// SPDX-License-Identifier: GPL-2.0-or-later
/*
 * Copyright (C) 2015 Josh Poimboeuf <jpoimboe@redhat.com>
 */

/*
 * This file reads all the special sections which have alternate instructions
 * which can be patched in or redirected to at runtime.
 */

#include <stdlib.h>
#include <string.h>

#include "builtin.h"
#include "special.h"
#include "warn.h"

#define EX_ENTRY_SIZE           12
#define EX_ORIG_OFFSET          0
#define EX_NEW_OFFSET           4

#define JUMP_ENTRY_SIZE         16
#define JUMP_ORIG_OFFSET        0
#define JUMP_NEW_OFFSET         4

#define ALT_ENTRY_SIZE          13
#define ALT_ORIG_OFFSET         0
#define ALT_NEW_OFFSET          4
#define ALT_FEATURE_OFFSET      8
#define ALT_ORIG_LEN_OFFSET     10
#define ALT_NEW_LEN_OFFSET      11

#define X86_FEATURE_POPCNT (4*32+23)
#define X86_FEATURE_SMAP   (9*32+20)

struct special_entry {
        const char *sec;
        bool group, jump_or_nop;
        unsigned char size, orig, new;
        unsigned char orig_len, new_len; /* group only */
        unsigned char feature; /* ALTERNATIVE macro CPU feature */
};

struct special_entry entries[] = {
        {
                .sec = ".altinstructions",
                .group = true,
                .size = ALT_ENTRY_SIZE,
                .orig = ALT_ORIG_OFFSET,
                .orig_len = ALT_ORIG_LEN_OFFSET,
                .new = ALT_NEW_OFFSET,
                .new_len = ALT_NEW_LEN_OFFSET,
                .feature = ALT_FEATURE_OFFSET,
        },
        {
                .sec = "__jump_table",
                .jump_or_nop = true,
                .size = JUMP_ENTRY_SIZE,
                .orig = JUMP_ORIG_OFFSET,
                .new = JUMP_NEW_OFFSET,
        },
        {
                .sec = "__ex_table",
                .size = EX_ENTRY_SIZE,
                .orig = EX_ORIG_OFFSET,
                .new = EX_NEW_OFFSET,
        },
        {},
};

static int get_alt_entry(struct elf *elf, struct special_entry *entry,
                         struct section *sec, int idx,
                         struct special_alt *alt)
{
        struct rela *orig_rela, *new_rela;
        unsigned long offset;

        offset = idx * entry->size;

        alt->group = entry->group;
        alt->jump_or_nop = entry->jump_or_nop;

        if (alt->group) {
                alt->orig_len = *(unsigned char *)(sec->data->d_buf + offset +
                                                   entry->orig_len);
                alt->new_len = *(unsigned char *)(sec->data->d_buf + offset +
                                                  entry->new_len);
        }

        if (entry->feature) {
                unsigned short feature;

                feature = *(unsigned short *)(sec->data->d_buf + offset +
                                              entry->feature);

                /*
                 * It has been requested that we don't validate the !POPCNT
                 * feature path which is a "very very small percentage of
                 * machines".
                 */
                if (feature == X86_FEATURE_POPCNT)
                        alt->skip_orig = true;

                /*
                 * If UACCESS validation is enabled; force that alternative;
                 * otherwise force it the other way.
                 *
                 * What we want to avoid is having both the original and the
                 * alternative code flow at the same time, in that case we can
                 * find paths that see the STAC but take the NOP instead of
                 * CLAC and the other way around.
                 */
                if (feature == X86_FEATURE_SMAP) {
                        if (uaccess)
                                alt->skip_orig = true;
                        else
                                alt->skip_alt = true;
                }
        }

        orig_rela = find_rela_by_dest(sec, offset + entry->orig);
        if (!orig_rela) {
                WARN_FUNC("can't find orig rela", sec, offset + entry->orig);
                return -1;
        }
        if (orig_rela->sym->type != STT_SECTION) {
                WARN_FUNC("don't know how to handle non-section rela symbol %s",
                           sec, offset + entry->orig, orig_rela->sym->name);
                return -1;
        }

        alt->orig_sec = orig_rela->sym->sec;
        alt->orig_off = orig_rela->addend;

        if (!entry->group || alt->new_len) {
                new_rela = find_rela_by_dest(sec, offset + entry->new);
                if (!new_rela) {
                        WARN_FUNC("can't find new rela",
                                  sec, offset + entry->new);
                        return -1;
                }

                alt->new_sec = new_rela->sym->sec;
                alt->new_off = (unsigned int)new_rela->addend;

                /* _ASM_EXTABLE_EX hack */
                if (alt->new_off >= 0x7ffffff0)
                        alt->new_off -= 0x7ffffff0;
        }

        return 0;
}

/*
 * Read all the special sections and create a list of special_alt structs which
 * describe all the alternate instructions which can be patched in or
 * redirected to at runtime.
 */
int special_get_alts(struct elf *elf, struct list_head *alts)
{
        struct special_entry *entry;
        struct section *sec;
        unsigned int nr_entries;
        struct special_alt *alt;
        int idx, ret;

        INIT_LIST_HEAD(alts);

        for (entry = entries; entry->sec; entry++) {
                sec = find_section_by_name(elf, entry->sec);
                if (!sec)
                        continue;

                if (sec->len % entry->size != 0) {
                        WARN("%s size not a multiple of %d",
                             sec->name, entry->size);
                        return -1;
                }

                nr_entries = sec->len / entry->size;

                for (idx = 0; idx < nr_entries; idx++) {
                        alt = malloc(sizeof(*alt));
                        if (!alt) {
                                WARN("malloc failed");
                                return -1;
                        }
                        memset(alt, 0, sizeof(*alt));

                        ret = get_alt_entry(elf, entry, sec, idx, alt);
                        if (ret)
                                return ret;

                        list_add_tail(&alt->list, alts);
                }
        }

        return 0;
}