL;
                        }
                        break;
                case SHT_NOBITS:
                        break;
                case SHT_REL:
                        err = linker_sanity_check_elf_relos(obj, sec);
                        if (err)
                                return err;
                        break;
                case SHT_LLVM_ADDRSIG:
                        break;
                default:
                        pr_warn("ELF section #%zu (%s) has unrecognized type %zu in %s\n",
                                sec->sec_idx, sec->sec_name, (size_t)sec->shdr->sh_type, obj->filename);
                        return -EINVAL;
                }
        }

        return 0;
}

static int linker_sanity_check_elf_symtab(struct src_obj *obj, struct src_sec *sec)
{
        struct src_sec *link_sec;
        Elf64_Sym *sym;
        int i, n;

        if (sec->shdr->sh_entsize != sizeof(Elf64_Sym))
                return -EINVAL;
        if (sec->shdr->sh_size % sec->shdr->sh_entsize != 0)
                return -EINVAL;

        if (!sec->shdr->sh_link || sec->shdr->sh_link >= obj->sec_cnt) {
                pr_warn("ELF SYMTAB section #%zu points to missing STRTAB section #%zu in %s\n",
                        sec->sec_idx, (size_t)sec->shdr->sh_link, obj->filename);
                return -EINVAL;
        }
        link_sec = &obj->secs[sec->shdr->sh_link];
        if (link_sec->shdr->sh_type != SHT_STRTAB) {
                pr_warn("ELF SYMTAB section #%zu points to invalid STRTAB section #%zu in %s\n",
                        sec->sec_idx, (size_t)sec->shdr->sh_link, obj->filename);
                return -EINVAL;
        }

        n = sec->shdr->sh_size / sec->shdr->sh_entsize;
        sym = sec->data->d_buf;
        for (i = 0; i < n; i++, sym++) {
                int sym_type = ELF64_ST_TYPE(sym->st_info);
                int sym_bind = ELF64_ST_BIND(sym->st_info);
                int sym_vis = ELF64_ST_VISIBILITY(sym->st_other);

                if (i == 0) {
                        if (sym->st_name != 0 || sym->st_info != 0
                            || sym->st_other != 0 || sym->st_shndx != 0
                            || sym->st_value != 0 || sym->st_size != 0) {
                                pr_warn("ELF sym #0 is invalid in %s\n", obj->filename);
                                return -EINVAL;
                        }
                        continue;
                }
                if (sym_bind != STB_LOCAL && sym_bind != STB_GLOBAL && sym_bind != STB_WEAK) {
                        pr_warn("ELF sym #%d in section #%zu has unsupported symbol binding %d\n",
                                i, sec->sec_idx, sym_bind);
                        return -EINVAL;
                }
                if (sym_vis != STV_DEFAULT && sym_vis != STV_HIDDEN) {
                        pr_warn("ELF sym #%d in section #%zu has unsupported symbol visibility %d\n",
                                i, sec->sec_idx, sym_vis);
                        return -EINVAL;
                }
                if (sym->st_shndx == 0) {
                        if (sym_type != STT_NOTYPE || sym_bind == STB_LOCAL
                            || sym->st_value != 0 || sym->st_size != 0) {
                                pr_warn("ELF sym #%d is invalid extern symbol in %s\n",
                                        i, obj->filename);

                                return -EINVAL;
                        }
                        continue;
                }
                if (sym->st_shndx < SHN_LORESERVE && sym->st_shndx >= obj->sec_cnt) {
                        pr_warn("ELF sym #%d in section #%zu points to missing section #%zu in %s\n",
                                i, sec->sec_idx, (size_t)sym->st_shndx, obj->filename);
                        return -EINVAL;
                }
                if (sym_type == STT_SECTION) {
                        if (sym->st_value != 0)
                                return -EINVAL;
                        continue;
                }
        }

        return 0;
}

static int linker_sanity_check_elf_relos(struct src_obj *obj, struct src_sec *sec)
{
        struct src_sec *link_sec, *sym_sec;
        Elf64_Rel *relo;
        int i, n;

        if (sec->shdr->sh_entsize != sizeof(Elf64_Rel))
                return -EINVAL;
        if (sec->shdr->sh_size % sec->shdr->sh_entsize != 0)
                return -EINVAL;

        /* SHT_REL's sh_link should point to SYMTAB */
        if (sec->shdr->sh_link != obj->symtab_sec_idx) {
                pr_warn("ELF relo section #%zu points to invalid SYMTAB section #%zu in %s\n",
                        sec->sec_idx, (size_t)sec->shdr->sh_link, obj->filename);
                return -EINVAL;
        }

        /* SHT_REL's sh_info points to relocated section */
        if (!sec->shdr->sh_info || sec->shdr->sh_info >= obj->sec_cnt) {
                pr_warn("ELF relo section #%zu points to missing section #%zu in %s\n",
                        sec->sec_idx, (size_t)sec->shdr->sh_info, obj->filename);
                return -EINVAL;
        }
        link_sec = &obj->secs[sec->shdr->sh_info];

        /* .rel<secname> -> <secname> pattern is followed */
        if (strncmp(sec->sec_name, ".rel", sizeof(".rel") - 1) != 0
            || strcmp(sec->sec_name + sizeof(".rel") - 1, link_sec->sec_name) != 0) {
                pr_warn("ELF relo section #%zu name has invalid name in %s\n",
                        sec->sec_idx, obj->filename);
                return -EINVAL;
        }

        /* don't further validate relocations for ignored sections */
        if (link_sec->skipped)
                return 0;

        /* relocatable section is data or instructions */
        if (link_sec->shdr->sh_type != SHT_PROGBITS && link_sec->shdr->sh_type != SHT_NOBITS) {
                pr_warn("ELF relo section #%zu points to invalid section #%zu in %s\n",
                        sec->sec_idx, (size_t)sec->shdr->sh_info, obj->filename);
                return -EINVAL;
        }

        /* check sanity of each relocation */
        n = sec->shdr->sh_size / sec->shdr->sh_entsize;
        relo = sec->data->d_buf;
        sym_sec = &obj->secs[obj->symtab_sec_idx];
        for (i = 0; i < n; i++, relo++) {
                size_t sym_idx = ELF64_R_SYM(relo->r_info);
                size_t sym_type = ELF64_R_TYPE(relo->r_info);

                if (sym_type != R_BPF_64_64 && sym_type != R_BPF_64_32 &&
                    sym_type != R_BPF_64_ABS64 && sym_type != R_BPF_64_ABS32) {
                        pr_warn("ELF relo #%d in section #%zu has unexpected type %zu in %s\n",
                                i, sec->sec_idx, sym_type, obj->filename);
                        return -EINVAL;
                }

                if (!sym_idx || sym_idx * sizeof(Elf64_Sym) >= sym_sec->shdr->sh_size) {
                        pr_warn("ELF relo #%d in section #%zu points to invalid symbol #%zu in %s\n",
                                i, sec->sec_idx, sym_idx, obj->filename);
                        return -EINVAL;
                }

                if (link_sec->shdr->sh_flags & SHF_EXECINSTR) {
                        if (relo->r_offset % sizeof(struct bpf_insn) != 0) {
                                pr_warn("ELF relo #%d in section #%zu points to missing symbol #%zu in %s\n",
                                        i, sec->sec_idx, sym_idx, obj->filename);
                                return -EINVAL;
                        }
                }
        }

        return 0;
}

static int check_btf_type_id(__u32 *type_id, void *ctx)
{
        struct btf *btf = ctx;

        if (*type_id >= btf__type_cnt(btf))
                return -EINVAL;

        return 0;
}

static int check_btf_str_off(__u32 *str_off, void *ctx)
{
        struct btf *btf = ctx;
        const char *s;

        s = btf__str_by_offset(btf, *str_off);

        if (!s)
                return -EINVAL;

        return 0;
}

static int linker_sanity_check_btf(struct src_obj *obj)
{
        struct btf_type *t;
        int i, n, err = 0;

        if (!obj->btf)
                return 0;

        n = btf__type_cnt(obj->btf);
        for (i = 1; i < n; i++) {
                t = btf_type_by_id(obj->btf, i);

                err = err ?: btf_type_visit_type_ids(t, check_btf_type_id, obj->btf);
                err = err ?: btf_type_visit_str_offs(t, check_btf_str_off, obj->btf);
                if (err)
                        return err;
        }

        return 0;
}

static int linker_sanity_check_btf_ext(struct src_obj *obj)
{
        int err = 0;

        if (!obj->btf_ext)
                return 0;

        /* can't use .BTF.ext without .BTF */
        if (!obj->btf)
                return -EINVAL;

        err = err ?: btf_ext_visit_type_ids(obj->btf_ext, check_btf_type_id, obj->btf);
        err = err ?: btf_ext_visit_str_offs(obj->btf_ext, check_btf_str_off, obj->btf);
        if (err)
                return err;

        return 0;
}

static int init_sec(struct bpf_linker *linker, struct dst_sec *dst_sec, struct src_sec *src_sec)
{
        Elf_Scn *scn;
        Elf_Data *data;
        Elf64_Shdr *shdr;
        int name_off;

        dst_sec->sec_sz = 0;
        dst_sec->sec_idx = 0;
        dst_sec->ephemeral = src_sec->ephemeral;

