// SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)

/*
 * BTF-to-C type converter.
 *
 * Copyright (c) 2019 Facebook
 */

#include <stdbool.h>
#include <stddef.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <linux/err.h>
#include <linux/btf.h>
#include "btf.h"
#include "hashmap.h"
#include "libbpf.h"
#include "libbpf_internal.h"

static const char PREFIXES[] = "\t\t\t\t\t\t\t\t\t\t\t\t\t";
static const size_t PREFIX_CNT = sizeof(PREFIXES) - 1;

static const char *pfx(int lvl)
{
        return lvl >= PREFIX_CNT ? PREFIXES : &PREFIXES[PREFIX_CNT - lvl];
}

enum btf_dump_type_order_state {
        NOT_ORDERED,
        ORDERING,
        ORDERED,
};

enum btf_dump_type_emit_state {
        NOT_EMITTED,
        EMITTING,
        EMITTED,
};

/* per-type auxiliary state */
struct btf_dump_type_aux_state {
        /* topological sorting state */
        enum btf_dump_type_order_state order_state: 2;
        /* emitting state used to determine the need for forward declaration */
        enum btf_dump_type_emit_state emit_state: 2;
        /* whether forward declaration was already emitted */
        __u8 fwd_emitted: 1;
        /* whether unique non-duplicate name was already assigned */
        __u8 name_resolved: 1;
};

struct btf_dump {
        const struct btf *btf;
        const struct btf_ext *btf_ext;
        btf_dump_printf_fn_t printf_fn;
        struct btf_dump_opts opts;

        /* per-type auxiliary state */
        struct btf_dump_type_aux_state *type_states;
        /* per-type optional cached unique name, must be freed, if present */
        const char **cached_names;

        /* topo-sorted list of dependent type definitions */
        __u32 *emit_queue;
        int emit_queue_cap;
        int emit_queue_cnt;

        /*
         * stack of type declarations (e.g., chain of modifiers, arrays,
         * funcs, etc)
         */
        __u32 *decl_stack;
        int decl_stack_cap;
        int decl_stack_cnt;

        /* maps struct/union/enum name to a number of name occurrences */
        struct hashmap *type_names;
        /*
         * maps typedef identifiers and enum value names to a number of such
         * name occurrences
         */
        struct hashmap *ident_names;
};

static size_t str_hash_fn(const void *key, void *ctx)
{
        const char *s = key;
        size_t h = 0;

        while (*s) {
                h = h * 31 + *s;
                s++;
        }
        return h;
}

static bool str_equal_fn(const void *a, const void *b, void *ctx)
{
        return strcmp(a, b) == 0;
}

static __u16 btf_kind_of(const struct btf_type *t)
{
        return BTF_INFO_KIND(t->info);
}

static __u16 btf_vlen_of(const struct btf_type *t)
{
        return BTF_INFO_VLEN(t->info);
}

static bool btf_kflag_of(const struct btf_type *t)
{
        return BTF_INFO_KFLAG(t->info);
}

static const char *btf_name_of(const struct btf_dump *d, __u32 name_off)
{
        return btf__name_by_offset(d->btf, name_off);
}

static void btf_dump_printf(const struct btf_dump *d, const char *fmt, ...)
{
        va_list args;

        va_start(args, fmt);
        d->printf_fn(d->opts.ctx, fmt, args);
        va_end(args);
}

struct btf_dump *btf_dump__new(const struct btf *btf,
                               const struct btf_ext *btf_ext,
                               const struct btf_dump_opts *opts,
                               btf_dump_printf_fn_t printf_fn)
{
        struct btf_dump *d;
        int err;

        d = calloc(1, sizeof(struct btf_dump));
        if (!d)
                return ERR_PTR(-ENOMEM);

        d->btf = btf;
        d->btf_ext = btf_ext;
        d->printf_fn = printf_fn;
        d->opts.ctx = opts ? opts->ctx : NULL;

        d->type_names = hashmap__new(str_hash_fn, str_equal_fn, NULL);
        if (IS_ERR(d->type_names)) {
                err = PTR_ERR(d->type_names);
                d->type_names = NULL;
                btf_dump__free(d);
                return ERR_PTR(err);
        }
        d->ident_names = hashmap__new(str_hash_fn, str_equal_fn, NULL);
        if (IS_ERR(d->ident_names)) {
                err = PTR_ERR(d->ident_names);
                d->ident_names = NULL;
                btf_dump__free(d);
                return ERR_PTR(err);
        }

        return d;
}

void btf_dump__free(struct btf_dump *d)
{
        int i, cnt;

        if (!d)
                return;

        free(d->type_states);
        if (d->cached_names) {
                /* any set cached name is owned by us and should be freed */
                for (i = 0, cnt = btf__get_nr_types(d->btf); i <= cnt; i++) {
                        if (d->cached_names[i])
                                free((void *)d->cached_names[i]);
                }
        }
        free(d->cached_names);
        free(d->emit_queue);
        free(d->decl_stack);
        hashmap__free(d->type_names);
        hashmap__free(d->ident_names);

        free(d);
}

static int btf_dump_order_type(struct btf_dump *d, __u32 id, bool through_ptr);
static void btf_dump_emit_type(struct btf_dump *d, __u32 id, __u32 cont_id);

/*
 * Dump BTF type in a compilable C syntax, including all the necessary
 * dependent types, necessary for compilation. If some of the dependent types
 * were already emitted as part of previous btf_dump__dump_type() invocation
 * for another type, they won't be emitted again. This API allows callers to
 * filter out BTF types according to user-defined criterias and emitted only
 * minimal subset of types, necessary to compile everything. Full struct/union
 * definitions will still be emitted, even if the only usage is through
 * pointer and could be satisfied with just a forward declaration.
 *
 * Dumping is done in two high-level passes:
 *   1. Topologically sort type definitions to satisfy C rules of compilation.
 *   2. Emit type definitions in C syntax.
 *
 * Returns 0 on success; <0, otherwise.
 */
int btf_dump__dump_type(struct btf_dump *d, __u32 id)
{
        int err, i;

        if (id > btf__get_nr_types(d->btf))
                return -EINVAL;

