tools/objtool/special.c

   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
  20 struct 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
  29 struct 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
  57 void __weak arch_handle_alternative(unsigned short feature, struct special_alt *alt)
  58 {
  59 }
  60
  61 static 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  */
 147 int 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 }