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1// SPDX-License-Identifier: GPL-2.0-or-later
2/*
3 * Copyright (C) 2015 Josh Poimboeuf <[email protected]>
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
19struct 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 unsigned char key; /* jump_label key */
26};
27
28static const 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 .key = JUMP_KEY_OFFSET,
46 },
47 {
48 .sec = "__ex_table",
49 .size = EX_ENTRY_SIZE,
50 .orig = EX_ORIG_OFFSET,
51 .new = EX_NEW_OFFSET,
52 },
53 {},
54};
55
56void __weak arch_handle_alternative(struct special_alt *alt)
57{
58}
59
60static void reloc_to_sec_off(struct reloc *reloc, struct section **sec,
61 unsigned long *off)
62{
63 *sec = reloc->sym->sec;
64 *off = reloc->sym->offset + reloc_addend(reloc);
65}
66
67static int get_alt_entry(struct elf *elf, const struct special_entry *entry,
68 struct section *sec, int idx,
69 struct special_alt *alt)
70{
71 struct reloc *orig_reloc, *new_reloc;
72 unsigned long offset;
73
74 offset = idx * entry->size;
75
76 alt->group = entry->group;
77 alt->jump_or_nop = entry->jump_or_nop;
78
79 if (alt->group) {
80 alt->orig_len = *(unsigned char *)(sec->data->d_buf + offset +
81 entry->orig_len);
82 alt->new_len = *(unsigned char *)(sec->data->d_buf + offset +
83 entry->new_len);
84 alt->feature = *(unsigned int *)(sec->data->d_buf + offset +
85 entry->feature);
86 }
87
88 orig_reloc = find_reloc_by_dest(elf, sec, offset + entry->orig);
89 if (!orig_reloc) {
90 ERROR_FUNC(sec, offset + entry->orig, "can't find orig reloc");
91 return -1;
92 }
93
94 reloc_to_sec_off(orig_reloc, &alt->orig_sec, &alt->orig_off);
95
96 arch_handle_alternative(alt);
97
98 if (!entry->group || alt->new_len) {
99 new_reloc = find_reloc_by_dest(elf, sec, offset + entry->new);
100 if (!new_reloc) {
101 ERROR_FUNC(sec, offset + entry->new, "can't find new reloc");
102 return -1;
103 }
104
105 reloc_to_sec_off(new_reloc, &alt->new_sec, &alt->new_off);
106
107 /* _ASM_EXTABLE_EX hack */
108 if (alt->new_off >= 0x7ffffff0)
109 alt->new_off -= 0x7ffffff0;
110 }
111
112 if (entry->key) {
113 struct reloc *key_reloc;
114
115 key_reloc = find_reloc_by_dest(elf, sec, offset + entry->key);
116 if (!key_reloc) {
117 ERROR_FUNC(sec, offset + entry->key, "can't find key reloc");
118 return -1;
119 }
120 alt->key_addend = reloc_addend(key_reloc);
121 }
122
123 return 0;
124}
125
126/*
127 * Read all the special sections and create a list of special_alt structs which
128 * describe all the alternate instructions which can be patched in or
129 * redirected to at runtime.
130 */
131int special_get_alts(struct elf *elf, struct list_head *alts)
132{
133 const struct special_entry *entry;
134 struct section *sec;
135 unsigned int nr_entries;
136 struct special_alt *alt;
137 int idx;
138
139 INIT_LIST_HEAD(alts);
140
141 for (entry = entries; entry->sec; entry++) {
142 sec = find_section_by_name(elf, entry->sec);
143 if (!sec)
144 continue;
145
146 if (sec_size(sec) % entry->size != 0) {
147 ERROR("%s size not a multiple of %d", sec->name, entry->size);
148 return -1;
149 }
150
151 nr_entries = sec_size(sec) / entry->size;
152
153 for (idx = 0; idx < nr_entries; idx++) {
154 alt = malloc(sizeof(*alt));
155 if (!alt) {
156 ERROR_GLIBC("malloc failed");
157 return -1;
158 }
159 memset(alt, 0, sizeof(*alt));
160
161 if (get_alt_entry(elf, entry, sec, idx, alt))
162 return -1;
163
164 list_add_tail(&alt->list, alts);
165 }
166 }
167
168 return 0;
169}