        /* type states are lazily allocated, as they might not be needed */
        if (!d->type_states) {
                d->type_states = calloc(1 + btf__get_nr_types(d->btf),
                                        sizeof(d->type_states[0]));
                if (!d->type_states)
                        return -ENOMEM;
                d->cached_names = calloc(1 + btf__get_nr_types(d->btf),
                                         sizeof(d->cached_names[0]));
                if (!d->cached_names)
                        return -ENOMEM;

                /* VOID is special */
                d->type_states[0].order_state = ORDERED;
                d->type_states[0].emit_state = EMITTED;
        }

        d->emit_queue_cnt = 0;
        err = btf_dump_order_type(d, id, false);
        if (err < 0)
                return err;

        for (i = 0; i < d->emit_queue_cnt; i++)
                btf_dump_emit_type(d, d->emit_queue[i], 0 /*top-level*/);

        return 0;
}

static int btf_dump_add_emit_queue_id(struct btf_dump *d, __u32 id)
{
        __u32 *new_queue;
        size_t new_cap;

        if (d->emit_queue_cnt >= d->emit_queue_cap) {
                new_cap = max(16, d->emit_queue_cap * 3 / 2);
                new_queue = realloc(d->emit_queue,
                                    new_cap * sizeof(new_queue[0]));
                if (!new_queue)
                        return -ENOMEM;
                d->emit_queue = new_queue;
                d->emit_queue_cap = new_cap;
        }

        d->emit_queue[d->emit_queue_cnt++] = id;
        return 0;
}

/*
 * Determine order of emitting dependent types and specified type to satisfy
 * C compilation rules.  This is done through topological sorting with an
 * additional complication which comes from C rules. The main idea for C is
 * that if some type is "embedded" into a struct/union, it's size needs to be
 * known at the time of definition of containing type. E.g., for:
 *
 *      struct A {};
 *      struct B { struct A x; }
 *
 * struct A *HAS* to be defined before struct B, because it's "embedded",
 * i.e., it is part of struct B layout. But in the following case:
 *
 *      struct A;
 *      struct B { struct A *x; }
 *      struct A {};
 *
 * it's enough to just have a forward declaration of struct A at the time of
 * struct B definition, as struct B has a pointer to struct A, so the size of
 * field x is known without knowing struct A size: it's sizeof(void *).
 *
 * Unfortunately, there are some trickier cases we need to handle, e.g.:
 *
 *      struct A {}; // if this was forward-declaration: compilation error
 *      struct B {
 *              struct { // anonymous struct
 *                      struct A y;
 *              } *x;
 *      };
 *
 * In this case, struct B's field x is a pointer, so it's size is known
 * regardless of the size of (anonymous) struct it points to. But because this
 * struct is anonymous and thus defined inline inside struct B, *and* it
 * embeds struct A, compiler requires full definition of struct A to be known
 * before struct B can be defined. This creates a transitive dependency
 * between struct A and struct B. If struct A was forward-declared before
 * struct B definition and fully defined after struct B definition, that would
 * trigger compilation error.
 *
 * All this means that while we are doing topological sorting on BTF type
 * graph, we need to determine relationships between different types (graph
 * nodes):
 *   - weak link (relationship) between X and Y, if Y *CAN* be
 *   forward-declared at the point of X definition;
 *   - strong link, if Y *HAS* to be fully-defined before X can be defined.
 *
 * The rule is as follows. Given a chain of BTF types from X to Y, if there is
 * BTF_KIND_PTR type in the chain and at least one non-anonymous type
 * Z (excluding X, including Y), then link is weak. Otherwise, it's strong.
 * Weak/strong relationship is determined recursively during DFS traversal and
 * is returned as a result from btf_dump_order_type().
 *
 * btf_dump_order_type() is trying to avoid unnecessary forward declarations,
 * but it is not guaranteeing that no extraneous forward declarations will be
 * emitted.
 *
 * To avoid extra work, algorithm marks some of BTF types as ORDERED, when
 * it's done with them, but not for all (e.g., VOLATILE, CONST, RESTRICT,
 * ARRAY, FUNC_PROTO), as weak/strong semantics for those depends on the
 * entire graph path, so depending where from one came to that BTF type, it
 * might cause weak or strong ordering. For types like STRUCT/UNION/INT/ENUM,
 * once they are processed, there is no need to do it again, so they are
 * marked as ORDERED. We can mark PTR as ORDERED as well, as it semi-forces
 * weak link, unless subsequent referenced STRUCT/UNION/ENUM is anonymous. But
 * in any case, once those are processed, no need to do it again, as the
 * result won't change.
 *
 * Returns:
 *   - 1, if type is part of strong link (so there is strong topological
 *   ordering requirements);
 *   - 0, if type is part of weak link (so can be satisfied through forward
 *   declaration);
 *   - <0, on error (e.g., unsatisfiable type loop detected).
 */
static int btf_dump_order_type(struct btf_dump *d, __u32 id, bool through_ptr)
{
        /*
         * Order state is used to detect strong link cycles, but only for BTF
         * kinds that are or could be an independent definition (i.e.,
         * stand-alone fwd decl, enum, typedef, struct, union). Ptrs, arrays,
         * func_protos, modifiers are just means to get to these definitions.
         * Int/void don't need definitions, they are assumed to be always
         * properly defined.  We also ignore datasec, var, and funcs for now.
         * So for all non-defining kinds, we never even set ordering state,
         * for defining kinds we set ORDERING and subsequently ORDERED if it
         * forms a strong link.
         */
        struct btf_dump_type_aux_state *tstate = &d->type_states[id];
        const struct btf_type *t;
        __u16 kind, vlen;
        int err, i;

