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 unsigned char key; /* jump_label key */ 27 }; 28 29 struct special_entry entries[] = { 30 { 31 .sec = ".altinstructions", 32 .group = true, 33 .size = ALT_ENTRY_SIZE, 34 .orig = ALT_ORIG_OFFSET, 35 .orig_len = ALT_ORIG_LEN_OFFSET, 36 .new = ALT_NEW_OFFSET, 37 .new_len = ALT_NEW_LEN_OFFSET, 38 .feature = ALT_FEATURE_OFFSET, 39 }, 40 { 41 .sec = "__jump_table", 42 .jump_or_nop = true, 43 .size = JUMP_ENTRY_SIZE, 44 .orig = JUMP_ORIG_OFFSET, 45 .new = JUMP_NEW_OFFSET, 46 .key = JUMP_KEY_OFFSET, 47 }, 48 { 49 .sec = "__ex_table", 50 .size = EX_ENTRY_SIZE, 51 .orig = EX_ORIG_OFFSET, 52 .new = EX_NEW_OFFSET, 53 }, 54 {}, 55 }; 56 57 void __weak arch_handle_alternative(unsigned short feature, struct special_alt *alt) 58 { 59 } 60 61 static bool reloc2sec_off(struct reloc *reloc, struct section **sec, unsigned long *off) 62 { 63 switch (reloc->sym->type) { 64 case STT_FUNC: 65 *sec = reloc->sym->sec; 66 *off = reloc->sym->offset + reloc->addend; 67 return true; 68 69 case STT_SECTION: 70 *sec = reloc->sym->sec; 71 *off = reloc->addend; 72 return true; 73 74 default: 75 return false; 76 } 77 } 78 79 static int get_alt_entry(struct elf *elf, struct special_entry *entry, 80 struct section *sec, int idx, 81 struct special_alt *alt) 82 { 83 struct reloc *orig_reloc, *new_reloc; 84 unsigned long offset; 85 86 offset = idx * entry->size; 87 88 alt->group = entry->group; 89 alt->jump_or_nop = entry->jump_or_nop; 90 91 if (alt->group) { 92 alt->orig_len = *(unsigned char *)(sec->data->d_buf + offset + 93 entry->orig_len); 94 alt->new_len = *(unsigned char *)(sec->data->d_buf + offset + 95 entry->new_len); 96 } 97 98 if (entry->feature) { 99 unsigned short feature; 100 101 feature = bswap_if_needed(*(unsigned short *)(sec->data->d_buf + 102 offset + 103 entry->feature)); 104 arch_handle_alternative(feature, alt); 105 } 106 107 orig_reloc = find_reloc_by_dest(elf, sec, offset + entry->orig); 108 if (!orig_reloc) { 109 WARN_FUNC("can't find orig reloc", sec, offset + entry->orig); 110 return -1; 111 } 112 if (!reloc2sec_off(orig_reloc, &alt->orig_sec, &alt->orig_off)) { 113 WARN_FUNC("don't know how to handle reloc symbol type %d: %s", 114 sec, offset + entry->orig, 115 orig_reloc->sym->type, 116 orig_reloc->sym->name); 117 return -1; 118 } 119 120 if (!entry->group || alt->new_len) { 121 new_reloc = find_reloc_by_dest(elf, sec, offset + entry->new); 122 if (!new_reloc) { 123 WARN_FUNC("can't find new reloc", 124 sec, offset + entry->new); 125 return -1; 126 } 127 128 /* 129 * Skip retpoline .altinstr_replacement... we already rewrite the 130 * instructions for retpolines anyway, see arch_is_retpoline() 131 * usage in add_{call,jump}_destinations(). 132 */ 133 if (arch_is_retpoline(new_reloc->sym)) 134 return 1; 135 136 if (!reloc2sec_off(new_reloc, &alt->new_sec, &alt->new_off)) { 137 WARN_FUNC("don't know how to handle reloc symbol type %d: %s", 138 sec, offset + entry->new, 139 new_reloc->sym->type, 140 new_reloc->sym->name); 141 return -1; 142 } 143 144 /* _ASM_EXTABLE_EX hack */ 145 if (alt->new_off >= 0x7ffffff0) 146 alt->new_off -= 0x7ffffff0; 147 } 148 149 if (entry->key) { 150 struct reloc *key_reloc; 151 152 key_reloc = find_reloc_by_dest(elf, sec, offset + entry->key); 153 if (!key_reloc) { 154 WARN_FUNC("can't find key reloc", 155 sec, offset + entry->key); 156 return -1; 157 } 158 alt->key_addend = key_reloc->addend; 159 } 160 161 return 0; 162 } 163 164 /* 165 * Read all the special sections and create a list of special_alt structs which 166 * describe all the alternate instructions which can be patched in or 167 * redirected to at runtime. 168 */ 169 int special_get_alts(struct elf *elf, struct list_head *alts) 170 { 171 struct special_entry *entry; 172 struct section *sec; 173 unsigned int nr_entries; 174 struct special_alt *alt; 175 int idx, ret; 176 177 INIT_LIST_HEAD(alts); 178 179 for (entry = entries; entry->sec; entry++) { 180 sec = find_section_by_name(elf, entry->sec); 181 if (!sec) 182 continue; 183 184 if (sec->len % entry->size != 0) { 185 WARN("%s size not a multiple of %d", 186 sec->name, entry->size); 187 return -1; 188 } 189 190 nr_entries = sec->len / entry->size; 191 192 for (idx = 0; idx < nr_entries; idx++) { 193 alt = malloc(sizeof(*alt)); 194 if (!alt) { 195 WARN("malloc failed"); 196 return -1; 197 } 198 memset(alt, 0, sizeof(*alt)); 199 200 ret = get_alt_entry(elf, entry, sec, idx, alt); 201 if (ret > 0) 202 continue; 203 if (ret < 0) 204 return ret; 205 206 list_add_tail(&alt->list, alts); 207 } 208 } 209 210 return 0; 211 } 212