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 bool reloc2sec_off(struct reloc *reloc, struct section **sec, unsigned long *off)
  62 {
  63         switch (reloc->sym->type) {
  64         case STT_FUNC:
  65                 *sec = reloc->sym->sec;
  66                 *off = reloc->sym->offset + reloc->addend;
  67                 return true;
  68
  69         case STT_SECTION:
  70                 *sec = reloc->sym->sec;
  71                 *off = reloc->addend;
  72                 return true;
  73
  74         default:
  75                 return false;
  76         }
  77 }
  78
  79 static int get_alt_entry(struct elf *elf, struct special_entry *entry,
  80                          struct section *sec, int idx,
  81                          struct special_alt *alt)
  82 {
  83         struct reloc *orig_reloc, *new_reloc;
  84         unsigned long offset;
  85
  86         offset = idx * entry->size;
  87
  88         alt->group = entry->group;
  89         alt->jump_or_nop = entry->jump_or_nop;
  90
  91         if (alt->group) {
  92                 alt->orig_len = *(unsigned char *)(sec->data->d_buf + offset +
  93                                                    entry->orig_len);
  94                 alt->new_len = *(unsigned char *)(sec->data->d_buf + offset +
  95                                                   entry->new_len);
  96         }
  97
  98         if (entry->feature) {
  99                 unsigned short feature;
 100
 101                 feature = bswap_if_needed(*(unsigned short *)(sec->data->d_buf +
 102                                                               offset +
 103                                                               entry->feature));
 104                 arch_handle_alternative(feature, alt);
 105         }
 106
 107         orig_reloc = find_reloc_by_dest(elf, sec, offset + entry->orig);
 108         if (!orig_reloc) {
 109                 WARN_FUNC("can't find orig reloc", sec, offset + entry->orig);
 110                 return -1;
 111         }
 112         if (!reloc2sec_off(orig_reloc, &alt->orig_sec, &alt->orig_off)) {
 113                 WARN_FUNC("don't know how to handle reloc symbol type %d: %s",
 114                            sec, offset + entry->orig,
 115                            orig_reloc->sym->type,
 116                            orig_reloc->sym->name);
 117                 return -1;
 118         }
 119
 120         if (!entry->group || alt->new_len) {
 121                 new_reloc = find_reloc_by_dest(elf, sec, offset + entry->new);
 122                 if (!new_reloc) {
 123                         WARN_FUNC("can't find new reloc",
 124                                   sec, offset + entry->new);
 125                         return -1;
 126                 }
 127
 128                 /*
 129                  * Skip retpoline .altinstr_replacement... we already rewrite the
 130                  * instructions for retpolines anyway, see arch_is_retpoline()
 131                  * usage in add_{call,jump}_destinations().
 132                  */
 133                 if (arch_is_retpoline(new_reloc->sym))
 134                         return 1;
 135
 136                 if (!reloc2sec_off(new_reloc, &alt->new_sec, &alt->new_off)) {
 137                         WARN_FUNC("don't know how to handle reloc symbol type %d: %s",
 138                                   sec, offset + entry->new,
 139                                   new_reloc->sym->type,
 140                                   new_reloc->sym->name);
 141                         return -1;
 142                 }
 143
 144                 /* _ASM_EXTABLE_EX hack */
 145                 if (alt->new_off >= 0x7ffffff0)
 146                         alt->new_off -= 0x7ffffff0;
 147         }
 148
 149         if (entry->key) {
 150                 struct reloc *key_reloc;
 151
 152                 key_reloc = find_reloc_by_dest(elf, sec, offset + entry->key);
 153                 if (!key_reloc) {
 154                         WARN_FUNC("can't find key reloc",
 155                                   sec, offset + entry->key);
 156                         return -1;
 157                 }
 158                 alt->key_addend = key_reloc->addend;
 159         }
 160
 161         return 0;
 162 }
 163
 164 /*
 165  * Read all the special sections and create a list of special_alt structs which
 166  * describe all the alternate instructions which can be patched in or
 167  * redirected to at runtime.
 168  */
 169 int special_get_alts(struct elf *elf, struct list_head *alts)
 170 {
 171         struct special_entry *entry;
 172         struct section *sec;
 173         unsigned int nr_entries;
 174         struct special_alt *alt;
 175         int idx, ret;
 176
 177         INIT_LIST_HEAD(alts);
 178
 179         for (entry = entries; entry->sec; entry++) {
 180                 sec = find_section_by_name(elf, entry->sec);
 181                 if (!sec)
 182                         continue;
 183
 184                 if (sec->len % entry->size != 0) {
 185                         WARN("%s size not a multiple of %d",
 186                              sec->name, entry->size);
 187                         return -1;
 188                 }
 189
 190                 nr_entries = sec->len / entry->size;
 191
 192                 for (idx = 0; idx < nr_entries; idx++) {
 193                         alt = malloc(sizeof(*alt));
 194                         if (!alt) {
 195                                 WARN("malloc failed");
 196                                 return -1;
 197                         }
 198                         memset(alt, 0, sizeof(*alt));
 199
 200                         ret = get_alt_entry(elf, entry, sec, idx, alt);
 201                         if (ret > 0)
 202                                 continue;
 203                         if (ret < 0)
 204                                 return ret;
 205
 206                         list_add_tail(&alt->list, alts);
 207                 }
 208         }
 209
 210         return 0;
 211 }