        /* return true, letting typedefs know that it's ok to be emitted */
        if (tstate->order_state == ORDERED)
                return 1;

        t = btf__type_by_id(d->btf, id);
        kind = btf_kind_of(t);

        if (tstate->order_state == ORDERING) {
                /* type loop, but resolvable through fwd declaration */
                if ((kind == BTF_KIND_STRUCT || kind == BTF_KIND_UNION) &&
                    through_ptr && t->name_off != 0)
                        return 0;
                pr_warning("unsatisfiable type cycle, id:[%u]\n", id);
                return -ELOOP;
        }

        switch (kind) {
        case BTF_KIND_INT:
                tstate->order_state = ORDERED;
                return 0;

        case BTF_KIND_PTR:
                err = btf_dump_order_type(d, t->type, true);
                tstate->order_state = ORDERED;
                return err;

        case BTF_KIND_ARRAY: {
                const struct btf_array *a = (void *)(t + 1);

                return btf_dump_order_type(d, a->type, through_ptr);
        }
        case BTF_KIND_STRUCT:
        case BTF_KIND_UNION: {
                const struct btf_member *m = (void *)(t + 1);
                /*
                 * struct/union is part of strong link, only if it's embedded
                 * (so no ptr in a path) or it's anonymous (so has to be
                 * defined inline, even if declared through ptr)
                 */
                if (through_ptr && t->name_off != 0)
                        return 0;

                tstate->order_state = ORDERING;

                vlen = btf_vlen_of(t);
                for (i = 0; i < vlen; i++, m++) {
                        err = btf_dump_order_type(d, m->type, false);
                        if (err < 0)
                                return err;
                }

                if (t->name_off != 0) {
                        err = btf_dump_add_emit_queue_id(d, id);
                        if (err < 0)
                                return err;
                }

                tstate->order_state = ORDERED;
                return 1;
        }
        case BTF_KIND_ENUM:
        case BTF_KIND_FWD:
                if (t->name_off != 0) {
                        err = btf_dump_add_emit_queue_id(d, id);
                        if (err)
                                return err;
                }
                tstate->order_state = ORDERED;
                return 1;

        case BTF_KIND_TYPEDEF: {
                int is_strong;

                is_strong = btf_dump_order_type(d, t->type, through_ptr);
                if (is_strong < 0)
                        return is_strong;

                /* typedef is similar to struct/union w.r.t. fwd-decls */
                if (through_ptr && !is_strong)
                        return 0;

                /* typedef is always a named definition */
                err = btf_dump_add_emit_queue_id(d, id);
                if (err)
                        return err;

                d->type_states[id].order_state = ORDERED;
                return 1;
        }
        case BTF_KIND_VOLATILE:
        case BTF_KIND_CONST:
        case BTF_KIND_RESTRICT:
                return btf_dump_order_type(d, t->type, through_ptr);

        case BTF_KIND_FUNC_PROTO: {
                const struct btf_param *p = (void *)(t + 1);
                bool is_strong;

                err = btf_dump_order_type(d, t->type, through_ptr);
                if (err < 0)
                        return err;
                is_strong = err > 0;

                vlen = btf_vlen_of(t);
                for (i = 0; i < vlen; i++, p++) {
                        err = btf_dump_order_type(d, p->type, through_ptr);
                        if (err < 0)
                                return err;
                        if (err > 0)
                                is_strong = true;
                }
                return is_strong;
        }
        case BTF_KIND_FUNC:
        case BTF_KIND_VAR:
        case BTF_KIND_DATASEC:
                d->type_states[id].order_state = ORDERED;
                return 0;

        default:
                return -EINVAL;
        }
}

static void btf_dump_emit_struct_fwd(struct btf_dump *d, __u32 id,
                                     const struct btf_type *t);
static void btf_dump_emit_struct_def(struct btf_dump *d, __u32 id,
                                     const struct btf_type *t, int lvl);

static void btf_dump_emit_enum_fwd(struct btf_dump *d, __u32 id,
                                   const struct btf_type *t);
static void btf_dump_emit_enum_def(struct btf_dump *d, __u32 id,
                                   const struct btf_type *t, int lvl);

static void btf_dump_emit_fwd_def(struct btf_dump *d, __u32 id,
                                  const struct btf_type *t);

static void btf_dump_emit_typedef_def(struct btf_dump *d, __u32 id,
                                      const struct btf_type *t, int lvl);

/* a local view into a shared stack */
struct id_stack {
        const __u32 *ids;
        int cnt;
};

static void btf_dump_emit_type_decl(struct btf_dump *d, __u32 id,
                                    const char *fname, int lvl);
static void btf_dump_emit_type_chain(struct btf_dump *d,
                                     struct id_stack *decl_stack,
                                     const char *fname, int lvl);

static const char *btf_dump_type_name(struct btf_dump *d, __u32 id);
static const char *btf_dump_ident_name(struct btf_dump *d, __u32 id);
static size_t btf_dump_name_dups(struct btf_dump *d, struct hashmap *name_map,
                                 const char *orig_name);

static bool btf_dump_is_blacklisted(struct btf_dump *d, __u32 id)
{
        const struct btf_type *t = btf__type_by_id(d->btf, id);