        /* ephemeral sections are just thin section shells lacking most parts */
        if (src_sec->ephemeral)
                return 0;

        scn = elf_newscn(linker->elf);
        if (!scn)
                return -ENOMEM;
        data = elf_newdata(scn);

        if (!data)
                return -ENOMEM;
        shdr = elf64_getshdr(scn);
        if (!shdr)
                return -ENOMEM;

        dst_sec->scn = scn;
        dst_sec->shdr = shdr;
        dst_sec->data = data;
        dst_sec->sec_idx = elf_ndxscn(scn);

        name_off = strset__add_str(linker->strtab_strs, src_sec->sec_name);
        if (name_off < 0)
                return name_off;

        shdr->sh_name = name_off;
        shdr->sh_type = src_sec->shdr->sh_type;
        shdr->sh_flags = src_sec->shdr->sh_flags;
        shdr->sh_size = 0;
        /* sh_link and sh_info have different meaning for different types of
         * sections, so we leave it up to the caller code to fill them in, if
         * necessary
         */
        shdr->sh_link = 0;
        shdr->sh_info = 0;
        shdr->sh_addralign = src_sec->shdr->sh_addralign;
        shdr->sh_entsize = src_sec->shdr->sh_entsize;

        data->d_type = src_sec->data->d_type;
        data->d_size = 0;
        data->d_buf = NULL;
        data->d_align = src_sec->data->d_align;
        data->d_off = 0;

        return 0;
}

static struct dst_sec *find_dst_sec_by_name(struct bpf_linker *linker, const char *sec_name)
{
        struct dst_sec *sec;
        int i;

        for (i = 1; i < linker->sec_cnt; i++) {
                sec = &linker->secs[i];

                if (strcmp(sec->sec_name, sec_name) == 0)
                        return sec;
        }

        return NULL;
}

static bool secs_match(struct dst_sec *dst, struct src_sec *src)
{
        if (dst->ephemeral || src->ephemeral)
                return true;

        if (dst->shdr->sh_type != src->shdr->sh_type) {
                pr_warn("sec %s types mismatch\n", dst->sec_name);
                return false;
        }
        if (dst->shdr->sh_flags != src->shdr->sh_flags) {
                pr_warn("sec %s flags mismatch\n", dst->sec_name);
                return false;
        }
        if (dst->shdr->sh_entsize != src->shdr->sh_entsize) {
                pr_warn("sec %s entsize mismatch\n", dst->sec_name);
                return false;
        }

        return true;
}

static bool sec_content_is_same(struct dst_sec *dst_sec, struct src_sec *src_sec)
{
        if (dst_sec->sec_sz != src_sec->shdr->sh_size)
                return false;
        if (memcmp(dst_sec->raw_data, src_sec->data->d_buf, dst_sec->sec_sz) != 0)
                return false;
        return true;
}

static int extend_sec(struct bpf_linker *linker, struct dst_sec *dst, struct src_sec *src)
{
        void *tmp;
        size_t dst_align, src_align;
        size_t dst_align_sz, dst_final_sz;
        int err;

        /* Ephemeral source section doesn't contribute anything to ELF
         * section data.
         */
        if (src->ephemeral)
                return 0;

        /* Some sections (like .maps) can contain both externs (and thus be
         * ephemeral) and non-externs (map definitions). So it's possible that
         * it has to be "upgraded" from ephemeral to non-ephemeral when the
         * first non-ephemeral entity appears. In such case, we add ELF
         * section, data, etc.
         */
        if (dst->ephemeral) {
                err = init_sec(linker, dst, src);
                if (err)
                        return err;
        }

        dst_align = dst->shdr->sh_addralign;
        src_align = src->shdr->sh_addralign;
        if (dst_align == 0)
                dst_align = 1;
        if (dst_align < src_align)
                dst_align = src_align;

        dst_align_sz = (dst->sec_sz + dst_align - 1) / dst_align * dst_align;

        /* no need to re-align final size */
        dst_final_sz = dst_align_sz + src->shdr->sh_size;

        if (src->shdr->sh_type != SHT_NOBITS) {
                tmp = realloc(dst->raw_data, dst_final_sz);
                if (!tmp)
                        return -ENOMEM;
                dst->raw_data = tmp;

                /* pad dst section, if it's alignment forced size increase */
                memset(dst->raw_data + dst->sec_sz, 0, dst_align_sz - dst->sec_sz);
                /* now copy src data at a properly aligned offset */
                memcpy(dst->raw_data + dst_align_sz, src->data->d_buf, src->shdr->sh_size);
        }

        dst->sec_sz = dst_final_sz;
        dst->shdr->sh_size = dst_final_sz;
        dst->data->d_size = dst_final_sz;

        dst->shdr->sh_addralign = dst_align;
        dst->data->d_align = dst_align;

        src->dst_off = dst_align_sz;

        return 0;
}

static bool is_data_sec(struct src_sec *sec)
{
        if (!sec || sec->skipped)
                return false;
        /* ephemeral sections are data sections, e.g., .kconfig, .ksyms */
        if (sec->ephemeral)
                return true;
        return sec->shdr->sh_type == SHT_PROGBITS || sec->shdr->sh_type == SHT_NOBITS;
}

static bool is_relo_sec(struct src_sec *sec)
{
        if (!sec || sec->skipped || sec->ephemeral)
                return false;
        return sec->shdr->sh_type == SHT_REL;
}

static int linker_append_sec_data(struct bpf_linker *linker, struct src_obj *obj)
{
        int i, err;

        for (i = 1; i < obj->sec_cnt; i++) {
                struct src_sec *src_sec;
                struct dst_sec *dst_sec;

                src_sec = &obj->secs[i];
                if (!is_data_sec(src_sec))
                        continue;

                dst_sec = find_dst_sec_by_name(linker, src_sec->sec_name);
                if (!dst_sec) {
                        dst_sec = add_dst_sec(linker, src_sec->sec_name);
                        if (!dst_sec)
                                return -ENOMEM;
                        err = init_sec(linker, dst_sec, src_sec);
                        if (err) {
                                pr_warn("failed to init section '%s'\n", src_sec->sec_name);
                                return err;
                        }
                } else {
                        if (!secs_match(dst_sec, src_sec)) {
                                pr_warn("ELF sections %s are incompatible\n", src_sec->sec_name);
                                return -1;
                        }

                        /* "license" and "version" sections are deduped */
                        if (strcmp(src_sec->sec_name, "license") == 0
                            || strcmp(src_sec->sec_name, "version") == 0) {
                                if (!sec_content_is_same(dst_sec, src_sec)) {
                                        pr_warn("non-identical contents of section '%s' are not supported\n", src_sec->sec_name);
                                        return -EINVAL;
                                }
                                src_sec->skipped = true;
                                src_sec->dst_id = dst_sec->id;
                                continue;
                        }
                }

                /* record mapped section index */
                src_sec->dst_id = dst_sec->id;

                err = extend_sec(linker, dst_sec, src_sec);
                if (err)
                        return err;
        }

        return 0;
}

static int linker_append_elf_syms(struct bpf_linker *linker, struct src_obj *obj)
{
        struct src_sec *symtab = &obj->secs[obj->symtab_sec_idx];
        Elf64_Sym *sym = symtab->data->d_buf;
        int i, n = symtab->shdr->sh_size / symtab->shdr->sh_entsize, err;
        int str_sec_idx = symtab->shdr->sh_link;
        const char *sym_name;

        obj->sym_map = calloc(n + 1, sizeof(*obj->sym_map));
        if (!obj->sym_map)
                return -ENOMEM;

        for (i = 0; i < n; i++, sym++) {
                /* We already validated all-zero symbol #0 and we already
                 * appended it preventively to the final SYMTAB, so skip it.
                 */
                if (i == 0)
                        continue;

                sym_name = elf_strptr(obj->elf, str_sec_idx, sym->st_name);
                if (!sym_name) {
                        pr_warn("can't fetch symbol name for symbol #%d in '%s'\n", i, obj->filename);
                        return -EINVAL;
                }

                err = linker_append_elf_sym(linker, obj, sym, sym_name, i);
                if (err)
                        return err;
        }

        return 0;
}

static Elf64_Sym *get_sym_by_idx(struct bpf_linker *linker, size_t sym_idx)
{
        struct dst_sec *symtab = &linker->secs[linker->symtab_sec_idx];
        Elf64_Sym *syms = symtab->raw_data;

        return &syms[sym_idx];
}

static struct glob_sym *find_glob_sym(struct bpf_linker *linker, const char *sym_name)
{
        struct glob_sym *glob_sym;
        const char *name;
        int i;

        for (i = 0; i < linker->glob_sym_cnt; i++) {
                glob_sym = &linker->glob_syms[i];
                name = strset__data(linker->strtab_strs) + glob_sym->name_off;

                if (strcmp(name, sym_name) == 0)
                        return glob_sym;
        }

        return NULL;
}

static struct glob_sym *add_glob_sym(struct bpf_linker *linker)
{
        struct glob_sym *syms, *sym;

        syms = libbpf_reallocarray(linker->glob_syms, linker->glob_sym_cnt + 1,
                                   sizeof(*linker->glob_syms));
        if (!syms)
                return NULL;

        sym = &syms[linker->glob_sym_cnt];
        memset(sym, 0, sizeof(*sym));
        sym->var_idx = -1;

        linker->glob_syms = syms;
        linker->glob_sym_cnt++;

        return sym;
}

static bool glob_sym_btf_matches(const char *sym_name, bool exact,
                                 const struct btf *btf1, __u32 id1,
                                 const struct btf *btf2, __u32 id2)
{
        const struct btf_type *t1, *t2;
        bool is_static1, is_static2;
        const char *n1, *n2;
        int i, n;

recur:
        n1 = n2 = NULL;
        t1 = skip_mods_and_typedefs(btf1, id1, &id1);
        t2 = skip_mods_and_typedefs(btf2, id2, &id2);

