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 #include "arch_special.h"
  18
  19 struct special_entry {
  20         const char *sec;
  21         bool group, jump_or_nop;
  22         unsigned char size, orig, new;
  23         unsigned char orig_len, new_len; /* group only */
  24         unsigned char feature; /* ALTERNATIVE macro CPU feature */
  25 };
  26
  27 struct special_entry entries[] = {
  28         {
  29                 .sec = ".altinstructions",
  30                 .group = true,
  31                 .size = ALT_ENTRY_SIZE,
  32                 .orig = ALT_ORIG_OFFSET,
  33                 .orig_len = ALT_ORIG_LEN_OFFSET,
  34                 .new = ALT_NEW_OFFSET,
  35                 .new_len = ALT_NEW_LEN_OFFSET,
  36                 .feature = ALT_FEATURE_OFFSET,
  37         },
  38         {
  39                 .sec = "__jump_table",
  40                 .jump_or_nop = true,
  41                 .size = JUMP_ENTRY_SIZE,
  42                 .orig = JUMP_ORIG_OFFSET,
  43                 .new = JUMP_NEW_OFFSET,
  44         },
  45         {
  46                 .sec = "__ex_table",
  47                 .size = EX_ENTRY_SIZE,
  48                 .orig = EX_ORIG_OFFSET,
  49                 .new = EX_NEW_OFFSET,
  50         },
  51         {},
  52 };
  53
  54 void __weak arch_handle_alternative(unsigned short feature, struct special_alt *alt)
  55 {
  56 }
  57
  58 static int get_alt_entry(struct elf *elf, struct special_entry *entry,
  59                          struct section *sec, int idx,
  60                          struct special_alt *alt)
  61 {
  62         struct reloc *orig_reloc, *new_reloc;
  63         unsigned long offset;
  64
  65         offset = idx * entry->size;
  66
  67         alt->group = entry->group;
  68         alt->jump_or_nop = entry->jump_or_nop;
  69
  70         if (alt->group) {
  71                 alt->orig_len = *(unsigned char *)(sec->data->d_buf + offset +
  72                                                    entry->orig_len);
  73                 alt->new_len = *(unsigned char *)(sec->data->d_buf + offset +
  74                                                   entry->new_len);
  75         }
  76
  77         if (entry->feature) {
  78                 unsigned short feature;
  79
  80                 feature = *(unsigned short *)(sec->data->d_buf + offset +
  81                                               entry->feature);
  82                 arch_handle_alternative(feature, alt);
  83         }
  84
  85         orig_reloc = find_reloc_by_dest(elf, sec, offset + entry->orig);
  86         if (!orig_reloc) {
  87                 WARN_FUNC("can't find orig reloc", sec, offset + entry->orig);
  88                 return -1;
  89         }
  90         if (orig_reloc->sym->type != STT_SECTION) {
  91                 WARN_FUNC("don't know how to handle non-section reloc symbol %s",
  92                            sec, offset + entry->orig, orig_reloc->sym->name);
  93                 return -1;
  94         }
  95
  96         alt->orig_sec = orig_reloc->sym->sec;
  97         alt->orig_off = orig_reloc->addend;
  98
  99         if (!entry->group || alt->new_len) {
 100                 new_reloc = find_reloc_by_dest(elf, sec, offset + entry->new);
 101                 if (!new_reloc) {
 102                         WARN_FUNC("can't find new reloc",
 103                                   sec, offset + entry->new);
 104                         return -1;
 105                 }
 106
 107                 alt->new_sec = new_reloc->sym->sec;
 108                 alt->new_off = (unsigned int)new_reloc->addend;
 109
 110                 /* _ASM_EXTABLE_EX hack */
 111                 if (alt->new_off >= 0x7ffffff0)
 112                         alt->new_off -= 0x7ffffff0;
 113         }
 114
 115         return 0;
 116 }
 117
 118 /*
 119  * Read all the special sections and create a list of special_alt structs which
 120  * describe all the alternate instructions which can be patched in or
 121  * redirected to at runtime.
 122  */
 123 int special_get_alts(struct elf *elf, struct list_head *alts)
 124 {
 125         struct special_entry *entry;
 126         struct section *sec;
 127         unsigned int nr_entries;
 128         struct special_alt *alt;
 129         int idx, ret;
 130
 131         INIT_LIST_HEAD(alts);
 132
 133         for (entry = entries; entry->sec; entry++) {
 134                 sec = find_section_by_name(elf, entry->sec);
 135                 if (!sec)
 136                         continue;
 137
 138                 if (sec->len % entry->size != 0) {
 139                         WARN("%s size not a multiple of %d",
 140                              sec->name, entry->size);
 141                         return -1;
 142                 }
 143
 144                 nr_entries = sec->len / entry->size;
 145
 146                 for (idx = 0; idx < nr_entries; idx++) {
 147                         alt = malloc(sizeof(*alt));
 148                         if (!alt) {
 149                                 WARN("malloc failed");
 150                                 return -1;
 151                         }
 152                         memset(alt, 0, sizeof(*alt));
 153
 154                         ret = get_alt_entry(elf, entry, sec, idx, alt);
 155                         if (ret)
 156                                 return ret;
 157
 158                         list_add_tail(&alt->list, alts);
 159                 }
 160         }
 161
 162         return 0;
 163 }