        /* __builtin_va_list is a compiler built-in, which causes compilation
         * errors, when compiling w/ different compiler, then used to compile
         * original code (e.g., GCC to compile kernel, Clang to use generated
         * C header from BTF). As it is built-in, it should be already defined
         * properly internally in compiler.
         */
        if (t->name_off == 0)
                return false;
        return strcmp(btf_name_of(d, t->name_off), "__builtin_va_list") == 0;
}

/*
 * Emit C-syntax definitions of types from chains of BTF types.
 *
 * High-level handling of determining necessary forward declarations are handled
 * by btf_dump_emit_type() itself, but all nitty-gritty details of emitting type
 * declarations/definitions in C syntax  are handled by a combo of
 * btf_dump_emit_type_decl()/btf_dump_emit_type_chain() w/ delegation to
 * corresponding btf_dump_emit_*_{def,fwd}() functions.
 *
 * We also keep track of "containing struct/union type ID" to determine when
 * we reference it from inside and thus can avoid emitting unnecessary forward
 * declaration.
 *
 * This algorithm is designed in such a way, that even if some error occurs
 * (either technical, e.g., out of memory, or logical, i.e., malformed BTF
 * that doesn't comply to C rules completely), algorithm will try to proceed
 * and produce as much meaningful output as possible.
 */
static void btf_dump_emit_type(struct btf_dump *d, __u32 id, __u32 cont_id)
{
        struct btf_dump_type_aux_state *tstate = &d->type_states[id];
        bool top_level_def = cont_id == 0;
        const struct btf_type *t;
        __u16 kind;

        if (tstate->emit_state == EMITTED)
                return;

        t = btf__type_by_id(d->btf, id);
        kind = btf_kind_of(t);

        if (top_level_def && t->name_off == 0) {
                pr_warning("unexpected nameless definition, id:[%u]\n", id);
                return;
        }

        if (tstate->emit_state == EMITTING) {
                if (tstate->fwd_emitted)
                        return;

                switch (kind) {
                case BTF_KIND_STRUCT:
                case BTF_KIND_UNION:
                        /*
                         * if we are referencing a struct/union that we are
                         * part of - then no need for fwd declaration
                         */
                        if (id == cont_id)
                                return;
                        if (t->name_off == 0) {
                                pr_warning("anonymous struct/union loop, id:[%u]\n",
                                           id);
                                return;
                        }
                        btf_dump_emit_struct_fwd(d, id, t);
                        btf_dump_printf(d, ";\n\n");
                        tstate->fwd_emitted = 1;
                        break;
                case BTF_KIND_TYPEDEF:
                        /*
                         * for typedef fwd_emitted means typedef definition
                         * was emitted, but it can be used only for "weak"
                         * references through pointer only, not for embedding
                         */
                        if (!btf_dump_is_blacklisted(d, id)) {
                                btf_dump_emit_typedef_def(d, id, t, 0);
                                btf_dump_printf(d, ";\n\n");
                        };
                        tstate->fwd_emitted = 1;
                        break;
                default:
                        break;
                }

                return;
        }

        switch (kind) {
        case BTF_KIND_INT:
                tstate->emit_state = EMITTED;
                break;
        case BTF_KIND_ENUM:
                if (top_level_def) {
                        btf_dump_emit_enum_def(d, id, t, 0);
                        btf_dump_printf(d, ";\n\n");
                }
                tstate->emit_state = EMITTED;
                break;
        case BTF_KIND_PTR:
        case BTF_KIND_VOLATILE:
        case BTF_KIND_CONST:
        case BTF_KIND_RESTRICT:
                btf_dump_emit_type(d, t->type, cont_id);
                break;
        case BTF_KIND_ARRAY: {
                const struct btf_array *a = (void *)(t + 1);

                btf_dump_emit_type(d, a->type, cont_id);
                break;
        }
        case BTF_KIND_FWD:
                btf_dump_emit_fwd_def(d, id, t);
                btf_dump_printf(d, ";\n\n");
                tstate->emit_state = EMITTED;
                break;
        case BTF_KIND_TYPEDEF:
                tstate->emit_state = EMITTING;
                btf_dump_emit_type(d, t->type, id);
                /*
                 * typedef can server as both definition and forward
                 * declaration; at this stage someone depends on
                 * typedef as a forward declaration (refers to it
                 * through pointer), so unless we already did it,
                 * emit typedef as a forward declaration
                 */
                if (!tstate->fwd_emitted && !btf_dump_is_blacklisted(d, id)) {
                        btf_dump_emit_typedef_def(d, id, t, 0);
                        btf_dump_printf(d, ";\n\n");
                }
                tstate->emit_state = EMITTED;
                break;
        case BTF_KIND_STRUCT:
        case BTF_KIND_UNION:
                tstate->emit_state = EMITTING;
                /* if it's a top-level struct/union definition or struct/union
                 * is anonymous, then in C we'll be emitting all fields and
                 * their types (as opposed to just `struct X`), so we need to
                 * make sure that all types, referenced from struct/union
                 * members have necessary forward-declarations, where
                 * applicable
                 */
                if (top_level_def || t->name_off == 0) {
                        const struct btf_member *m = (void *)(t + 1);
                        __u16 vlen = btf_vlen_of(t);
                        int i, new_cont_id;