        /* check if only one side is FWD, otherwise handle with common logic */
        if (!exact && btf_is_fwd(t1) != btf_is_fwd(t2)) {
                n1 = btf__str_by_offset(btf1, t1->name_off);
                n2 = btf__str_by_offset(btf2, t2->name_off);
                if (strcmp(n1, n2) != 0) {
                        pr_warn("global '%s': incompatible forward declaration names '%s' and '%s'\n",
                                sym_name, n1, n2);
                        return false;
                }
                /* validate if FWD kind matches concrete kind */
                if (btf_is_fwd(t1)) {
                        if (btf_kflag(t1) && btf_is_union(t2))
                                return true;
                        if (!btf_kflag(t1) && btf_is_struct(t2))
                                return true;
                        pr_warn("global '%s': incompatible %s forward declaration and concrete kind %s\n",
                                sym_name, btf_kflag(t1) ? "union" : "struct", btf_kind_str(t2));
                } else {
                        if (btf_kflag(t2) && btf_is_union(t1))
                                return true;
                        if (!btf_kflag(t2) && btf_is_struct(t1))
                                return true;
                        pr_warn("global '%s': incompatible %s forward declaration and concrete kind %s\n",
                                sym_name, btf_kflag(t2) ? "union" : "struct", btf_kind_str(t1));
                }
                return false;
        }

        if (btf_kind(t1) != btf_kind(t2)) {
                pr_warn("global '%s': incompatible BTF kinds %s and %s\n",
                        sym_name, btf_kind_str(t1), btf_kind_str(t2));
                return false;
        }

        switch (btf_kind(t1)) {
        case BTF_KIND_STRUCT:
        case BTF_KIND_UNION:
        case BTF_KIND_ENUM:
        case BTF_KIND_FWD:
        case BTF_KIND_FUNC:
        case BTF_KIND_VAR:
                n1 = btf__str_by_offset(btf1, t1->name_off);
                n2 = btf__str_by_offset(btf2, t2->name_off);
                if (strcmp(n1, n2) != 0) {
                        pr_warn("global '%s': incompatible %s names '%s' and '%s'\n",
                                sym_name, btf_kind_str(t1), n1, n2);
                        return false;
                }
                break;
        default:
                break;
        }

        switch (btf_kind(t1)) {
        case BTF_KIND_UNKN: /* void */
        case BTF_KIND_FWD:
                return true;
        case BTF_KIND_INT:
        case BTF_KIND_FLOAT:
        case BTF_KIND_ENUM:
                /* ignore encoding for int and enum values for enum */
                if (t1->size != t2->size) {
                        pr_warn("global '%s': incompatible %s '%s' size %u and %u\n",
                                sym_name, btf_kind_str(t1), n1, t1->size, t2->size);
                        return false;
                }
                return true;
        case BTF_KIND_PTR:
                /* just validate overall shape of the referenced type, so no
                 * contents comparison for struct/union, and allowd fwd vs
                 * struct/union
                 */
                exact = false;
                id1 = t1->type;
                id2 = t2->type;
                goto recur;
        case BTF_KIND_ARRAY:
                /* ignore index type and array size */
                id1 = btf_array(t1)->type;
                id2 = btf_array(t2)->type;
                goto recur;
        case BTF_KIND_FUNC:
                /* extern and global linkages are compatible */
                is_static1 = btf_func_linkage(t1) == BTF_FUNC_STATIC;
                is_static2 = btf_func_linkage(t2) == BTF_FUNC_STATIC;
                if (is_static1 != is_static2) {
                        pr_warn("global '%s': incompatible func '%s' linkage\n", sym_name, n1);
                        return false;
                }

                id1 = t1->type;
                id2 = t2->type;
                goto recur;
        case BTF_KIND_VAR:
                /* extern and global linkages are compatible */
                is_static1 = btf_var(t1)->linkage == BTF_VAR_STATIC;
                is_static2 = btf_var(t2)->linkage == BTF_VAR_STATIC;
                if (is_static1 != is_static2) {
                        pr_warn("global '%s': incompatible var '%s' linkage\n", sym_name, n1);
                        return false;
                }

                id1 = t1->type;
                id2 = t2->type;
                goto recur;
        case BTF_KIND_STRUCT:
        case BTF_KIND_UNION: {
                const struct btf_member *m1, *m2;

                if (!exact)
                        return true;

                if (btf_vlen(t1) != btf_vlen(t2)) {
                        pr_warn("global '%s': incompatible number of %s fields %u and %u\n",
                                sym_name, btf_kind_str(t1), btf_vlen(t1), btf_vlen(t2));
                        return false;
                }

                n = btf_vlen(t1);
                m1 = btf_members(t1);
                m2 = btf_members(t2);
                for (i = 0; i < n; i++, m1++, m2++) {
                        n1 = btf__str_by_offset(btf1, m1->name_off);
                        n2 = btf__str_by_offset(btf2, m2->name_off);
                        if (strcmp(n1, n2) != 0) {
                                pr_warn("global '%s': incompatible field #%d names '%s' and '%s'\n",
                                        sym_name, i, n1, n2);
                                return false;
                        }
                        if (m1->offset != m2->offset) {
                                pr_warn("global '%s': incompatible field #%d ('%s') offsets\n",
                                        sym_name, i, n1);
                                return false;
                        }
                        if (!glob_sym_btf_matches(sym_name, exact, btf1, m1->type, btf2, m2->type))
                                return false;
                }

                return true;
        }
        case BTF_KIND_FUNC_PROTO: {
                const struct btf_param *m1, *m2;

                if (btf_vlen(t1) != btf_vlen(t2)) {
                        pr_warn("global '%s': incompatible number of %s params %u and %u\n",
                                sym_name, btf_kind_str(t1), btf_vlen(t1), btf_vlen(t2));
                        return false;
                }

                n = btf_vlen(t1);
                m1 = btf_params(t1);
                m2 = btf_params(t2);
                for (i = 0; i < n; i++, m1++, m2++) {
                        /* ignore func arg names */
                        if (!glob_sym_btf_matches(sym_name, exact, btf1, m1->type, btf2, m2->type))
                                return false;
                }

                /* now check return type as well */
                id1 = t1->type;
                id2 = t2->type;
                goto recur;
        }

        /* skip_mods_and_typedefs() make this impossible */
        case BTF_KIND_TYPEDEF:
        case BTF_KIND_VOLATILE:
        case BTF_KIND_CONST:
        case BTF_KIND_RESTRICT:
        /* DATASECs are never compared with each other */
        case BTF_KIND_DATASEC:
        default:
                pr_warn("global '%s': unsupported BTF kind %s\n",
                        sym_name, btf_kind_str(t1));
                return false;
        }
}

static bool map_defs_match(const char *sym_name,
                           const struct btf *main_btf,
                           const struct btf_map_def *main_def,
                           const struct btf_map_def *main_inner_def,
                           const struct btf *extra_btf,
                           const struct btf_map_def *extra_def,
                           const struct btf_map_def *extra_inner_def)
{
        const char *reason;

        if (main_def->map_type != extra_def->map_type) {
                reason = "type";
                goto mismatch;
        }

        /* check key type/size match */
        if (main_def->key_size != extra_def->key_size) {
                reason = "key_size";
                goto mismatch;
        }
        if (!!main_def->key_type_id != !!extra_def->key_type_id) {
                reason = "key type";
                goto mismatch;
        }
        if ((main_def->parts & MAP_DEF_KEY_TYPE)
             && !glob_sym_btf_matches(sym_name, true /*exact*/,
                                      main_btf, main_def->key_type_id,
                                      extra_btf, extra_def->key_type_id)) {
                reason = "key type";
                goto mismatch;
        }

        /* validate value type/size match */
        if (main_def->value_size != extra_def->value_size) {
                reason = "value_size";
                goto mismatch;
        }
        if (!!main_def->value_type_id != !!extra_def->value_type_id) {
                reason = "value type";
                goto mismatch;
        }
        if ((main_def->parts & MAP_DEF_VALUE_TYPE)
             && !glob_sym_btf_matches(sym_name, true /*exact*/,
                                      main_btf, main_def->value_type_id,
                                      extra_btf, extra_def->value_type_id)) {
                reason = "key type";
                goto mismatch;
        }

        if (main_def->max_entries != extra_def->max_entries) {
                reason = "max_entries";
                goto mismatch;
        }
        if (main_def->map_flags != extra_def->map_flags) {
                reason = "map_flags";
                goto mismatch;
        }
        if (main_def->numa_node != extra_def->numa_node) {
                reason = "numa_node";
                goto mismatch;
        }
        if (main_def->pinning != extra_def->pinning) {
                reason = "pinning";
                goto mismatch;
        }

        if ((main_def->parts & MAP_DEF_INNER_MAP) != (extra_def->parts & MAP_DEF_INNER_MAP)) {
                reason = "inner map";
                goto mismatch;
        }

        if (main_def->parts & MAP_DEF_INNER_MAP) {
                char inner_map_name[128];

                snprintf(inner_map_name, sizeof(inner_map_name), "%s.inner", sym_name);

                return map_defs_match(inner_map_name,
                                      main_btf, main_inner_def, NULL,
                                      extra_btf, extra_inner_def, NULL);
        }

        return true;

mismatch:
        pr_warn("global '%s': map %s mismatch\n", sym_name, reason);
        return false;
}

static bool glob_map_defs_match(const char *sym_name,
                                struct bpf_linker *linker, struct glob_sym *glob_sym,
                                struct src_obj *obj, Elf64_Sym *sym, int btf_id)
{
        struct btf_map_def dst_def = {}, dst_inner_def = {};
        struct btf_map_def src_def = {}, src_inner_def = {};
        const struct btf_type *t;
        int err;

        t = btf__type_by_id(obj->btf, btf_id);
        if (!btf_is_var(t)) {
                pr_warn("global '%s': invalid map definition type [%d]\n", sym_name, btf_id);
                return false;
        }
        t = skip_mods_and_typedefs(obj->btf, t->type, NULL);

        err = parse_btf_map_def(sym_name, obj->btf, t, true /*strict*/, &src_def, &src_inner_def);
        if (err) {
                pr_warn("global '%s': invalid map definition\n", sym_name);
                return false;
        }

