linux/tools/objtool/special.c
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   1// SPDX-License-Identifier: GPL-2.0-or-later
   2/*
   3 * Copyright (C) 2015 Josh Poimboeuf <jpoimboe@redhat.com>
   4 */
   5
   6/*
   7 * This file reads all the special sections which have alternate instructions
   8 * which can be patched in or redirected to at runtime.
   9 */
  10
  11#include <stdlib.h>
  12#include <string.h>
  13
  14#include <arch/special.h>
  15#include <objtool/builtin.h>
  16#include <objtool/special.h>
  17#include <objtool/warn.h>
  18#include <objtool/endianness.h>
  19
  20struct special_entry {
  21        const char *sec;
  22        bool group, jump_or_nop;
  23        unsigned char size, orig, new;
  24        unsigned char orig_len, new_len; /* group only */
  25        unsigned char feature; /* ALTERNATIVE macro CPU feature */
  26        unsigned char key; /* jump_label key */
  27};
  28
  29struct special_entry entries[] = {
  30        {
  31                .sec = ".altinstructions",
  32                .group = true,
  33                .size = ALT_ENTRY_SIZE,
  34                .orig = ALT_ORIG_OFFSET,
  35                .orig_len = ALT_ORIG_LEN_OFFSET,
  36                .new = ALT_NEW_OFFSET,
  37                .new_len = ALT_NEW_LEN_OFFSET,
  38                .feature = ALT_FEATURE_OFFSET,
  39        },
  40        {
  41                .sec = "__jump_table",
  42                .jump_or_nop = true,
  43                .size = JUMP_ENTRY_SIZE,
  44                .orig = JUMP_ORIG_OFFSET,
  45                .new = JUMP_NEW_OFFSET,
  46                .key = JUMP_KEY_OFFSET,
  47        },
  48        {
  49                .sec = "__ex_table",
  50                .size = EX_ENTRY_SIZE,
  51                .orig = EX_ORIG_OFFSET,
  52                .new = EX_NEW_OFFSET,
  53        },
  54        {},
  55};
  56
  57void __weak arch_handle_alternative(unsigned short feature, struct special_alt *alt)
  58{
  59}
  60
  61static int get_alt_entry(struct elf *elf, struct special_entry *entry,
  62                         struct section *sec, int idx,
  63                         struct special_alt *alt)
  64{
  65        struct reloc *orig_reloc, *new_reloc;
  66        unsigned long offset;
  67
  68        offset = idx * entry->size;
  69
  70        alt->group = entry->group;
  71        alt->jump_or_nop = entry->jump_or_nop;
  72
  73        if (alt->group) {
  74                alt->orig_len = *(unsigned char *)(sec->data->d_buf + offset +
  75                                                   entry->orig_len);
  76                alt->new_len = *(unsigned char *)(sec->data->d_buf + offset +
  77                                                  entry->new_len);
  78        }
  79
  80        if (entry->feature) {
  81                unsigned short feature;
  82
  83                feature = bswap_if_needed(*(unsigned short *)(sec->data->d_buf +
  84                                                              offset +
  85                                                              entry->feature));
  86                arch_handle_alternative(feature, alt);
  87        }
  88
  89        orig_reloc = find_reloc_by_dest(elf, sec, offset + entry->orig);
  90        if (!orig_reloc) {
  91                WARN_FUNC("can't find orig reloc", sec, offset + entry->orig);
  92                return -1;
  93        }
  94        if (orig_reloc->sym->type != STT_SECTION) {
  95                WARN_FUNC("don't know how to handle non-section reloc symbol %s",
  96                           sec, offset + entry->orig, orig_reloc->sym->name);
  97                return -1;
  98        }
  99
 100        alt->orig_sec = orig_reloc->sym->sec;
 101        alt->orig_off = orig_reloc->addend;
 102
 103        if (!entry->group || alt->new_len) {
 104                new_reloc = find_reloc_by_dest(elf, sec, offset + entry->new);
 105                if (!new_reloc) {
 106                        WARN_FUNC("can't find new reloc",
 107                                  sec, offset + entry->new);
 108                        return -1;
 109                }
 110
 111                /*
 112                 * Skip retpoline .altinstr_replacement... we already rewrite the
 113                 * instructions for retpolines anyway, see arch_is_retpoline()
 114                 * usage in add_{call,jump}_destinations().
 115                 */
 116                if (arch_is_retpoline(new_reloc->sym))
 117                        return 1;
 118
 119                alt->new_sec = new_reloc->sym->sec;
 120                alt->new_off = (unsigned int)new_reloc->addend;
 121
 122                /* _ASM_EXTABLE_EX hack */
 123                if (alt->new_off >= 0x7ffffff0)
 124                        alt->new_off -= 0x7ffffff0;
 125        }
 126
 127        if (entry->key) {
 128                struct reloc *key_reloc;
 129
 130                key_reloc = find_reloc_by_dest(elf, sec, offset + entry->key);
 131                if (!key_reloc) {
 132                        WARN_FUNC("can't find key reloc",
 133                                  sec, offset + entry->key);
 134                        return -1;
 135                }
 136                alt->key_addend = key_reloc->addend;
 137        }
 138
 139        return 0;
 140}
 141
 142/*
 143 * Read all the special sections and create a list of special_alt structs which
 144 * describe all the alternate instructions which can be patched in or
 145 * redirected to at runtime.
 146 */
 147int special_get_alts(struct elf *elf, struct list_head *alts)
 148{
 149        struct special_entry *entry;
 150        struct section *sec;
 151        unsigned int nr_entries;
 152        struct special_alt *alt;
 153        int idx, ret;
 154
 155        INIT_LIST_HEAD(alts);
 156
 157        for (entry = entries; entry->sec; entry++) {
 158                sec = find_section_by_name(elf, entry->sec);
 159                if (!sec)
 160                        continue;
 161
 162                if (sec->len % entry->size != 0) {
 163                        WARN("%s size not a multiple of %d",
 164                             sec->name, entry->size);
 165                        return -1;
 166                }
 167
 168                nr_entries = sec->len / entry->size;
 169
 170                for (idx = 0; idx < nr_entries; idx++) {
 171                        alt = malloc(sizeof(*alt));
 172                        if (!alt) {
 173                                WARN("malloc failed");
 174                                return -1;
 175                        }
 176                        memset(alt, 0, sizeof(*alt));
 177
 178                        ret = get_alt_entry(elf, entry, sec, idx, alt);
 179                        if (ret > 0)
 180                                continue;
 181                        if (ret < 0)
 182                                return ret;
 183
 184                        list_add_tail(&alt->list, alts);
 185                }
 186        }
 187
 188        return 0;
 189}
 190