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 "builtin.h"
  15 #include "special.h"
  16 #include "warn.h"
  17
  18 #define EX_ENTRY_SIZE           12
  19 #define EX_ORIG_OFFSET          0
  20 #define EX_NEW_OFFSET           4
  21
  22 #define JUMP_ENTRY_SIZE         16
  23 #define JUMP_ORIG_OFFSET        0
  24 #define JUMP_NEW_OFFSET         4
  25
  26 #define ALT_ENTRY_SIZE          13
  27 #define ALT_ORIG_OFFSET         0
  28 #define ALT_NEW_OFFSET          4
  29 #define ALT_FEATURE_OFFSET      8
  30 #define ALT_ORIG_LEN_OFFSET     10
  31 #define ALT_NEW_LEN_OFFSET      11
  32
  33 #define X86_FEATURE_POPCNT (4*32+23)
  34 #define X86_FEATURE_SMAP   (9*32+20)
  35
  36 struct special_entry {
  37         const char *sec;
  38         bool group, jump_or_nop;
  39         unsigned char size, orig, new;
  40         unsigned char orig_len, new_len; /* group only */
  41         unsigned char feature; /* ALTERNATIVE macro CPU feature */
  42 };
  43
  44 struct special_entry entries[] = {
  45         {
  46                 .sec = ".altinstructions",
  47                 .group = true,
  48                 .size = ALT_ENTRY_SIZE,
  49                 .orig = ALT_ORIG_OFFSET,
  50                 .orig_len = ALT_ORIG_LEN_OFFSET,
  51                 .new = ALT_NEW_OFFSET,
  52                 .new_len = ALT_NEW_LEN_OFFSET,
  53                 .feature = ALT_FEATURE_OFFSET,
  54         },
  55         {
  56                 .sec = "__jump_table",
  57                 .jump_or_nop = true,
  58                 .size = JUMP_ENTRY_SIZE,
  59                 .orig = JUMP_ORIG_OFFSET,
  60                 .new = JUMP_NEW_OFFSET,
  61         },
  62         {
  63                 .sec = "__ex_table",
  64                 .size = EX_ENTRY_SIZE,
  65                 .orig = EX_ORIG_OFFSET,
  66                 .new = EX_NEW_OFFSET,
  67         },
  68         {},
  69 };
  70
  71 static int get_alt_entry(struct elf *elf, struct special_entry *entry,
  72                          struct section *sec, int idx,
  73                          struct special_alt *alt)
  74 {
  75         struct rela *orig_rela, *new_rela;
  76         unsigned long offset;
  77
  78         offset = idx * entry->size;
  79
  80         alt->group = entry->group;
  81         alt->jump_or_nop = entry->jump_or_nop;
  82
  83         if (alt->group) {
  84                 alt->orig_len = *(unsigned char *)(sec->data->d_buf + offset +
  85                                                    entry->orig_len);
  86                 alt->new_len = *(unsigned char *)(sec->data->d_buf + offset +
  87                                                   entry->new_len);
  88         }
  89
  90         if (entry->feature) {
  91                 unsigned short feature;
  92
  93                 feature = *(unsigned short *)(sec->data->d_buf + offset +
  94                                               entry->feature);
  95
  96                 /*
  97                  * It has been requested that we don't validate the !POPCNT
  98                  * feature path which is a "very very small percentage of
  99                  * machines".
 100                  */
 101                 if (feature == X86_FEATURE_POPCNT)
 102                         alt->skip_orig = true;
 103
 104                 /*
 105                  * If UACCESS validation is enabled; force that alternative;
 106                  * otherwise force it the other way.
 107                  *
 108                  * What we want to avoid is having both the original and the
 109                  * alternative code flow at the same time, in that case we can
 110                  * find paths that see the STAC but take the NOP instead of
 111                  * CLAC and the other way around.
 112                  */
 113                 if (feature == X86_FEATURE_SMAP) {
 114                         if (uaccess)
 115                                 alt->skip_orig = true;
 116                         else
 117                                 alt->skip_alt = true;
 118                 }
 119         }
 120
 121         orig_rela = find_rela_by_dest(sec, offset + entry->orig);
 122         if (!orig_rela) {
 123                 WARN_FUNC("can't find orig rela", sec, offset + entry->orig);
 124                 return -1;
 125         }
 126         if (orig_rela->sym->type != STT_SECTION) {
 127                 WARN_FUNC("don't know how to handle non-section rela symbol %s",
 128                            sec, offset + entry->orig, orig_rela->sym->name);
 129                 return -1;
 130         }
 131
 132         alt->orig_sec = orig_rela->sym->sec;
 133         alt->orig_off = orig_rela->addend;
 134
 135         if (!entry->group || alt->new_len) {
 136                 new_rela = find_rela_by_dest(sec, offset + entry->new);
 137                 if (!new_rela) {
 138                         WARN_FUNC("can't find new rela",
 139                                   sec, offset + entry->new);
 140                         return -1;
 141                 }
 142
 143                 alt->new_sec = new_rela->sym->sec;
 144                 alt->new_off = (unsigned int)new_rela->addend;
 145
 146                 /* _ASM_EXTABLE_EX hack */
 147                 if (alt->new_off >= 0x7ffffff0)
 148                         alt->new_off -= 0x7ffffff0;
 149         }
 150
 151         return 0;
 152 }
 153
 154 /*
 155  * Read all the special sections and create a list of special_alt structs which
 156  * describe all the alternate instructions which can be patched in or
 157  * redirected to at runtime.
 158  */
 159 int special_get_alts(struct elf *elf, struct list_head *alts)
 160 {
 161         struct special_entry *entry;
 162         struct section *sec;
 163         unsigned int nr_entries;
 164         struct special_alt *alt;
 165         int idx, ret;
 166
 167         INIT_LIST_HEAD(alts);
 168
 169         for (entry = entries; entry->sec; entry++) {
 170                 sec = find_section_by_name(elf, entry->sec);
 171                 if (!sec)
 172                         continue;
 173
 174                 if (sec->len % entry->size != 0) {
 175                         WARN("%s size not a multiple of %d",
 176                              sec->name, entry->size);
 177                         return -1;
 178                 }
 179
 180                 nr_entries = sec->len / entry->size;
 181
 182                 for (idx = 0; idx < nr_entries; idx++) {
 183                         alt = malloc(sizeof(*alt));
 184                         if (!alt) {
 185                                 WARN("malloc failed");
 186                                 return -1;
 187                         }
 188                         memset(alt, 0, sizeof(*alt));
 189
 190                         ret = get_alt_entry(elf, entry, sec, idx, alt);
 191                         if (ret)
 192                                 return ret;
 193
 194                         list_add_tail(&alt->list, alts);
 195                 }
 196         }
 197
 198         return 0;
 199 }