        /* re-parse existing map definition */
        t = btf__type_by_id(linker->btf, glob_sym->btf_id);
        t = skip_mods_and_typedefs(linker->btf, t->type, NULL);
        err = parse_btf_map_def(sym_name, linker->btf, t, true /*strict*/, &dst_def, &dst_inner_def);
        if (err) {
                /* this should not happen, because we already validated it */
                pr_warn("global '%s': invalid dst map definition\n", sym_name);
                return false;
        }

        /* Currently extern map definition has to be complete and match
         * concrete map definition exactly. This restriction might be lifted
         * in the future.
         */
        return map_defs_match(sym_name, linker->btf, &dst_def, &dst_inner_def,
                              obj->btf, &src_def, &src_inner_def);
}

static bool glob_syms_match(const char *sym_name,
                            struct bpf_linker *linker, struct glob_sym *glob_sym,
                            struct src_obj *obj, Elf64_Sym *sym, size_t sym_idx, int btf_id)
{
        const struct btf_type *src_t;

        /* if we are dealing with externs, BTF types describing both global
         * and extern VARs/FUNCs should be completely present in all files
         */
        if (!glob_sym->btf_id || !btf_id) {
                pr_warn("BTF info is missing for global symbol '%s'\n", sym_name);
                return false;
        }

        src_t = btf__type_by_id(obj->btf, btf_id);
        if (!btf_is_var(src_t) && !btf_is_func(src_t)) {
                pr_warn("only extern variables and functions are supported, but got '%s' for '%s'\n",
                        btf_kind_str(src_t), sym_name);
                return false;
        }

        /* deal with .maps definitions specially */
        if (glob_sym->sec_id && strcmp(linker->secs[glob_sym->sec_id].sec_name, MAPS_ELF_SEC) == 0)
                return glob_map_defs_match(sym_name, linker, glob_sym, obj, sym, btf_id);

        if (!glob_sym_btf_matches(sym_name, true /*exact*/,
                                  linker->btf, glob_sym->btf_id, obj->btf, btf_id))
                return false;

        return true;
}

static bool btf_is_non_static(const struct btf_type *t)
{
        return (btf_is_var(t) && btf_var(t)->linkage != BTF_VAR_STATIC)
               || (btf_is_func(t) && btf_func_linkage(t) != BTF_FUNC_STATIC);
}

static int find_glob_sym_btf(struct src_obj *obj, Elf64_Sym *sym, const char *sym_name,
                             int *out_btf_sec_id, int *out_btf_id)
{
        int i, j, n, m, btf_id = 0;
        const struct btf_type *t;
        const struct btf_var_secinfo *vi;
        const char *name;

        if (!obj->btf) {
                pr_warn("failed to find BTF info for object '%s'\n", obj->filename);
                return -EINVAL;
        }

        n = btf__type_cnt(obj->btf);
        for (i = 1; i < n; i++) {
                t = btf__type_by_id(obj->btf, i);

                /* some global and extern FUNCs and VARs might not be associated with any
                 * DATASEC, so try to detect them in the same pass
                 */
                if (btf_is_non_static(t)) {
                        name = btf__str_by_offset(obj->btf, t->name_off);
                        if (strcmp(name, sym_name) != 0)
                                continue;

                        /* remember and still try to find DATASEC */
                        btf_id = i;
                        continue;
                }

                if (!btf_is_datasec(t))
                        continue;

                vi = btf_var_secinfos(t);
                for (j = 0, m = btf_vlen(t); j < m; j++, vi++) {
                        t = btf__type_by_id(obj->btf, vi->type);
                        name = btf__str_by_offset(obj->btf, t->name_off);

                        if (strcmp(name, sym_name) != 0)
                                continue;
                        if (btf_is_var(t) && btf_var(t)->linkage == BTF_VAR_STATIC)
                                continue;
                        if (btf_is_func(t) && btf_func_linkage(t) == BTF_FUNC_STATIC)
                                continue;

                        if (btf_id && btf_id != vi->type) {
                                pr_warn("global/extern '%s' BTF is ambiguous: both types #%d and #%u match\n",
                                        sym_name, btf_id, vi->type);
                                return -EINVAL;
                        }

                        *out_btf_sec_id = i;
                        *out_btf_id = vi->type;

                        return 0;
                }
        }

        /* free-floating extern or global FUNC */
        if (btf_id) {
                *out_btf_sec_id = 0;
                *out_btf_id = btf_id;
                return 0;
        }

        pr_warn("failed to find BTF info for global/extern symbol '%s'\n", sym_name);
        return -ENOENT;
}

static struct src_sec *find_src_sec_by_name(struct src_obj *obj, const char *sec_name)
{
        struct src_sec *sec;
        int i;

        for (i = 1; i < obj->sec_cnt; i++) {
                sec = &obj->secs[i];

                if (strcmp(sec->sec_name, sec_name) == 0)
                        return sec;
        }

        return NULL;
}

static int complete_extern_btf_info(struct btf *dst_btf, int dst_id,
                                    struct btf *src_btf, int src_id)
{
        struct btf_type *dst_t = btf_type_by_id(dst_btf, dst_id);
        struct btf_type *src_t = btf_type_by_id(src_btf, src_id);
        struct btf_param *src_p, *dst_p;
        const char *s;
        int i, n, off;

        /* We already made sure that source and destination types (FUNC or
         * VAR) match in terms of types and argument names.
         */
        if (btf_is_var(dst_t)) {
                btf_var(dst_t)->linkage = BTF_VAR_GLOBAL_ALLOCATED;
                return 0;
        }

        dst_t->info = btf_type_info(BTF_KIND_FUNC, BTF_FUNC_GLOBAL, 0);

        /* now onto FUNC_PROTO types */
        src_t = btf_type_by_id(src_btf, src_t->type);
        dst_t = btf_type_by_id(dst_btf, dst_t->type);

        /* Fill in all the argument names, which for extern FUNCs are missing.
         * We'll end up with two copies of FUNCs/VARs for externs, but that
         * will be taken care of by BTF dedup at the very end.
         * It might be that BTF types for extern in one file has less/more BTF
         * information (e.g., FWD instead of full STRUCT/UNION information),
         * but that should be (in most cases, subject to BTF dedup rules)
         * handled and resolved by BTF dedup algorithm as well, so we won't
         * worry about it. Our only job is to make sure that argument names
         * are populated on both sides, otherwise BTF dedup will pedantically
         * consider them different.
         */
        src_p = btf_params(src_t);
        dst_p = btf_params(dst_t);
        for (i = 0, n = btf_vlen(dst_t); i < n; i++, src_p++, dst_p++) {
                if (!src_p->name_off)
                        continue;

                /* src_btf has more complete info, so add name to dst_btf */
                s = btf__str_by_offset(src_btf, src_p->name_off);
                off = btf__add_str(dst_btf, s);
                if (off < 0)
                        return off;
                dst_p->name_off = off;
        }
        return 0;
}

static void sym_update_bind(Elf64_Sym *sym, int sym_bind)
{
        sym->st_info = ELF64_ST_INFO(sym_bind, ELF64_ST_TYPE(sym->st_info));
}

static void sym_update_type(Elf64_Sym *sym, int sym_type)
{
        sym->st_info = ELF64_ST_INFO(ELF64_ST_BIND(sym->st_info), sym_type);
}

static void sym_update_visibility(Elf64_Sym *sym, int sym_vis)
{
        /* libelf doesn't provide setters for ST_VISIBILITY,
         * but it is stored in the lower 2 bits of st_other
         */
        sym->st_other &= ~0x03;
        sym->st_other |= sym_vis;
}

static int linker_append_elf_sym(struct bpf_linker *linker, struct src_obj *obj,
                                 Elf64_Sym *sym, const char *sym_name, int src_sym_idx)
{
        struct src_sec *src_sec = NULL;
        struct dst_sec *dst_sec = NULL;
        struct glob_sym *glob_sym = NULL;
        int name_off, sym_type, sym_bind, sym_vis, err;
        int btf_sec_id = 0, btf_id = 0;
        size_t dst_sym_idx;
        Elf64_Sym *dst_sym;
        bool sym_is_extern;

        sym_type = ELF64_ST_TYPE(sym->st_info);
        sym_bind = ELF64_ST_BIND(sym->st_info);
        sym_vis = ELF64_ST_VISIBILITY(sym->st_other);
        sym_is_extern = sym->st_shndx == SHN_UNDEF;

        if (sym_is_extern) {
                if (!obj->btf) {
                        pr_warn("externs without BTF info are not supported\n");
                        return -ENOTSUP;
                }
        } else if (sym->st_shndx < SHN_LORESERVE) {
                src_sec = &obj->secs[sym->st_shndx];
                if (src_sec->skipped)
                        return 0;
                dst_sec = &linker->secs[src_sec->dst_id];

