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 "builtin.h"
15#include "special.h"
16#include "warn.h"
17#include "arch_special.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};
26
27struct special_entry entries[] = {
28	{
29		.sec = ".altinstructions",
30		.group = true,
31		.size = ALT_ENTRY_SIZE,
32		.orig = ALT_ORIG_OFFSET,
33		.orig_len = ALT_ORIG_LEN_OFFSET,
34		.new = ALT_NEW_OFFSET,
35		.new_len = ALT_NEW_LEN_OFFSET,
36		.feature = ALT_FEATURE_OFFSET,
37	},
38	{
39		.sec = "__jump_table",
40		.jump_or_nop = true,
41		.size = JUMP_ENTRY_SIZE,
42		.orig = JUMP_ORIG_OFFSET,
43		.new = JUMP_NEW_OFFSET,
44	},
45	{
46		.sec = "__ex_table",
47		.size = EX_ENTRY_SIZE,
48		.orig = EX_ORIG_OFFSET,
49		.new = EX_NEW_OFFSET,
50	},
51	{},
52};
53
54void __weak arch_handle_alternative(unsigned short feature, struct special_alt *alt)
55{
56}
57
58static void reloc_to_sec_off(struct reloc *reloc, struct section **sec,
59			     unsigned long *off)
60{
61	*sec = reloc->sym->sec;
62	*off = reloc->sym->offset + reloc->addend;
63}
64
65static int get_alt_entry(struct elf *elf, struct special_entry *entry,
66			 struct section *sec, int idx,
67			 struct special_alt *alt)
68{
69	struct reloc *orig_reloc, *new_reloc;
70	unsigned long offset;
71
72	offset = idx * entry->size;
73
74	alt->group = entry->group;
75	alt->jump_or_nop = entry->jump_or_nop;
76
77	if (alt->group) {
78		alt->orig_len = *(unsigned char *)(sec->data->d_buf + offset +
79						   entry->orig_len);
80		alt->new_len = *(unsigned char *)(sec->data->d_buf + offset +
81						  entry->new_len);
82	}
83
84	if (entry->feature) {
85		unsigned short feature;
86
87		feature = *(unsigned short *)(sec->data->d_buf + offset +
88					      entry->feature);
89		arch_handle_alternative(feature, alt);
90	}
91
92	orig_reloc = find_reloc_by_dest(elf, sec, offset + entry->orig);
93	if (!orig_reloc) {
94		WARN_FUNC("can't find orig reloc", sec, offset + entry->orig);
95		return -1;
96	}
97
98	reloc_to_sec_off(orig_reloc, &alt->orig_sec, &alt->orig_off);
99
100	if (!entry->group || alt->new_len) {
101		new_reloc = find_reloc_by_dest(elf, sec, offset + entry->new);
102		if (!new_reloc) {
103			WARN_FUNC("can't find new reloc",
104				  sec, offset + entry->new);
105			return -1;
106		}
107
108		reloc_to_sec_off(new_reloc, &alt->new_sec, &alt->new_off);
109
110		/* _ASM_EXTABLE_EX hack */
111		if (alt->new_off >= 0x7ffffff0)
112			alt->new_off -= 0x7ffffff0;
113	}
114
115	return 0;
116}
117
118/*
119 * Read all the special sections and create a list of special_alt structs which
120 * describe all the alternate instructions which can be patched in or
121 * redirected to at runtime.
122 */
123int special_get_alts(struct elf *elf, struct list_head *alts)
124{
125	struct special_entry *entry;
126	struct section *sec;
127	unsigned int nr_entries;
128	struct special_alt *alt;
129	int idx, ret;
130
131	INIT_LIST_HEAD(alts);
132
133	for (entry = entries; entry->sec; entry++) {
134		sec = find_section_by_name(elf, entry->sec);
135		if (!sec)
136			continue;
137
138		if (sec->len % entry->size != 0) {
139			WARN("%s size not a multiple of %d",
140			     sec->name, entry->size);
141			return -1;
142		}
143
144		nr_entries = sec->len / entry->size;
145
146		for (idx = 0; idx < nr_entries; idx++) {
147			alt = malloc(sizeof(*alt));
148			if (!alt) {
149				WARN("malloc failed");
150				return -1;
151			}
152			memset(alt, 0, sizeof(*alt));
153
154			ret = get_alt_entry(elf, entry, sec, idx, alt);
155			if (ret > 0)
156				continue;
157			if (ret < 0)
158				return ret;
159
160			list_add_tail(&alt->list, alts);
161		}
162	}
163
164	return 0;
165}
166