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 
arch_handle_alternative(struct special_alt * alt)57 void __weak arch_handle_alternative(struct special_alt *alt)
58 {
59 }
60 
reloc_to_sec_off(struct reloc * reloc,struct section ** sec,unsigned long * off)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 
get_alt_entry(struct elf * elf,const struct special_entry * entry,struct section * sec,int idx,struct special_alt * alt)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 		ERROR_FUNC(sec, offset + entry->orig, "can't find orig reloc");
90 		return -1;
91 	}
92 
93 	reloc_to_sec_off(orig_reloc, &alt->orig_sec, &alt->orig_off);
94 
95 	arch_handle_alternative(alt);
96 
97 	if (!entry->group || alt->new_len) {
98 		new_reloc = find_reloc_by_dest(elf, sec, offset + entry->new);
99 		if (!new_reloc) {
100 			ERROR_FUNC(sec, offset + entry->new, "can't find new reloc");
101 			return -1;
102 		}
103 
104 		reloc_to_sec_off(new_reloc, &alt->new_sec, &alt->new_off);
105 
106 		/* _ASM_EXTABLE_EX hack */
107 		if (alt->new_off >= 0x7ffffff0)
108 			alt->new_off -= 0x7ffffff0;
109 	}
110 
111 	if (entry->key) {
112 		struct reloc *key_reloc;
113 
114 		key_reloc = find_reloc_by_dest(elf, sec, offset + entry->key);
115 		if (!key_reloc) {
116 			ERROR_FUNC(sec, offset + entry->key, "can't find key reloc");
117 			return -1;
118 		}
119 		alt->key_addend = reloc_addend(key_reloc);
120 	}
121 
122 	return 0;
123 }
124 
125 /*
126  * Read all the special sections and create a list of special_alt structs which
127  * describe all the alternate instructions which can be patched in or
128  * redirected to at runtime.
129  */
special_get_alts(struct elf * elf,struct list_head * alts)130 int special_get_alts(struct elf *elf, struct list_head *alts)
131 {
132 	const struct special_entry *entry;
133 	struct section *sec;
134 	unsigned int nr_entries;
135 	struct special_alt *alt;
136 	int idx, ret;
137 
138 	INIT_LIST_HEAD(alts);
139 
140 	for (entry = entries; entry->sec; entry++) {
141 		sec = find_section_by_name(elf, entry->sec);
142 		if (!sec)
143 			continue;
144 
145 		if (sec->sh.sh_size % entry->size != 0) {
146 			ERROR("%s size not a multiple of %d", sec->name, entry->size);
147 			return -1;
148 		}
149 
150 		nr_entries = sec->sh.sh_size / entry->size;
151 
152 		for (idx = 0; idx < nr_entries; idx++) {
153 			alt = malloc(sizeof(*alt));
154 			if (!alt) {
155 				ERROR_GLIBC("malloc failed");
156 				return -1;
157 			}
158 			memset(alt, 0, sizeof(*alt));
159 
160 			ret = get_alt_entry(elf, entry, sec, idx, alt);
161 			if (ret > 0)
162 				continue;
163 			if (ret < 0)
164 				return ret;
165 
166 			list_add_tail(&alt->list, alts);
167 		}
168 	}
169 
170 	return 0;
171 }
172