                /* allow only one STT_SECTION symbol per section */
                if (sym_type == STT_SECTION && dst_sec->sec_sym_idx) {
                        obj->sym_map[src_sym_idx] = dst_sec->sec_sym_idx;
                        return 0;
                }
        }

        if (sym_bind == STB_LOCAL)
                goto add_sym;

        /* find matching BTF info */
        err = find_glob_sym_btf(obj, sym, sym_name, &btf_sec_id, &btf_id);
        if (err)
                return err;

        if (sym_is_extern && btf_sec_id) {
                const char *sec_name = NULL;
                const struct btf_type *t;

                t = btf__type_by_id(obj->btf, btf_sec_id);
                sec_name = btf__str_by_offset(obj->btf, t->name_off);

                /* Clang puts unannotated extern vars into
                 * '.extern' BTF DATASEC. Treat them the same
                 * as unannotated extern funcs (which are
                 * currently not put into any DATASECs).
                 * Those don't have associated src_sec/dst_sec.
                 */
                if (strcmp(sec_name, BTF_EXTERN_SEC) != 0) {
                        src_sec = find_src_sec_by_name(obj, sec_name);
                        if (!src_sec) {
                                pr_warn("failed to find matching ELF sec '%s'\n", sec_name);
                                return -ENOENT;
                        }
                        dst_sec = &linker->secs[src_sec->dst_id];
                }
        }

        glob_sym = find_glob_sym(linker, sym_name);
        if (glob_sym) {
                /* Preventively resolve to existing symbol. This is
                 * needed for further relocation symbol remapping in
                 * the next step of linking.
                 */
                obj->sym_map[src_sym_idx] = glob_sym->sym_idx;

                /* If both symbols are non-externs, at least one of
                 * them has to be STB_WEAK, otherwise they are in
                 * a conflict with each other.
                 */
                if (!sym_is_extern && !glob_sym->is_extern
                    && !glob_sym->is_weak && sym_bind != STB_WEAK) {
                        pr_warn("conflicting non-weak symbol #%d (%s) definition in '%s'\n",
                                src_sym_idx, sym_name, obj->filename);
                        return -EINVAL;
                }

                if (!glob_syms_match(sym_name, linker, glob_sym, obj, sym, src_sym_idx, btf_id))
                        return -EINVAL;

                dst_sym = get_sym_by_idx(linker, glob_sym->sym_idx);

                /* If new symbol is strong, then force dst_sym to be strong as
                 * well; this way a mix of weak and non-weak extern
                 * definitions will end up being strong.
                 */
                if (sym_bind == STB_GLOBAL) {
                        /* We still need to preserve type (NOTYPE or
                         * OBJECT/FUNC, depending on whether the symbol is
                         * extern or not)
                         */
                        sym_update_bind(dst_sym, STB_GLOBAL);
                        glob_sym->is_weak = false;
                }

                /* Non-default visibility is "contaminating", with stricter
                 * visibility overwriting more permissive ones, even if more
                 * permissive visibility comes from just an extern definition.
                 * Currently only STV_DEFAULT and STV_HIDDEN are allowed and
                 * ensured by ELF symbol sanity checks above.
                 */
                if (sym_vis > ELF64_ST_VISIBILITY(dst_sym->st_other))
                        sym_update_visibility(dst_sym, sym_vis);

                /* If the new symbol is extern, then regardless if
                 * existing symbol is extern or resolved global, just
                 * keep the existing one untouched.
                 */
                if (sym_is_extern)
                        return 0;

                /* If existing symbol is a strong resolved symbol, bail out,
                 * because we lost resolution battle have nothing to
                 * contribute. We already checked abover that there is no
                 * strong-strong conflict. We also already tightened binding
                 * and visibility, so nothing else to contribute at that point.
                 */
                if (!glob_sym->is_extern && sym_bind == STB_WEAK)
                        return 0;

                /* At this point, new symbol is strong non-extern,
                 * so overwrite glob_sym with new symbol information.
                 * Preserve binding and visibility.
                 */
                sym_update_type(dst_sym, sym_type);
                dst_sym->st_shndx = dst_sec->sec_idx;
                dst_sym->st_value = src_sec->dst_off + sym->st_value;
                dst_sym->st_size = sym->st_size;

                /* see comment below about dst_sec->id vs dst_sec->sec_idx */
                glob_sym->sec_id = dst_sec->id;
                glob_sym->is_extern = false;

                if (complete_extern_btf_info(linker->btf, glob_sym->btf_id,
                                             obj->btf, btf_id))
                        return -EINVAL;

                /* request updating VAR's/FUNC's underlying BTF type when appending BTF type */
                glob_sym->underlying_btf_id = 0;

                obj->sym_map[src_sym_idx] = glob_sym->sym_idx;
                return 0;
        }

add_sym:
        name_off = strset__add_str(linker->strtab_strs, sym_name);
        if (name_off < 0)
                return name_off;

        dst_sym = add_new_sym(linker, &dst_sym_idx);
        if (!dst_sym)
                return -ENOMEM;

        dst_sym->st_name = name_off;
        dst_sym->st_info = sym->st_info;
        dst_sym->st_other = sym->st_other;
        dst_sym->st_shndx = dst_sec ? dst_sec->sec_idx : sym->st_shndx;
        dst_sym->st_value = (src_sec ? src_sec->dst_off : 0) + sym->st_value;
        dst_sym->st_size = sym->st_size;

        obj->sym_map[src_sym_idx] = dst_sym_idx;

        if (sym_type == STT_SECTION && dst_sym) {
                dst_sec->sec_sym_idx = dst_sym_idx;
                dst_sym->st_value = 0;
        }

        if (sym_bind != STB_LOCAL) {
                glob_sym = add_glob_sym(linker);
                if (!glob_sym)
                        return -ENOMEM;

                glob_sym->sym_idx = dst_sym_idx;
                /* we use dst_sec->id (and not dst_sec->sec_idx), because
                 * ephemeral sections (.kconfig, .ksyms, etc) don't have
                 * sec_idx (as they don't have corresponding ELF section), but
                 * still have id. .extern doesn't have even ephemeral section
                 * associated with it, so dst_sec->id == dst_sec->sec_idx == 0.
                 */
                glob_sym->sec_id = dst_sec ? dst_sec->id : 0;
                glob_sym->name_off = name_off;
                /* we will fill btf_id in during BTF merging step */
                glob_sym->btf_id = 0;
                glob_sym->is_extern = sym_is_extern;
                glob_sym->is_weak = sym_bind == STB_WEAK;
        }

        return 0;
}

static int linker_append_elf_relos(struct bpf_linker *linker, struct src_obj *obj)
{

        struct src_sec *src_symtab = &obj->secs[obj->symtab_sec_idx];
        struct dst_sec *dst_symtab = &linker->secs[linker->symtab_sec_idx];
        int i, err;

        for (i = 1; i < obj->sec_cnt; i++) {
                struct src_sec *src_sec, *src_linked_sec;
                struct dst_sec *dst_sec, *dst_linked_sec;
                Elf64_Rel *src_rel, *dst_rel;
                int j, n;

                src_sec = &obj->secs[i];
                if (!is_relo_sec(src_sec))
                        continue;

                /* shdr->sh_info points to relocatable section */
                src_linked_sec = &obj->secs[src_sec->shdr->sh_info];
                if (src_linked_sec->skipped)
                        continue;

                dst_sec = find_dst_sec_by_name(linker, src_sec->sec_name);
                if (!dst_sec) {
                        dst_sec = add_dst_sec(linker, src_sec->sec_name);
                        if (!dst_sec)
                                return -ENOMEM;
                        err = init_sec(linker, dst_sec, src_sec);
                        if (err) {
                                pr_warn("failed to init section '%s'\n", src_sec->sec_name);
                                return err;
                        }
                } else if (!secs_match(dst_sec, src_sec)) {
                        pr_warn("sections %s are not compatible\n", src_sec->sec_name);
                        return -1;
                }

                /* shdr->sh_link points to SYMTAB */
                dst_sec->shdr->sh_link = linker->symtab_sec_idx;

                /* shdr->sh_info points to relocated section */
                dst_linked_sec = &linker->secs[src_linked_sec->dst_id];
                dst_sec->shdr->sh_info = dst_linked_sec->sec_idx;

                src_sec->dst_id = dst_sec->id;
                err = extend_sec(linker, dst_sec, src_sec);
                if (err)
                        return err;

                src_rel = src_sec->data->d_buf;
                dst_rel = dst_sec->raw_data + src_sec->dst_off;
                n = src_sec->shdr->sh_size / src_sec->shdr->sh_entsize;
                for (j = 0; j < n; j++, src_rel++, dst_rel++) {
                        size_t src_sym_idx = ELF64_R_SYM(src_rel->r_info);
                        size_t sym_type = ELF64_R_TYPE(src_rel->r_info);
                        Elf64_Sym *src_sym, *dst_sym;
                        size_t dst_sym_idx;

                        src_sym_idx = ELF64_R_SYM(src_rel->r_info);
                        src_sym = src_symtab->data->d_buf + sizeof(*src_sym) * src_sym_idx;

                        dst_sym_idx = obj->sym_map[src_sym_idx];
                        dst_sym = dst_symtab->raw_data + sizeof(*dst_sym) * dst_sym_idx;
                        dst_rel->r_offset += src_linked_sec->dst_off;
                        sym_type = ELF64_R_TYPE(src_rel->r_info);
                        dst_rel->r_info = ELF64_R_INFO(dst_sym_idx, sym_type);

