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