                        new_cont_id = t->name_off == 0 ? cont_id : id;
                        for (i = 0; i < vlen; i++, m++)
                                btf_dump_emit_type(d, m->type, new_cont_id);
                } else if (!tstate->fwd_emitted && id != cont_id) {
                        btf_dump_emit_struct_fwd(d, id, t);
                        btf_dump_printf(d, ";\n\n");
                        tstate->fwd_emitted = 1;
                }

                if (top_level_def) {
                        btf_dump_emit_struct_def(d, id, t, 0);
                        btf_dump_printf(d, ";\n\n");
                        tstate->emit_state = EMITTED;
                } else {
                        tstate->emit_state = NOT_EMITTED;
                }
                break;
        case BTF_KIND_FUNC_PROTO: {
                const struct btf_param *p = (void *)(t + 1);
                __u16 vlen = btf_vlen_of(t);
                int i;

                btf_dump_emit_type(d, t->type, cont_id);
                for (i = 0; i < vlen; i++, p++)
                        btf_dump_emit_type(d, p->type, cont_id);

                break;
        }
        default:
                break;
        }
}

static int btf_align_of(const struct btf *btf, __u32 id)
{
        const struct btf_type *t = btf__type_by_id(btf, id);
        __u16 kind = btf_kind_of(t);

        switch (kind) {
        case BTF_KIND_INT:
        case BTF_KIND_ENUM:
                return min(sizeof(void *), t->size);
        case BTF_KIND_PTR:
                return sizeof(void *);
        case BTF_KIND_TYPEDEF:
        case BTF_KIND_VOLATILE:
        case BTF_KIND_CONST:
        case BTF_KIND_RESTRICT:
                return btf_align_of(btf, t->type);
        case BTF_KIND_ARRAY: {
                const struct btf_array *a = (void *)(t + 1);

                return btf_align_of(btf, a->type);
        }
        case BTF_KIND_STRUCT:
        case BTF_KIND_UNION: {
                const struct btf_member *m = (void *)(t + 1);
                __u16 vlen = btf_vlen_of(t);
                int i, align = 1;

                for (i = 0; i < vlen; i++, m++)
                        align = max(align, btf_align_of(btf, m->type));

                return align;
        }
        default:
                pr_warning("unsupported BTF_KIND:%u\n", btf_kind_of(t));
                return 1;
        }
}

static bool btf_is_struct_packed(const struct btf *btf, __u32 id,
                                 const struct btf_type *t)
{
        const struct btf_member *m;
        int align, i, bit_sz;
        __u16 vlen;
        bool kflag;

        align = btf_align_of(btf, id);
        /* size of a non-packed struct has to be a multiple of its alignment*/
        if (t->size % align)
                return true;

        m = (void *)(t + 1);
        kflag = btf_kflag_of(t);
        vlen = btf_vlen_of(t);
        /* all non-bitfield fields have to be naturally aligned */
        for (i = 0; i < vlen; i++, m++) {
                align = btf_align_of(btf, m->type);
                bit_sz = kflag ? BTF_MEMBER_BITFIELD_SIZE(m->offset) : 0;
                if (bit_sz == 0 && m->offset % (8 * align) != 0)
                        return true;
        }

        /*
         * if original struct was marked as packed, but its layout is
         * naturally aligned, we'll detect that it's not packed
         */
        return false;
}

static int chip_away_bits(int total, int at_most)
{
        return total % at_most ? : at_most;
}

static void btf_dump_emit_bit_padding(const struct btf_dump *d,
                                      int cur_off, int m_off, int m_bit_sz,
                                      int align, int lvl)
{
        int off_diff = m_off - cur_off;
        int ptr_bits = sizeof(void *) * 8;

        if (off_diff <= 0)
                /* no gap */
                return;
        if (m_bit_sz == 0 && off_diff < align * 8)
                /* natural padding will take care of a gap */
                return;

        while (off_diff > 0) {
                const char *pad_type;
                int pad_bits;

                if (ptr_bits > 32 && off_diff > 32) {
                        pad_type = "long";
                        pad_bits = chip_away_bits(off_diff, ptr_bits);
                } else if (off_diff > 16) {
                        pad_type = "int";
                        pad_bits = chip_away_bits(off_diff, 32);
                } else if (off_diff > 8) {
                        pad_type = "short";
                        pad_bits = chip_away_bits(off_diff, 16);
                } else {
                        pad_type = "char";
                        pad_bits = chip_away_bits(off_diff, 8);
                }
                btf_dump_printf(d, "\n%s%s: %d;", pfx(lvl), pad_type, pad_bits);
                off_diff -= pad_bits;
        }
}

static void btf_dump_emit_struct_fwd(struct btf_dump *d, __u32 id,
                                     const struct btf_type *t)
{
        btf_dump_printf(d, "%s %s",
                        btf_kind_of(t) == BTF_KIND_STRUCT ? "struct" : "union",
                        btf_dump_type_name(d, id));
}

static void btf_dump_emit_struct_def(struct btf_dump *d,
                                     __u32 id,
                                     const struct btf_type *t,
                                     int lvl)
{
        const struct btf_member *m = (void *)(t + 1);
        bool kflag = btf_kflag_of(t), is_struct;
        int align, i, packed, off = 0;
        __u16 vlen = btf_vlen_of(t);

        is_struct = btf_kind_of(t) == BTF_KIND_STRUCT;
        packed = is_struct ? btf_is_struct_packed(d->btf, id, t) : 0;
        align = packed ? 1 : btf_align_of(d->btf, id);

        btf_dump_printf(d, "%s%s%s {",
                        is_struct ? "struct" : "union",
                        t->name_off ? " " : "",
                        btf_dump_type_name(d, id));

        for (i = 0; i < vlen; i++, m++) {
                const char *fname;
                int m_off, m_sz;

                fname = btf_name_of(d, m->name_off);
                m_sz = kflag ? BTF_MEMBER_BITFIELD_SIZE(m->offset) : 0;
                m_off = kflag ? BTF_MEMBER_BIT_OFFSET(m->offset) : m->offset;
                align = packed ? 1 : btf_align_of(d->btf, m->type);