                        if (ELF64_ST_TYPE(src_sym->st_info) == STT_SECTION) {
                                struct src_sec *sec = &obj->secs[src_sym->st_shndx];
                                struct bpf_insn *insn;

                                if (src_linked_sec->shdr->sh_flags & SHF_EXECINSTR) {
                                        /* calls to the very first static function inside
                                         * .text section at offset 0 will
                                         * reference section symbol, not the
                                         * function symbol. Fix that up,
                                         * otherwise it won't be possible to
                                         * relocate calls to two different
                                         * static functions with the same name
                                         * (rom two different object files)
                                         */
                                        insn = dst_linked_sec->raw_data + dst_rel->r_offset;
                                        if (insn->code == (BPF_JMP | BPF_CALL))
                                                insn->imm += sec->dst_off / sizeof(struct bpf_insn);
                                        else
                                                insn->imm += sec->dst_off;
                                } else {
                                        pr_warn("relocation against STT_SECTION in non-exec section is not supported!\n");
                                        return -EINVAL;
                                }
                        }

                }
        }

        return 0;
}

static Elf64_Sym *find_sym_by_name(struct src_obj *obj, size_t sec_idx,
                                   int sym_type, const char *sym_name)
{
        struct src_sec *symtab = &obj->secs[obj->symtab_sec_idx];
        Elf64_Sym *sym = symtab->data->d_buf;
        int i, n = symtab->shdr->sh_size / symtab->shdr->sh_entsize;
        int str_sec_idx = symtab->shdr->sh_link;
        const char *name;

        for (i = 0; i < n; i++, sym++) {
                if (sym->st_shndx != sec_idx)
                        continue;
                if (ELF64_ST_TYPE(sym->st_info) != sym_type)
                        continue;

                name = elf_strptr(obj->elf, str_sec_idx, sym->st_name);
                if (!name)
                        return NULL;

                if (strcmp(sym_name, name) != 0)
                        continue;

                return sym;
        }

        return NULL;
}

static int linker_fixup_btf(struct src_obj *obj)
{
        const char *sec_name;
        struct src_sec *sec;
        int i, j, n, m;

        if (!obj->btf)
                return 0;

        n = btf__type_cnt(obj->btf);
        for (i = 1; i < n; i++) {
                struct btf_var_secinfo *vi;
                struct btf_type *t;

                t = btf_type_by_id(obj->btf, i);
                if (btf_kind(t) != BTF_KIND_DATASEC)
                        continue;

                sec_name = btf__str_by_offset(obj->btf, t->name_off);
                sec = find_src_sec_by_name(obj, sec_name);
                if (sec) {
                        /* record actual section size, unless ephemeral */
                        if (sec->shdr)
                                t->size = sec->shdr->sh_size;
                } else {
                        /* BTF can have some sections that are not represented
                         * in ELF, e.g., .kconfig, .ksyms, .extern, which are used
                         * for special extern variables.
                         *
                         * For all but one such special (ephemeral)
                         * sections, we pre-create "section shells" to be able
                         * to keep track of extra per-section metadata later
                         * (e.g., those BTF extern variables).
                         *
                         * .extern is even more special, though, because it
                         * contains extern variables that need to be resolved
                         * by static linker, not libbpf and kernel. When such
                         * externs are resolved, we are going to remove them
                         * from .extern BTF section and might end up not
                         * needing it at all. Each resolved extern should have
                         * matching non-extern VAR/FUNC in other sections.
                         *
                         * We do support leaving some of the externs
                         * unresolved, though, to support cases of building
                         * libraries, which will later be linked against final
                         * BPF applications. So if at finalization we still
                         * see unresolved externs, we'll create .extern
                         * section on our own.
                         */
                        if (strcmp(sec_name, BTF_EXTERN_SEC) == 0)
                                continue;

                        sec = add_src_sec(obj, sec_name);
                        if (!sec)
                                return -ENOMEM;

                        sec->ephemeral = true;
                        sec->sec_idx = 0; /* will match UNDEF shndx in ELF */
                }

                /* remember ELF section and its BTF type ID match */
                sec->sec_type_id = i;

                /* fix up variable offsets */
                vi = btf_var_secinfos(t);
                for (j = 0, m = btf_vlen(t); j < m; j++, vi++) {
                        const struct btf_type *vt = btf__type_by_id(obj->btf, vi->type);
                        const char *var_name = btf__str_by_offset(obj->btf, vt->name_off);
                        int var_linkage = btf_var(vt)->linkage;
                        Elf64_Sym *sym;

                        /* no need to patch up static or extern vars */
                        if (var_linkage != BTF_VAR_GLOBAL_ALLOCATED)
                                continue;

                        sym = find_sym_by_name(obj, sec->sec_idx, STT_OBJECT, var_name);
                        if (!sym) {
                                pr_warn("failed to find symbol for variable '%s' in section '%s'\n", var_name, sec_name);
                                return -ENOENT;
                        }

                        vi->offset = sym->st_value;
                }
        }

        return 0;
}

static int remap_type_id(__u32 *type_id, void *ctx)
{
        int *id_map = ctx;
        int new_id = id_map[*type_id];

        /* Error out if the type wasn't remapped. Ignore VOID which stays VOID. */
        if (new_id == 0 && *type_id != 0) {
                pr_warn("failed to find new ID mapping for original BTF type ID %u\n", *type_id);
                return -EINVAL;
        }

        *type_id = id_map[*type_id];

        return 0;
}

static int linker_append_btf(struct bpf_linker *linker, struct src_obj *obj)
{
        const struct btf_type *t;
        int i, j, n, start_id, id;
        const char *name;

        if (!obj->btf)
                return 0;

        start_id = btf__type_cnt(linker->btf);
        n = btf__type_cnt(obj->btf);

        obj->btf_type_map = calloc(n + 1, sizeof(int));
        if (!obj->btf_type_map)
                return -ENOMEM;

        for (i = 1; i < n; i++) {
                struct glob_sym *glob_sym = NULL;

                t = btf__type_by_id(obj->btf, i);

                /* DATASECs are handled specially below */
                if (btf_kind(t) == BTF_KIND_DATASEC)
                        continue;

                if (btf_is_non_static(t)) {
                        /* there should be glob_sym already */
                        name = btf__str_by_offset(obj->btf, t->name_off);
                        glob_sym = find_glob_sym(linker, name);

                        /* VARs without corresponding glob_sym are those that
                         * belong to skipped/deduplicated sections (i.e.,
                         * license and version), so just skip them
                         */
                        if (!glob_sym)
                                continue;

                        /* linker_append_elf_sym() might have requested
                         * updating underlying type ID, if extern was resolved
                         * to strong symbol or weak got upgraded to non-weak
                         */
                        if (glob_sym->underlying_btf_id == 0)
                                glob_sym->underlying_btf_id = -t->type;

                        /* globals from previous object files that match our
                         * VAR/FUNC already have a corresponding associated
                         * BTF type, so just make sure to use it
                         */
                        if (glob_sym->btf_id) {
                                /* reuse existing BTF type for global var/func */
                                obj->btf_type_map[i] = glob_sym->btf_id;
                                continue;
                        }
                }

                id = btf__add_type(linker->btf, obj->btf, t);
                if (id < 0) {
                        pr_warn("failed to append BTF type #%d from file '%s'\n", i, obj->filename);
                        return id;
                }

                obj->btf_type_map[i] = id;

                /* record just appended BTF type for var/func */
                if (glob_sym) {
                        glob_sym->btf_id = id;
                        glob_sym->underlying_btf_id = -t->type;
                }
        }

        /* remap all the types except DATASECs */
        n = btf__type_cnt(linker->btf);
        for (i = start_id; i < n; i++) {
                struct btf_type *dst_t = btf_type_by_id(linker->btf, i);

                if (btf_type_visit_type_ids(dst_t, remap_type_id, obj->btf_type_map))
                        return -EINVAL;
        }

        /* Rewrite VAR/FUNC underlying types (i.e., FUNC's FUNC_PROTO and VAR's
         * actual type), if necessary
         */
        for (i = 0; i < linker->glob_sym_cnt; i++) {
                struct glob_sym *glob_sym = &linker->glob_syms[i];
                struct btf_type *glob_t;

                if (glob_sym->underlying_btf_id >= 0)
                        continue;

                glob_sym->underlying_btf_id = obj->btf_type_map[-glob_sym->underlying_btf_id];

                glob_t = btf_type_by_id(linker->btf, glob_sym->btf_id);
                glob_t->type = glob_sym->underlying_btf_id;
        }

        /* append DATASEC info */
        for (i = 1; i < obj->sec_cnt; i++) {
                struct src_sec *src_sec;
                struct dst_sec *dst_sec;
                const struct btf_var_secinfo *src_var;
                struct btf_var_secinfo *dst_var;

                src_sec = &obj->secs[i];
                if (!src_sec->sec_type_id || src_sec->skipped)
                        continue;
                dst_sec = &linker->secs[src_sec->dst_id];

