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 "builtin.h"
  27 #include "special.h"
  28 #include "warn.h"
  29
  30 #define EX_ENTRY_SIZE           12
  31 #define EX_ORIG_OFFSET          0
  32 #define EX_NEW_OFFSET           4
  33
  34 #define JUMP_ENTRY_SIZE         16
  35 #define JUMP_ORIG_OFFSET        0
  36 #define JUMP_NEW_OFFSET         4
  37
  38 #define ALT_ENTRY_SIZE          13
  39 #define ALT_ORIG_OFFSET         0
  40 #define ALT_NEW_OFFSET          4
  41 #define ALT_FEATURE_OFFSET      8
  42 #define ALT_ORIG_LEN_OFFSET     10
  43 #define ALT_NEW_LEN_OFFSET      11
  44
  45 #define X86_FEATURE_POPCNT (4*32+23)
  46 #define X86_FEATURE_SMAP   (9*32+20)
  47
  48 struct special_entry {
  49         const char *sec;
  50         bool group, jump_or_nop;
  51         unsigned char size, orig, new;
  52         unsigned char orig_len, new_len; /* group only */
  53         unsigned char feature; /* ALTERNATIVE macro CPU feature */
  54 };
  55
  56 struct special_entry entries[] = {
  57         {
  58                 .sec = ".altinstructions",
  59                 .group = true,
  60                 .size = ALT_ENTRY_SIZE,
  61                 .orig = ALT_ORIG_OFFSET,
  62                 .orig_len = ALT_ORIG_LEN_OFFSET,
  63                 .new = ALT_NEW_OFFSET,
  64                 .new_len = ALT_NEW_LEN_OFFSET,
  65                 .feature = ALT_FEATURE_OFFSET,
  66         },
  67         {
  68                 .sec = "__jump_table",
  69                 .jump_or_nop = true,
  70                 .size = JUMP_ENTRY_SIZE,
  71                 .orig = JUMP_ORIG_OFFSET,
  72                 .new = JUMP_NEW_OFFSET,
  73         },
  74         {
  75                 .sec = "__ex_table",
  76                 .size = EX_ENTRY_SIZE,
  77                 .orig = EX_ORIG_OFFSET,
  78                 .new = EX_NEW_OFFSET,
  79         },
  80         {},
  81 };
  82
  83 static int get_alt_entry(struct elf *elf, struct special_entry *entry,
  84                          struct section *sec, int idx,
  85                          struct special_alt *alt)
  86 {
  87         struct rela *orig_rela, *new_rela;
  88         unsigned long offset;
  89
  90         offset = idx * entry->size;
  91
  92         alt->group = entry->group;
  93         alt->jump_or_nop = entry->jump_or_nop;
  94
  95         if (alt->group) {
  96                 alt->orig_len = *(unsigned char *)(sec->data->d_buf + offset +
  97                                                    entry->orig_len);
  98                 alt->new_len = *(unsigned char *)(sec->data->d_buf + offset +
  99                                                   entry->new_len);
 100         }
 101
 102         if (entry->feature) {
 103                 unsigned short feature;
 104
 105                 feature = *(unsigned short *)(sec->data->d_buf + offset +
 106                                               entry->feature);
 107
 108                 /*
 109                  * It has been requested that we don't validate the !POPCNT
 110                  * feature path which is a "very very small percentage of
 111                  * machines".
 112                  */
 113                 if (feature == X86_FEATURE_POPCNT)
 114                         alt->skip_orig = true;
 115
 116                 /*
 117                  * If UACCESS validation is enabled; force that alternative;
 118                  * otherwise force it the other way.
 119                  *
 120                  * What we want to avoid is having both the original and the
 121                  * alternative code flow at the same time, in that case we can
 122                  * find paths that see the STAC but take the NOP instead of
 123                  * CLAC and the other way around.
 124                  */
 125                 if (feature == X86_FEATURE_SMAP) {
 126                         if (uaccess)
 127                                 alt->skip_orig = true;
 128                         else
 129                                 alt->skip_alt = true;
 130                 }
 131         }
 132
 133         orig_rela = find_rela_by_dest(sec, offset + entry->orig);
 134         if (!orig_rela) {
 135                 WARN_FUNC("can't find orig rela", sec, offset + entry->orig);
 136                 return -1;
 137         }
 138         if (orig_rela->sym->type != STT_SECTION) {
 139                 WARN_FUNC("don't know how to handle non-section rela symbol %s",
 140                            sec, offset + entry->orig, orig_rela->sym->name);
 141                 return -1;
 142         }
 143
 144         alt->orig_sec = orig_rela->sym->sec;
 145         alt->orig_off = orig_rela->addend;
 146
 147         if (!entry->group || alt->new_len) {
 148                 new_rela = find_rela_by_dest(sec, offset + entry->new);
 149                 if (!new_rela) {
 150                         WARN_FUNC("can't find new rela",
 151                                   sec, offset + entry->new);
 152                         return -1;
 153                 }
 154
 155                 alt->new_sec = new_rela->sym->sec;
 156                 alt->new_off = (unsigned int)new_rela->addend;
 157
 158                 /* _ASM_EXTABLE_EX hack */
 159                 if (alt->new_off >= 0x7ffffff0)
 160                         alt->new_off -= 0x7ffffff0;
 161         }
 162
 163         return 0;
 164 }
 165
 166 /*
 167  * Read all the special sections and create a list of special_alt structs which
 168  * describe all the alternate instructions which can be patched in or
 169  * redirected to at runtime.
 170  */
 171 int special_get_alts(struct elf *elf, struct list_head *alts)
 172 {
 173         struct special_entry *entry;
 174         struct section *sec;
 175         unsigned int nr_entries;
 176         struct special_alt *alt;
 177         int idx, ret;
 178
 179         INIT_LIST_HEAD(alts);
 180
 181         for (entry = entries; entry->sec; entry++) {
 182                 sec = find_section_by_name(elf, entry->sec);
 183                 if (!sec)
 184                         continue;
 185
 186                 if (sec->len % entry->size != 0) {
 187                         WARN("%s size not a multiple of %d",
 188                              sec->name, entry->size);
 189                         return -1;
 190                 }
 191
 192                 nr_entries = sec->len / entry->size;
 193
 194                 for (idx = 0; idx < nr_entries; idx++) {
 195                         alt = malloc(sizeof(*alt));
 196                         if (!alt) {
 197                                 WARN("malloc failed");
 198                                 return -1;
 199                         }
 200                         memset(alt, 0, sizeof(*alt));
 201
 202                         ret = get_alt_entry(elf, entry, sec, idx, alt);
 203                         if (ret)
 204                                 return ret;
 205
 206                         list_add_tail(&alt->list, alts);
 207                 }
 208         }
 209
 210         return 0;
 211 }