                btf_dump_emit_bit_padding(d, off, m_off, m_sz, align, lvl + 1);
                btf_dump_printf(d, "\n%s", pfx(lvl + 1));
                btf_dump_emit_type_decl(d, m->type, fname, lvl + 1);

                if (m_sz) {
                        btf_dump_printf(d, ": %d", m_sz);
                        off = m_off + m_sz;
                } else {
                        m_sz = max(0, btf__resolve_size(d->btf, m->type));
                        off = m_off + m_sz * 8;
                }
                btf_dump_printf(d, ";");
        }

        if (vlen)
                btf_dump_printf(d, "\n");
        btf_dump_printf(d, "%s}", pfx(lvl));
        if (packed)
                btf_dump_printf(d, " __attribute__((packed))");
}

static void btf_dump_emit_enum_fwd(struct btf_dump *d, __u32 id,
                                   const struct btf_type *t)
{
        btf_dump_printf(d, "enum %s", btf_dump_type_name(d, id));
}

static void btf_dump_emit_enum_def(struct btf_dump *d, __u32 id,
                                   const struct btf_type *t,
                                   int lvl)
{
        const struct btf_enum *v = (void *)(t+1);
        __u16 vlen = btf_vlen_of(t);
        const char *name;
        size_t dup_cnt;
        int i;

        btf_dump_printf(d, "enum%s%s",
                        t->name_off ? " " : "",
                        btf_dump_type_name(d, id));

        if (vlen) {
                btf_dump_printf(d, " {");
                for (i = 0; i < vlen; i++, v++) {
                        name = btf_name_of(d, v->name_off);
                        /* enumerators share namespace with typedef idents */
                        dup_cnt = btf_dump_name_dups(d, d->ident_names, name);
                        if (dup_cnt > 1) {
                                btf_dump_printf(d, "\n%s%s___%zu = %d,",
                                                pfx(lvl + 1), name, dup_cnt,
                                                (__s32)v->val);
                        } else {
                                btf_dump_printf(d, "\n%s%s = %d,",
                                                pfx(lvl + 1), name,
                                                (__s32)v->val);
                        }
                }
                btf_dump_printf(d, "\n%s}", pfx(lvl));
        }
}

static void btf_dump_emit_fwd_def(struct btf_dump *d, __u32 id,
                                  const struct btf_type *t)
{
        const char *name = btf_dump_type_name(d, id);

        if (btf_kflag_of(t))
                btf_dump_printf(d, "union %s", name);
        else
                btf_dump_printf(d, "struct %s", name);
}

static void btf_dump_emit_typedef_def(struct btf_dump *d, __u32 id,
                                     const struct btf_type *t, int lvl)
{
        const char *name = btf_dump_ident_name(d, id);

        btf_dump_printf(d, "typedef ");
        btf_dump_emit_type_decl(d, t->type, name, lvl);
}

static int btf_dump_push_decl_stack_id(struct btf_dump *d, __u32 id)
{
        __u32 *new_stack;
        size_t new_cap;

        if (d->decl_stack_cnt >= d->decl_stack_cap) {
                new_cap = max(16, d->decl_stack_cap * 3 / 2);
                new_stack = realloc(d->decl_stack,
                                    new_cap * sizeof(new_stack[0]));
                if (!new_stack)
                        return -ENOMEM;
                d->decl_stack = new_stack;
                d->decl_stack_cap = new_cap;
        }

        d->decl_stack[d->decl_stack_cnt++] = id;

        return 0;
}

/*
 * Emit type declaration (e.g., field type declaration in a struct or argument
 * declaration in function prototype) in correct C syntax.
 *
 * For most types it's trivial, but there are few quirky type declaration
 * cases worth mentioning:
 *   - function prototypes (especially nesting of function prototypes);
 *   - arrays;
 *   - const/volatile/restrict for pointers vs other types.
 *
 * For a good discussion of *PARSING* C syntax (as a human), see
 * Peter van der Linden's "Expert C Programming: Deep C Secrets",
 * Ch.3 "Unscrambling Declarations in C".
 *
 * It won't help with BTF to C conversion much, though, as it's an opposite
 * problem. So we came up with this algorithm in reverse to van der Linden's
 * parsing algorithm. It goes from structured BTF representation of type
 * declaration to a valid compilable C syntax.
 *
 * For instance, consider this C typedef:
 *      typedef const int * const * arr[10] arr_t;
 * It will be represented in BTF with this chain of BTF types:
 *      [typedef] -> [array] -> [ptr] -> [const] -> [ptr] -> [const] -> [int]
 *
 * Notice how [const] modifier always goes before type it modifies in BTF type
 * graph, but in C syntax, const/volatile/restrict modifiers are written to
 * the right of pointers, but to the left of other types. There are also other
 * quirks, like function pointers, arrays of them, functions returning other
 * functions, etc.
 *
 * We handle that by pushing all the types to a stack, until we hit "terminal"
 * type (int/enum/struct/union/fwd). Then depending on the kind of a type on
 * top of a stack, modifiers are handled differently. Array/function pointers
 * have also wildly different syntax and how nesting of them are done. See
 * code for authoritative definition.
 *
 * To avoid allocating new stack for each independent chain of BTF types, we
 * share one bigger stack, with each chain working only on its own local view
 * of a stack frame. Some care is required to "pop" stack frames after
 * processing type declaration chain.
 */
static void btf_dump_emit_type_decl(struct btf_dump *d, __u32 id,
                                    const char *fname, int lvl)
{
        struct id_stack decl_stack;
        const struct btf_type *t;
        int err, stack_start;
        __u16 kind;

        stack_start = d->decl_stack_cnt;
        for (;;) {
                err = btf_dump_push_decl_stack_id(d, id);
                if (err < 0) {
                        /*
                         * if we don't have enough memory for entire type decl
                         * chain, restore stack, emit warning, and try to
                         * proceed nevertheless
                         */