                /* Mark section as having BTF regardless of the presence of
                 * variables. In some cases compiler might generate empty BTF
                 * with no variables information. E.g., when promoting local
                 * array/structure variable initial values and BPF object
                 * file otherwise has no read-only static variables in
                 * .rodata. We need to preserve such empty BTF and just set
                 * correct section size.
                 */
                dst_sec->has_btf = true;

                t = btf__type_by_id(obj->btf, src_sec->sec_type_id);
                src_var = btf_var_secinfos(t);
                n = btf_vlen(t);
                for (j = 0; j < n; j++, src_var++) {
                        void *sec_vars = dst_sec->sec_vars;
                        int new_id = obj->btf_type_map[src_var->type];
                        struct glob_sym *glob_sym = NULL;

                        t = btf_type_by_id(linker->btf, new_id);
                        if (btf_is_non_static(t)) {
                                name = btf__str_by_offset(linker->btf, t->name_off);
                                glob_sym = find_glob_sym(linker, name);
                                if (glob_sym->sec_id != dst_sec->id) {
                                        pr_warn("global '%s': section mismatch %d vs %d\n",
                                                name, glob_sym->sec_id, dst_sec->id);
                                        return -EINVAL;
                                }
                        }

                        /* If there is already a member (VAR or FUNC) mapped
                         * to the same type, don't add a duplicate entry.
                         * This will happen when multiple object files define
                         * the same extern VARs/FUNCs.
                         */
                        if (glob_sym && glob_sym->var_idx >= 0) {
                                __s64 sz;

                                dst_var = &dst_sec->sec_vars[glob_sym->var_idx];
                                /* Because underlying BTF type might have
                                 * changed, so might its size have changed, so
                                 * re-calculate and update it in sec_var.
                                 */
                                sz = btf__resolve_size(linker->btf, glob_sym->underlying_btf_id);
                                if (sz < 0) {
                                        pr_warn("global '%s': failed to resolve size of underlying type: %d\n",
                                                name, (int)sz);
                                        return -EINVAL;
                                }
                                dst_var->size = sz;
                                continue;
                        }

                        sec_vars = libbpf_reallocarray(sec_vars,
                                                       dst_sec->sec_var_cnt + 1,
                                                       sizeof(*dst_sec->sec_vars));
                        if (!sec_vars)
                                return -ENOMEM;

                        dst_sec->sec_vars = sec_vars;
                        dst_sec->sec_var_cnt++;

                        dst_var = &dst_sec->sec_vars[dst_sec->sec_var_cnt - 1];
                        dst_var->type = obj->btf_type_map[src_var->type];
                        dst_var->size = src_var->size;
                        dst_var->offset = src_sec->dst_off + src_var->offset;

                        if (glob_sym)
                                glob_sym->var_idx = dst_sec->sec_var_cnt - 1;
                }
        }

        return 0;
}

static void *add_btf_ext_rec(struct btf_ext_sec_data *ext_data, const void *src_rec)
{
        void *tmp;

        tmp = libbpf_reallocarray(ext_data->recs, ext_data->rec_cnt + 1, ext_data->rec_sz);
        if (!tmp)
                return NULL;
        ext_data->recs = tmp;

        tmp += ext_data->rec_cnt * ext_data->rec_sz;
        memcpy(tmp, src_rec, ext_data->rec_sz);

        ext_data->rec_cnt++;

        return tmp;
}

static int linker_append_btf_ext(struct bpf_linker *linker, struct src_obj *obj)
{
        const struct btf_ext_info_sec *ext_sec;
        const char *sec_name, *s;
        struct src_sec *src_sec;
        struct dst_sec *dst_sec;
        int rec_sz, str_off, i;

        if (!obj->btf_ext)
                return 0;

        rec_sz = obj->btf_ext->func_info.rec_size;
        for_each_btf_ext_sec(&obj->btf_ext->func_info, ext_sec) {
                struct bpf_func_info_min *src_rec, *dst_rec;

                sec_name = btf__name_by_offset(obj->btf, ext_sec->sec_name_off);
                src_sec = find_src_sec_by_name(obj, sec_name);
                if (!src_sec) {
                        pr_warn("can't find section '%s' referenced from .BTF.ext\n", sec_name);
                        return -EINVAL;
                }
                dst_sec = &linker->secs[src_sec->dst_id];

                if (dst_sec->func_info.rec_sz == 0)
                        dst_sec->func_info.rec_sz = rec_sz;
                if (dst_sec->func_info.rec_sz != rec_sz) {
                        pr_warn("incompatible .BTF.ext record sizes for section '%s'\n", sec_name);
                        return -EINVAL;
                }

                for_each_btf_ext_rec(&obj->btf_ext->func_info, ext_sec, i, src_rec) {
                        dst_rec = add_btf_ext_rec(&dst_sec->func_info, src_rec);
                        if (!dst_rec)
                                return -ENOMEM;

                        dst_rec->insn_off += src_sec->dst_off;
                        dst_rec->type_id = obj->btf_type_map[dst_rec->type_id];
                }
        }

        rec_sz = obj->btf_ext->line_info.rec_size;
        for_each_btf_ext_sec(&obj->btf_ext->line_info, ext_sec) {
                struct bpf_line_info_min *src_rec, *dst_rec;

                sec_name = btf__name_by_offset(obj->btf, ext_sec->sec_name_off);
                src_sec = find_src_sec_by_name(obj, sec_name);
                if (!src_sec) {
                        pr_warn("can't find section '%s' referenced from .BTF.ext\n", sec_name);
                        return -EINVAL;
                }
                dst_sec = &linker->secs[src_sec->dst_id];

                if (dst_sec->line_info.rec_sz == 0)
                        dst_sec->line_info.rec_sz = rec_sz;
                if (dst_sec->line_info.rec_sz != rec_sz) {
                        pr_warn("incompatible .BTF.ext record sizes for section '%s'\n", sec_name);
                        return -EINVAL;
                }

                for_each_btf_ext_rec(&obj->btf_ext->line_info, ext_sec, i, src_rec) {
                        dst_rec = add_btf_ext_rec(&dst_sec->line_info, src_rec);
                        if (!dst_rec)
                                return -ENOMEM;

                        dst_rec->insn_off += src_sec->dst_off;

                        s = btf__str_by_offset(obj->btf, src_rec->file_name_off);
                        str_off = btf__add_str(linker->btf, s);
                        if (str_off < 0)
                                return -ENOMEM;
                        dst_rec->file_name_off = str_off;

                        s = btf__str_by_offset(obj->btf, src_rec->line_off);
                        str_off = btf__add_str(linker->btf, s);
                        if (str_off < 0)
                                return -ENOMEM;
                        dst_rec->line_off = str_off;

                        /* dst_rec->line_col is fine */
                }
        }

        rec_sz = obj->btf_ext->core_relo_info.rec_size;
        for_each_btf_ext_sec(&obj->btf_ext->core_relo_info, ext_sec) {
                struct bpf_core_relo *src_rec, *dst_rec;

                sec_name = btf__name_by_offset(obj->btf, ext_sec->sec_name_off);
                src_sec = find_src_sec_by_name(obj, sec_name);
                if (!src_sec) {
                        pr_warn("can't find section '%s' referenced from .BTF.ext\n", sec_name);
                        return -EINVAL;
                }
                dst_sec = &linker->secs[src_sec->dst_id];

                if (dst_sec->core_relo_info.rec_sz == 0)
                        dst_sec->core_relo_info.rec_sz = rec_sz;
                if (dst_sec->core_relo_info.rec_sz != rec_sz) {
                        pr_warn("incompatible .BTF.ext record sizes for section '%s'\n", sec_name);
                        return -EINVAL;
                }

                for_each_btf_ext_rec(&obj->btf_ext->core_relo_info, ext_sec, i, src_rec) {
                        dst_rec = add_btf_ext_rec(&dst_sec->core_relo_info, src_rec);
                        if (!dst_rec)
                                return -ENOMEM;

                        dst_rec->insn_off += src_sec->dst_off;
                        dst_rec->type_id = obj->btf_type_map[dst_rec->type_id];

                        s = btf__str_by_offset(obj->btf, src_rec->access_str_off);
                        str_off = btf__add_str(linker->btf, s);
                        if (str_off < 0)
                                return -ENOMEM;
                        dst_rec->access_str_off = str_off;

                        /* dst_rec->kind is fine */
                }
        }

        return 0;
}

int bpf_linker__finalize(struct bpf_linker *linker)
{
        struct dst_sec *sec;
        size_t strs_sz;
        const void *strs;
        int err, i;

        if (!linker->elf)
                return libbpf_err(-EINVAL);

        err = finalize_btf(linker);
        if (err)
                return libbpf_err(err);

        /* Finalize strings */
        strs_sz = strset__data_size(linker->strtab_strs);
        strs = strset__data(linker->strtab_strs);

        sec = &linker->secs[linker->strtab_sec_idx];
        sec->data->d_align = 1;
        sec->data->d_off = 0LL;
        sec->data->d_buf = (void *)strs;
        sec->data->d_type = ELF_T_BYTE;
        sec->data->d_size = strs_sz;
        sec->shdr->sh_size = strs_sz;

        for (i = 1; i < linker->sec_cnt; i++) {
                sec = &linker->secs[i];

                /* STRTAB is handled specially above */
                if (sec->sec_idx == linker->strtab_sec_idx)
                        continue;

                /* special ephemeral sections (.ksyms, .kconfig, etc) */
                if (!sec->scn)
                        continue;

                sec->data->d_buf = sec->raw_data;
        }

        /* Finalize ELF layout */
        if (elf_update(linker->elf, ELF_C_NULL) < 0) {
                err = -errno;
                pr_warn_elf("failed to finalize ELF layout");
                return libbpf_err(err);
        }

