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
 20struct 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
 29static 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
 57void __weak arch_handle_alternative(unsigned short feature, struct special_alt *alt)
 58{
 59}
 60
 61static 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
 68static 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	if (entry->feature) {
 88		unsigned short feature;
 89
 90		feature = bswap_if_needed(elf,
 91					  *(unsigned short *)(sec->data->d_buf +
 92							      offset +
 93							      entry->feature));
 94		arch_handle_alternative(feature, alt);
 95	}
 96
 97	orig_reloc = find_reloc_by_dest(elf, sec, offset + entry->orig);
 98	if (!orig_reloc) {
 99		WARN_FUNC("can't find orig reloc", sec, offset + entry->orig);
100		return -1;
101	}
102
103	reloc_to_sec_off(orig_reloc, &alt->orig_sec, &alt->orig_off);
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 */
140int 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}