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