        /* Write out final ELF contents */
        if (elf_update(linker->elf, ELF_C_WRITE) < 0) {
                err = -errno;
                pr_warn_elf("failed to write ELF contents");
                return libbpf_err(err);
        }

        elf_end(linker->elf);
        close(linker->fd);

        linker->elf = NULL;
        linker->fd = -1;

        return 0;
}

static int emit_elf_data_sec(struct bpf_linker *linker, const char *sec_name,
                             size_t align, const void *raw_data, size_t raw_sz)
{
        Elf_Scn *scn;
        Elf_Data *data;
        Elf64_Shdr *shdr;
        int name_off;

        name_off = strset__add_str(linker->strtab_strs, sec_name);
        if (name_off < 0)
                return name_off;

        scn = elf_newscn(linker->elf);
        if (!scn)
                return -ENOMEM;
        data = elf_newdata(scn);
        if (!data)
                return -ENOMEM;
        shdr = elf64_getshdr(scn);
        if (!shdr)
                return -EINVAL;

        shdr->sh_name = name_off;
        shdr->sh_type = SHT_PROGBITS;
        shdr->sh_flags = 0;
        shdr->sh_size = raw_sz;
        shdr->sh_link = 0;
        shdr->sh_info = 0;
        shdr->sh_addralign = align;
        shdr->sh_entsize = 0;

        data->d_type = ELF_T_BYTE;
        data->d_size = raw_sz;
        data->d_buf = (void *)raw_data;
        data->d_align = align;
        data->d_off = 0;

        return 0;
}

static int finalize_btf(struct bpf_linker *linker)
{
        struct btf *btf = linker->btf;
        const void *raw_data;
        int i, j, id, err;
        __u32 raw_sz;

        /* bail out if no BTF data was produced */
        if (btf__type_cnt(linker->btf) == 1)
                return 0;

        for (i = 1; i < linker->sec_cnt; i++) {
                struct dst_sec *sec = &linker->secs[i];

                if (!sec->has_btf)
                        continue;

                id = btf__add_datasec(btf, sec->sec_name, sec->sec_sz);
                if (id < 0) {
                        pr_warn("failed to add consolidated BTF type for datasec '%s': %d\n",
                                sec->sec_name, id);
                        return id;
                }

                for (j = 0; j < sec->sec_var_cnt; j++) {
                        struct btf_var_secinfo *vi = &sec->sec_vars[j];

                        if (btf__add_datasec_var_info(btf, vi->type, vi->offset, vi->size))
                                return -EINVAL;
                }
        }

        err = finalize_btf_ext(linker);
        if (err) {
                pr_warn(".BTF.ext generation failed: %d\n", err);
                return err;
        }

        err = btf__dedup(linker->btf, linker->btf_ext, NULL);
        if (err) {
                pr_warn("BTF dedup failed: %d\n", err);
                return err;
        }

        /* Emit .BTF section */
        raw_data = btf__raw_data(linker->btf, &raw_sz);
        if (!raw_data)
                return -ENOMEM;

        err = emit_elf_data_sec(linker, BTF_ELF_SEC, 8, raw_data, raw_sz);
        if (err) {
                pr_warn("failed to write out .BTF ELF section: %d\n", err);
                return err;
        }

        /* Emit .BTF.ext section */
        if (linker->btf_ext) {
                raw_data = btf_ext__get_raw_data(linker->btf_ext, &raw_sz);
                if (!raw_data)
                        return -ENOMEM;

                err = emit_elf_data_sec(linker, BTF_EXT_ELF_SEC, 8, raw_data, raw_sz);
                if (err) {
                        pr_warn("failed to write out .BTF.ext ELF section: %d\n", err);
                        return err;
                }
        }

        return 0;
}

static int emit_btf_ext_data(struct bpf_linker *linker, void *output,
                             const char *sec_name, struct btf_ext_sec_data *sec_data)
{
        struct btf_ext_info_sec *sec_info;
        void *cur = output;
        int str_off;
        size_t sz;

        if (!sec_data->rec_cnt)
                return 0;

        str_off = btf__add_str(linker->btf, sec_name);
        if (str_off < 0)
                return -ENOMEM;

        sec_info = cur;
        sec_info->sec_name_off = str_off;
        sec_info->num_info = sec_data->rec_cnt;
        cur += sizeof(struct btf_ext_info_sec);

        sz = sec_data->rec_cnt * sec_data->rec_sz;
        memcpy(cur, sec_data->recs, sz);
        cur += sz;

        return cur - output;
}

static int finalize_btf_ext(struct bpf_linker *linker)
{
        size_t funcs_sz = 0, lines_sz = 0, core_relos_sz = 0, total_sz = 0;
        size_t func_rec_sz = 0, line_rec_sz = 0, core_relo_rec_sz = 0;
        struct btf_ext_header *hdr;
        void *data, *cur;
        int i, err, sz;

        /* validate that all sections have the same .BTF.ext record sizes
         * and calculate total data size for each type of data (func info,
         * line info, core relos)
         */
        for (i = 1; i < linker->sec_cnt; i++) {
                struct dst_sec *sec = &linker->secs[i];

                if (sec->func_info.rec_cnt) {
                        if (func_rec_sz == 0)
                                func_rec_sz = sec->func_info.rec_sz;
                        if (func_rec_sz != sec->func_info.rec_sz) {
                                pr_warn("mismatch in func_info record size %zu != %u\n",
                                        func_rec_sz, sec->func_info.rec_sz);
                                return -EINVAL;
                        }

                        funcs_sz += sizeof(struct btf_ext_info_sec) + func_rec_sz * sec->func_info.rec_cnt;
                }
                if (sec->line_info.rec_cnt) {
                        if (line_rec_sz == 0)
                                line_rec_sz = sec->line_info.rec_sz;
                        if (line_rec_sz != sec->line_info.rec_sz) {
                                pr_warn("mismatch in line_info record size %zu != %u\n",
                                        line_rec_sz, sec->line_info.rec_sz);
                                return -EINVAL;
                        }

                        lines_sz += sizeof(struct btf_ext_info_sec) + line_rec_sz * sec->line_info.rec_cnt;
                }
                if (sec->core_relo_info.rec_cnt) {
                        if (core_relo_rec_sz == 0)
                                core_relo_rec_sz = sec->core_relo_info.rec_sz;
                        if (core_relo_rec_sz != sec->core_relo_info.rec_sz) {
                                pr_warn("mismatch in core_relo_info record size %zu != %u\n",
                                        core_relo_rec_sz, sec->core_relo_info.rec_sz);
                                return -EINVAL;
                        }

                        core_relos_sz += sizeof(struct btf_ext_info_sec) + core_relo_rec_sz * sec->core_relo_info.rec_cnt;
                }
        }

        if (!funcs_sz && !lines_sz && !core_relos_sz)
                return 0;

        total_sz += sizeof(struct btf_ext_header);
        if (funcs_sz) {
                funcs_sz += sizeof(__u32); /* record size prefix */
                total_sz += funcs_sz;
        }
        if (lines_sz) {
                lines_sz += sizeof(__u32); /* record size prefix */
                total_sz += lines_sz;
        }
        if (core_relos_sz) {
                core_relos_sz += sizeof(__u32); /* record size prefix */
                total_sz += core_relos_sz;
        }

        cur = data = calloc(1, total_sz);
        if (!data)
                return -ENOMEM;

        hdr = cur;
        hdr->magic = BTF_MAGIC;
        hdr->version = BTF_VERSION;
        hdr->flags = 0;
        hdr->hdr_len = sizeof(struct btf_ext_header);
        cur += sizeof(struct btf_ext_header);

        /* All offsets are in bytes relative to the end of this header */
        hdr->func_info_off = 0;
        hdr->func_info_len = funcs_sz;
        hdr->line_info_off = funcs_sz;
        hdr->line_info_len = lines_sz;
        hdr->core_relo_off = funcs_sz + lines_sz;
        hdr->core_relo_len = core_relos_sz;

        if (funcs_sz) {
                *(__u32 *)cur = func_rec_sz;
                cur += sizeof(__u32);

                for (i = 1; i < linker->sec_cnt; i++) {
                        struct dst_sec *sec = &linker->secs[i];

                        sz = emit_btf_ext_data(linker, cur, sec->sec_name, &sec->func_info);
                        if (sz < 0) {
                                err = sz;
                                goto out;
                        }

                        cur += sz;
                }
        }

        if (lines_sz) {
                *(__u32 *)cur = line_rec_sz;
                cur += sizeof(__u32);

                for (i = 1; i < linker->sec_cnt; i++) {
                        struct dst_sec *sec = &linker->secs[i];

                        sz = emit_btf_ext_data(linker, cur, sec->sec_name, &sec->line_info);
                        if (sz < 0) {
                                err = sz;
                                goto out;
                        }

                        cur += sz;
                }
        }

        if (core_relos_sz) {
                *(__u32 *)cur = core_relo_rec_sz;
                cur += sizeof(__u32);

                for (i = 1; i < linker->sec_cnt; i++) {
                        struct dst_sec *sec = &linker->secs[i];

                        sz = emit_btf_ext_data(linker, cur, sec->sec_name, &sec->core_relo_info);
                        if (sz < 0) {
                                err = sz;
                                goto out;
                        }

                        cur += sz;
                }
        }

        linker->btf_ext = btf_ext__new(data, total_sz);
        err = libbpf_get_error(linker->btf_ext);
        if (err) {
                linker->btf_ext = NULL;
                pr_warn("failed to parse final .BTF.ext data: %d\n", err);
                goto out;
        }

out:
        free(data);
        return err;
}