                        pr_warning("not enough memory for decl stack:%d", err);
                        d->decl_stack_cnt = stack_start;
                        return;
                }

                /* VOID */
                if (id == 0)
                        break;

                t = btf__type_by_id(d->btf, id);
                kind = btf_kind_of(t);
                switch (kind) {
                case BTF_KIND_PTR:
                case BTF_KIND_VOLATILE:
                case BTF_KIND_CONST:
                case BTF_KIND_RESTRICT:
                case BTF_KIND_FUNC_PROTO:
                        id = t->type;
                        break;
                case BTF_KIND_ARRAY: {
                        const struct btf_array *a = (void *)(t + 1);

                        id = a->type;
                        break;
                }
                case BTF_KIND_INT:
                case BTF_KIND_ENUM:
                case BTF_KIND_FWD:
                case BTF_KIND_STRUCT:
                case BTF_KIND_UNION:
                case BTF_KIND_TYPEDEF:
                        goto done;
                default:
                        pr_warning("unexpected type in decl chain, kind:%u, id:[%u]\n",
                                   kind, id);
                        goto done;
                }
        }
done:
        /*
         * We might be inside a chain of declarations (e.g., array of function
         * pointers returning anonymous (so inlined) structs, having another
         * array field). Each of those needs its own "stack frame" to handle
         * emitting of declarations. Those stack frames are non-overlapping
         * portions of shared btf_dump->decl_stack. To make it a bit nicer to
         * handle this set of nested stacks, we create a view corresponding to
         * our own "stack frame" and work with it as an independent stack.
         * We'll need to clean up after emit_type_chain() returns, though.
         */
        decl_stack.ids = d->decl_stack + stack_start;
        decl_stack.cnt = d->decl_stack_cnt - stack_start;
        btf_dump_emit_type_chain(d, &decl_stack, fname, lvl);
        /*
         * emit_type_chain() guarantees that it will pop its entire decl_stack
         * frame before returning. But it works with a read-only view into
         * decl_stack, so it doesn't actually pop anything from the
         * perspective of shared btf_dump->decl_stack, per se. We need to
         * reset decl_stack state to how it was before us to avoid it growing
         * all the time.
         */
        d->decl_stack_cnt = stack_start;
}

static void btf_dump_emit_mods(struct btf_dump *d, struct id_stack *decl_stack)
{
        const struct btf_type *t;
        __u32 id;

        while (decl_stack->cnt) {
                id = decl_stack->ids[decl_stack->cnt - 1];
                t = btf__type_by_id(d->btf, id);

                switch (btf_kind_of(t)) {
                case BTF_KIND_VOLATILE:
                        btf_dump_printf(d, "volatile ");
                        break;
                case BTF_KIND_CONST:
                        btf_dump_printf(d, "const ");
                        break;
                case BTF_KIND_RESTRICT:
                        btf_dump_printf(d, "restrict ");
                        break;
                default:
                        return;
                }
                decl_stack->cnt--;
        }
}

static bool btf_is_mod_kind(const struct btf *btf, __u32 id)
{
        const struct btf_type *t = btf__type_by_id(btf, id);

        switch (btf_kind_of(t)) {
        case BTF_KIND_VOLATILE:
        case BTF_KIND_CONST:
        case BTF_KIND_RESTRICT:
                return true;
        default:
                return false;
        }
}

static void btf_dump_emit_name(const struct btf_dump *d,
                               const char *name, bool last_was_ptr)
{
        bool separate = name[0] && !last_was_ptr;

        btf_dump_printf(d, "%s%s", separate ? " " : "", name);
}

static void btf_dump_emit_type_chain(struct btf_dump *d,
                                     struct id_stack *decls,
                                     const char *fname, int lvl)
{
        /*
         * last_was_ptr is used to determine if we need to separate pointer
         * asterisk (*) from previous part of type signature with space, so
         * that we get `int ***`, instead of `int * * *`. We default to true
         * for cases where we have single pointer in a chain. E.g., in ptr ->
         * func_proto case. func_proto will start a new emit_type_chain call
         * with just ptr, which should be emitted as (*) or (*<fname>), so we
         * don't want to prepend space for that last pointer.
         */
        bool last_was_ptr = true;
        const struct btf_type *t;
        const char *name;
        __u16 kind;
        __u32 id;

        while (decls->cnt) {
                id = decls->ids[--decls->cnt];
                if (id == 0) {
                        /* VOID is a special snowflake */
                        btf_dump_emit_mods(d, decls);
                        btf_dump_printf(d, "void");
                        last_was_ptr = false;
                        continue;
                }

                t = btf__type_by_id(d->btf, id);
                kind = btf_kind_of(t);

