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 int get_alt_entry(struct elf *elf, struct special_entry *entry, 62 struct section *sec, int idx, 63 struct special_alt *alt) 64 { 65 struct reloc *orig_reloc, *new_reloc; 66 unsigned long offset; 67 68 offset = idx * entry->size; 69 70 alt->group = entry->group; 71 alt->jump_or_nop = entry->jump_or_nop; 72 73 if (alt->group) { 74 alt->orig_len = *(unsigned char *)(sec->data->d_buf + offset + 75 entry->orig_len); 76 alt->new_len = *(unsigned char *)(sec->data->d_buf + offset + 77 entry->new_len); 78 } 79 80 if (entry->feature) { 81 unsigned short feature; 82 83 feature = bswap_if_needed(*(unsigned short *)(sec->data->d_buf + 84 offset + 85 entry->feature)); 86 arch_handle_alternative(feature, alt); 87 } 88 89 orig_reloc = find_reloc_by_dest(elf, sec, offset + entry->orig); 90 if (!orig_reloc) { 91 WARN_FUNC("can't find orig reloc", sec, offset + entry->orig); 92 return -1; 93 } 94 if (orig_reloc->sym->type != STT_SECTION) { 95 WARN_FUNC("don't know how to handle non-section reloc symbol %s", 96 sec, offset + entry->orig, orig_reloc->sym->name); 97 return -1; 98 } 99 100 alt->orig_sec = orig_reloc->sym->sec; 101 alt->orig_off = orig_reloc->addend; 102 103 if (!entry->group || alt->new_len) { 104 new_reloc = find_reloc_by_dest(elf, sec, offset + entry->new); 105 if (!new_reloc) { 106 WARN_FUNC("can't find new reloc", 107 sec, offset + entry->new); 108 return -1; 109 } 110 111 /* 112 * Skip retpoline .altinstr_replacement... we already rewrite the 113 * instructions for retpolines anyway, see arch_is_retpoline() 114 * usage in add_{call,jump}_destinations(). 115 */ 116 if (arch_is_retpoline(new_reloc->sym)) 117 return 1; 118 119 alt->new_sec = new_reloc->sym->sec; 120 alt->new_off = (unsigned int)new_reloc->addend; 121 122 /* _ASM_EXTABLE_EX hack */ 123 if (alt->new_off >= 0x7ffffff0) 124 alt->new_off -= 0x7ffffff0; 125 } 126 127 if (entry->key) { 128 struct reloc *key_reloc; 129 130 key_reloc = find_reloc_by_dest(elf, sec, offset + entry->key); 131 if (!key_reloc) { 132 WARN_FUNC("can't find key reloc", 133 sec, offset + entry->key); 134 return -1; 135 } 136 alt->key_addend = key_reloc->addend; 137 } 138 139 return 0; 140 } 141 142 /* 143 * Read all the special sections and create a list of special_alt structs which 144 * describe all the alternate instructions which can be patched in or 145 * redirected to at runtime. 146 */ 147 int special_get_alts(struct elf *elf, struct list_head *alts) 148 { 149 struct special_entry *entry; 150 struct section *sec; 151 unsigned int nr_entries; 152 struct special_alt *alt; 153 int idx, ret; 154 155 INIT_LIST_HEAD(alts); 156 157 for (entry = entries; entry->sec; entry++) { 158 sec = find_section_by_name(elf, entry->sec); 159 if (!sec) 160 continue; 161 162 if (sec->len % entry->size != 0) { 163 WARN("%s size not a multiple of %d", 164 sec->name, entry->size); 165 return -1; 166 } 167 168 nr_entries = sec->len / entry->size; 169 170 for (idx = 0; idx < nr_entries; idx++) { 171 alt = malloc(sizeof(*alt)); 172 if (!alt) { 173 WARN("malloc failed"); 174 return -1; 175 } 176 memset(alt, 0, sizeof(*alt)); 177 178 ret = get_alt_entry(elf, entry, sec, idx, alt); 179 if (ret > 0) 180 continue; 181 if (ret < 0) 182 return ret; 183 184 list_add_tail(&alt->list, alts); 185 } 186 } 187 188 return 0; 189 } 190