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