                switch (kind) {
                case BTF_KIND_INT:
                        btf_dump_emit_mods(d, decls);
                        name = btf_name_of(d, t->name_off);
                        btf_dump_printf(d, "%s", name);
                        break;
                case BTF_KIND_STRUCT:
                case BTF_KIND_UNION:
                        btf_dump_emit_mods(d, decls);
                        /* inline anonymous struct/union */
                        if (t->name_off == 0)
                                btf_dump_emit_struct_def(d, id, t, lvl);
                        else
                                btf_dump_emit_struct_fwd(d, id, t);
                        break;
                case BTF_KIND_ENUM:
                        btf_dump_emit_mods(d, decls);
                        /* inline anonymous enum */
                        if (t->name_off == 0)
                                btf_dump_emit_enum_def(d, id, t, lvl);
                        else
                                btf_dump_emit_enum_fwd(d, id, t);
                        break;
                case BTF_KIND_FWD:
                        btf_dump_emit_mods(d, decls);
                        btf_dump_emit_fwd_def(d, id, t);
                        break;
                case BTF_KIND_TYPEDEF:
                        btf_dump_emit_mods(d, decls);
                        btf_dump_printf(d, "%s", btf_dump_ident_name(d, id));
                        break;
                case BTF_KIND_PTR:
                        btf_dump_printf(d, "%s", last_was_ptr ? "*" : " *");
                        break;
                case BTF_KIND_VOLATILE:
                        btf_dump_printf(d, " volatile");
                        break;
                case BTF_KIND_CONST:
                        btf_dump_printf(d, " const");
                        break;
                case BTF_KIND_RESTRICT:
                        btf_dump_printf(d, " restrict");
                        break;
                case BTF_KIND_ARRAY: {
                        const struct btf_array *a = (void *)(t + 1);
                        const struct btf_type *next_t;
                        __u32 next_id;
                        bool multidim;
                        /*
                         * GCC has a bug
                         * (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=8354)
                         * which causes it to emit extra const/volatile
                         * modifiers for an array, if array's element type has
                         * const/volatile modifiers. Clang doesn't do that.
                         * In general, it doesn't seem very meaningful to have
                         * a const/volatile modifier for array, so we are
                         * going to silently skip them here.
                         */
                        while (decls->cnt) {
                                next_id = decls->ids[decls->cnt - 1];
                                if (btf_is_mod_kind(d->btf, next_id))
                                        decls->cnt--;
                                else
                                        break;
                        }

                        if (decls->cnt == 0) {
                                btf_dump_emit_name(d, fname, last_was_ptr);
                                btf_dump_printf(d, "[%u]", a->nelems);
                                return;
                        }

                        next_t = btf__type_by_id(d->btf, next_id);
                        multidim = btf_kind_of(next_t) == BTF_KIND_ARRAY;
                        /* we need space if we have named non-pointer */
                        if (fname[0] && !last_was_ptr)
                                btf_dump_printf(d, " ");
                        /* no parentheses for multi-dimensional array */
                        if (!multidim)
                                btf_dump_printf(d, "(");
                        btf_dump_emit_type_chain(d, decls, fname, lvl);
                        if (!multidim)
                                btf_dump_printf(d, ")");
                        btf_dump_printf(d, "[%u]", a->nelems);
                        return;
                }
                case BTF_KIND_FUNC_PROTO: {
                        const struct btf_param *p = (void *)(t + 1);
                        __u16 vlen = btf_vlen_of(t);
                        int i;

                        btf_dump_emit_mods(d, decls);
                        if (decls->cnt) {
                                btf_dump_printf(d, " (");
                                btf_dump_emit_type_chain(d, decls, fname, lvl);
                                btf_dump_printf(d, ")");
                        } else {
                                btf_dump_emit_name(d, fname, last_was_ptr);
                        }
                        btf_dump_printf(d, "(");
                        /*
                         * Clang for BPF target generates func_proto with no
                         * args as a func_proto with a single void arg (e.g.,
                         * `int (*f)(void)` vs just `int (*f)()`). We are
                         * going to pretend there are no args for such case.
                         */
                        if (vlen == 1 && p->type == 0) {
                                btf_dump_printf(d, ")");
                                return;
                        }

                        for (i = 0; i < vlen; i++, p++) {
                                if (i > 0)
                                        btf_dump_printf(d, ", ");

                                /* last arg of type void is vararg */
                                if (i == vlen - 1 && p->type == 0) {
                                        btf_dump_printf(d, "...");
                                        break;
                                }

                                name = btf_name_of(d, p->name_off);
                                btf_dump_emit_type_decl(d, p->type, name, lvl);
                        }

                        btf_dump_printf(d, ")");
                        return;
                }
                default:
                        pr_warning("unexpected type in decl chain, kind:%u, id:[%u]\n",
                                   kind, id);
                        return;
                }

                last_was_ptr = kind == BTF_KIND_PTR;
        }

        btf_dump_emit_name(d, fname, last_was_ptr);
}

/* return number of duplicates (occurrences) of a given name */
static size_t btf_dump_name_dups(struct btf_dump *d, struct hashmap *name_map,
                                 const char *orig_name)
{
        size_t dup_cnt = 0;

        hashmap__find(name_map, orig_name, (void **)&dup_cnt);
        dup_cnt++;
        hashmap__set(name_map, orig_name, (void *)dup_cnt, NULL, NULL);

        return dup_cnt;
}

static const char *btf_dump_resolve_name(struct btf_dump *d, __u32 id,
                                         struct hashmap *name_map)
{
        struct btf_dump_type_aux_state *s = &d->type_states[id];
        const struct btf_type *t = btf__type_by_id(d->btf, id);
        const char *orig_name = btf_name_of(d, t->name_off);
        const char **cached_name = &d->cached_names[id];
        size_t dup_cnt;

        if (t->name_off == 0)
                return "";

        if (s->name_resolved)
                return *cached_name ? *cached_name : orig_name;

        dup_cnt = btf_dump_name_dups(d, name_map, orig_name);
        if (dup_cnt > 1) {
                const size_t max_len = 256;
                char new_name[max_len];

                snprintf(new_name, max_len, "%s___%zu", orig_name, dup_cnt);
                *cached_name = strdup(new_name);
        }

        s->name_resolved = 1;
        return *cached_name ? *cached_name : orig_name;
}

static const char *btf_dump_type_name(struct btf_dump *d, __u32 id)
{
        return btf_dump_resolve_name(d, id, d->type_names);
}

static const char *btf_dump_ident_name(struct btf_dump *d, __u32 id)
{
        return btf_dump_resolve_name(d, id, d->ident_names);
}