tools/objtool/special.c

   1 /*
   2  * Copyright (C) 2015 Josh Poimboeuf <jpoimboe@redhat.com>
   3  *
   4  * This program is free software; you can redistribute it and/or
   5  * modify it under the terms of the GNU General Public License
   6  * as published by the Free Software Foundation; either version 2
   7  * of the License, or (at your option) any later version.
   8  *
   9  * This program is distributed in the hope that it will be useful,
  10  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  11  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12  * GNU General Public License for more details.
  13  *
  14  * You should have received a copy of the GNU General Public License
  15  * along with this program; if not, see <http://www.gnu.org/licenses/>.
  16  */
  17
  18 /*
  19  * This file reads all the special sections which have alternate instructions
  20  * which can be patched in or redirected to at runtime.
  21  */
  22
  23 #include <stdlib.h>
  24 #include <string.h>
  25
  26 #include "special.h"
  27 #include "warn.h"
  28
  29 #define EX_ENTRY_SIZE           12
  30 #define EX_ORIG_OFFSET          0
  31 #define EX_NEW_OFFSET           4
  32
  33 #define JUMP_ENTRY_SIZE         24
  34 #define JUMP_ORIG_OFFSET        0
  35 #define JUMP_NEW_OFFSET         8
  36
  37 #define ALT_ENTRY_SIZE          13
  38 #define ALT_ORIG_OFFSET         0
  39 #define ALT_NEW_OFFSET          4
  40 #define ALT_FEATURE_OFFSET      8
  41 #define ALT_ORIG_LEN_OFFSET     10
  42 #define ALT_NEW_LEN_OFFSET      11
  43
  44 #define X86_FEATURE_POPCNT (4*32+23)
  45
  46 struct special_entry {
  47         const char *sec;
  48         bool group, jump_or_nop;
  49         unsigned char size, orig, new;
  50         unsigned char orig_len, new_len; /* group only */
  51         unsigned char feature; /* ALTERNATIVE macro CPU feature */
  52 };
  53
  54 struct special_entry entries[] = {
  55         {
  56                 .sec = ".altinstructions",
  57                 .group = true,
  58                 .size = ALT_ENTRY_SIZE,
  59                 .orig = ALT_ORIG_OFFSET,
  60                 .orig_len = ALT_ORIG_LEN_OFFSET,
  61                 .new = ALT_NEW_OFFSET,
  62                 .new_len = ALT_NEW_LEN_OFFSET,
  63                 .feature = ALT_FEATURE_OFFSET,
  64         },
  65         {
  66                 .sec = "__jump_table",
  67                 .jump_or_nop = true,
  68                 .size = JUMP_ENTRY_SIZE,
  69                 .orig = JUMP_ORIG_OFFSET,
  70                 .new = JUMP_NEW_OFFSET,
  71         },
  72         {
  73                 .sec = "__ex_table",
  74                 .size = EX_ENTRY_SIZE,
  75                 .orig = EX_ORIG_OFFSET,
  76                 .new = EX_NEW_OFFSET,
  77         },
  78         {},
  79 };
  80
  81 static int get_alt_entry(struct elf *elf, struct special_entry *entry,
  82                          struct section *sec, int idx,
  83                          struct special_alt *alt)
  84 {
  85         struct rela *orig_rela, *new_rela;
  86         unsigned long offset;
  87
  88         offset = idx * entry->size;
  89
  90         alt->group = entry->group;
  91         alt->jump_or_nop = entry->jump_or_nop;
  92
  93         if (alt->group) {
  94                 alt->orig_len = *(unsigned char *)(sec->data->d_buf + offset +
  95                                                    entry->orig_len);
  96                 alt->new_len = *(unsigned char *)(sec->data->d_buf + offset +
  97                                                   entry->new_len);
  98         }
  99
 100         if (entry->feature) {
 101                 unsigned short feature;
 102
 103                 feature = *(unsigned short *)(sec->data->d_buf + offset +
 104                                               entry->feature);
 105
 106                 /*
 107                  * It has been requested that we don't validate the !POPCNT
 108                  * feature path which is a "very very small percentage of
 109                  * machines".
 110                  */
 111                 if (feature == X86_FEATURE_POPCNT)
 112                         alt->skip_orig = true;
 113         }
 114
 115         orig_rela = find_rela_by_dest(sec, offset + entry->orig);
 116         if (!orig_rela) {
 117                 WARN_FUNC("can't find orig rela", sec, offset + entry->orig);
 118                 return -1;
 119         }
 120         if (orig_rela->sym->type != STT_SECTION) {
 121                 WARN_FUNC("don't know how to handle non-section rela symbol %s",
 122                            sec, offset + entry->orig, orig_rela->sym->name);
 123                 return -1;
 124         }
 125
 126         alt->orig_sec = orig_rela->sym->sec;
 127         alt->orig_off = orig_rela->addend;
 128
 129         if (!entry->group || alt->new_len) {
 130                 new_rela = find_rela_by_dest(sec, offset + entry->new);
 131                 if (!new_rela) {
 132                         WARN_FUNC("can't find new rela",
 133                                   sec, offset + entry->new);
 134                         return -1;
 135                 }
 136
 137                 alt->new_sec = new_rela->sym->sec;
 138                 alt->new_off = (unsigned int)new_rela->addend;
 139
 140                 /* _ASM_EXTABLE_EX hack */
 141                 if (alt->new_off >= 0x7ffffff0)
 142                         alt->new_off -= 0x7ffffff0;
 143         }
 144
 145         return 0;
 146 }
 147
 148 /*
 149  * Read all the special sections and create a list of special_alt structs which
 150  * describe all the alternate instructions which can be patched in or
 151  * redirected to at runtime.
 152  */
 153 int special_get_alts(struct elf *elf, struct list_head *alts)
 154 {
 155         struct special_entry *entry;
 156         struct section *sec;
 157         unsigned int nr_entries;
 158         struct special_alt *alt;
 159         int idx, ret;
 160
 161         INIT_LIST_HEAD(alts);
 162
 163         for (entry = entries; entry->sec; entry++) {
 164                 sec = find_section_by_name(elf, entry->sec);
 165                 if (!sec)
 166                         continue;
 167
 168                 if (sec->len % entry->size != 0) {
 169                         WARN("%s size not a multiple of %d",
 170                              sec->name, entry->size);
 171                         return -1;
 172                 }
 173
 174                 nr_entries = sec->len / entry->size;
 175
 176                 for (idx = 0; idx < nr_entries; idx++) {
 177                         alt = malloc(sizeof(*alt));
 178                         if (!alt) {
 179                                 WARN("malloc failed");
 180                                 return -1;
 181                         }
 182                         memset(alt, 0, sizeof(*alt));
 183
 184                         ret = get_alt_entry(elf, entry, sec, idx, alt);
 185                         if (ret)
 186                                 return ret;
 187
 188                         list_add_tail(&alt->list, alts);
 189                 }
 190         }
 191
 192         return 0;
 193 }