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 };
  27
  28 struct special_entry entries[] = {
  29         {
  30                 .sec = ".altinstructions",
  31                 .group = true,
  32                 .size = ALT_ENTRY_SIZE,
  33                 .orig = ALT_ORIG_OFFSET,
  34                 .orig_len = ALT_ORIG_LEN_OFFSET,
  35                 .new = ALT_NEW_OFFSET,
  36                 .new_len = ALT_NEW_LEN_OFFSET,
  37                 .feature = ALT_FEATURE_OFFSET,
  38         },
  39         {
  40                 .sec = "__jump_table",
  41                 .jump_or_nop = true,
  42                 .size = JUMP_ENTRY_SIZE,
  43                 .orig = JUMP_ORIG_OFFSET,
  44                 .new = JUMP_NEW_OFFSET,
  45         },
  46         {
  47                 .sec = "__ex_table",
  48                 .size = EX_ENTRY_SIZE,
  49                 .orig = EX_ORIG_OFFSET,
  50                 .new = EX_NEW_OFFSET,
  51         },
  52         {},
  53 };
  54
  55 void __weak arch_handle_alternative(unsigned short feature, struct special_alt *alt)
  56 {
  57 }
  58
  59 static int get_alt_entry(struct elf *elf, struct special_entry *entry,
  60                          struct section *sec, int idx,
  61                          struct special_alt *alt)
  62 {
  63         struct reloc *orig_reloc, *new_reloc;
  64         unsigned long offset;
  65
  66         offset = idx * entry->size;
  67
  68         alt->group = entry->group;
  69         alt->jump_or_nop = entry->jump_or_nop;
  70
  71         if (alt->group) {
  72                 alt->orig_len = *(unsigned char *)(sec->data->d_buf + offset +
  73                                                    entry->orig_len);
  74                 alt->new_len = *(unsigned char *)(sec->data->d_buf + offset +
  75                                                   entry->new_len);
  76         }
  77
  78         if (entry->feature) {
  79                 unsigned short feature;
  80
  81                 feature = bswap_if_needed(*(unsigned short *)(sec->data->d_buf +
  82                                                               offset +
  83                                                               entry->feature));
  84                 arch_handle_alternative(feature, alt);
  85         }
  86
  87         orig_reloc = find_reloc_by_dest(elf, sec, offset + entry->orig);
  88         if (!orig_reloc) {
  89                 WARN_FUNC("can't find orig reloc", sec, offset + entry->orig);
  90                 return -1;
  91         }
  92         if (orig_reloc->sym->type != STT_SECTION) {
  93                 WARN_FUNC("don't know how to handle non-section reloc symbol %s",
  94                            sec, offset + entry->orig, orig_reloc->sym->name);
  95                 return -1;
  96         }
  97
  98         alt->orig_sec = orig_reloc->sym->sec;
  99         alt->orig_off = orig_reloc->addend;
 100
 101         if (!entry->group || alt->new_len) {
 102                 new_reloc = find_reloc_by_dest(elf, sec, offset + entry->new);
 103                 if (!new_reloc) {
 104                         WARN_FUNC("can't find new reloc",
 105                                   sec, offset + entry->new);
 106                         return -1;
 107                 }
 108
 109                 /*
 110                  * Skip retpoline .altinstr_replacement... we already rewrite the
 111                  * instructions for retpolines anyway, see arch_is_retpoline()
 112                  * usage in add_{call,jump}_destinations().
 113                  */
 114                 if (arch_is_retpoline(new_reloc->sym))
 115                         return 1;
 116
 117                 alt->new_sec = new_reloc->sym->sec;
 118                 alt->new_off = (unsigned int)new_reloc->addend;
 119
 120                 /* _ASM_EXTABLE_EX hack */
 121                 if (alt->new_off >= 0x7ffffff0)
 122                         alt->new_off -= 0x7ffffff0;
 123         }
 124
 125         return 0;
 126 }
 127
 128 /*
 129  * Read all the special sections and create a list of special_alt structs which
 130  * describe all the alternate instructions which can be patched in or
 131  * redirected to at runtime.
 132  */
 133 int special_get_alts(struct elf *elf, struct list_head *alts)
 134 {
 135         struct special_entry *entry;
 136         struct section *sec;
 137         unsigned int nr_entries;
 138         struct special_alt *alt;
 139         int idx, ret;
 140
 141         INIT_LIST_HEAD(alts);
 142
 143         for (entry = entries; entry->sec; entry++) {
 144                 sec = find_section_by_name(elf, entry->sec);
 145                 if (!sec)
 146                         continue;
 147
 148                 if (sec->len % entry->size != 0) {
 149                         WARN("%s size not a multiple of %d",
 150                              sec->name, entry->size);
 151                         return -1;
 152                 }
 153
 154                 nr_entries = sec->len / entry->size;
 155
 156                 for (idx = 0; idx < nr_entries; idx++) {
 157                         alt = malloc(sizeof(*alt));
 158                         if (!alt) {
 159                                 WARN("malloc failed");
 160                                 return -1;
 161                         }
 162                         memset(alt, 0, sizeof(*alt));
 163
 164                         ret = get_alt_entry(elf, entry, sec, idx, alt);
 165                         if (ret > 0)
 166                                 continue;
 167                         if (ret < 0)
 168                                 return ret;
 169
 170                         list_add_tail(&alt->list, alts);
 171                 }
 172         }
 173
 174         return 0;
 175 }