1/*
  2 * Copyright (C) 2015 Josh Poimboeuf <jpoimboe@redhat.com>
  3 *
  4 * This program is free software; you can redistribute it and/or
  5 * modify it under the terms of the GNU General Public License
  6 * as published by the Free Software Foundation; either version 2
  7 * of the License, or (at your option) any later version.
  8 *
  9 * This program is distributed in the hope that it will be useful,
 10 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 11 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 12 * GNU General Public License for more details.
 13 *
 14 * You should have received a copy of the GNU General Public License
 15 * along with this program; if not, see <http://www.gnu.org/licenses/>.
 16 */
 17
 18/*
 19 * This file reads all the special sections which have alternate instructions
 20 * which can be patched in or redirected to at runtime.
 21 */
 22
 23#include <stdlib.h>
 24#include <string.h>
 25
 26#include "special.h"
 27#include "warn.h"
 28
 29#define EX_ENTRY_SIZE		12
 30#define EX_ORIG_OFFSET		0
 31#define EX_NEW_OFFSET		4
 32
 33#define JUMP_ENTRY_SIZE		24
 34#define JUMP_ORIG_OFFSET	0
 35#define JUMP_NEW_OFFSET		8
 36
 37#define ALT_ENTRY_SIZE		13
 38#define ALT_ORIG_OFFSET		0
 39#define ALT_NEW_OFFSET		4
 40#define ALT_FEATURE_OFFSET	8
 41#define ALT_ORIG_LEN_OFFSET	10
 42#define ALT_NEW_LEN_OFFSET	11
 43
 44#define X86_FEATURE_POPCNT (4*32+23)
 45
 46struct special_entry {
 47	const char *sec;
 48	bool group, jump_or_nop;
 49	unsigned char size, orig, new;
 50	unsigned char orig_len, new_len; /* group only */
 51	unsigned char feature; /* ALTERNATIVE macro CPU feature */
 52};
 53
 54struct special_entry entries[] = {
 55	{
 56		.sec = ".altinstructions",
 57		.group = true,
 58		.size = ALT_ENTRY_SIZE,
 59		.orig = ALT_ORIG_OFFSET,
 60		.orig_len = ALT_ORIG_LEN_OFFSET,
 61		.new = ALT_NEW_OFFSET,
 62		.new_len = ALT_NEW_LEN_OFFSET,
 63		.feature = ALT_FEATURE_OFFSET,
 64	},
 65	{
 66		.sec = "__jump_table",
 67		.jump_or_nop = true,
 68		.size = JUMP_ENTRY_SIZE,
 69		.orig = JUMP_ORIG_OFFSET,
 70		.new = JUMP_NEW_OFFSET,
 71	},
 72	{
 73		.sec = "__ex_table",
 74		.size = EX_ENTRY_SIZE,
 75		.orig = EX_ORIG_OFFSET,
 76		.new = EX_NEW_OFFSET,
 77	},
 78	{},
 79};
 80
 81static int get_alt_entry(struct elf *elf, struct special_entry *entry,
 82			 struct section *sec, int idx,
 83			 struct special_alt *alt)
 84{
 85	struct rela *orig_rela, *new_rela;
 86	unsigned long offset;
 87
 88	offset = idx * entry->size;
 89
 90	alt->group = entry->group;
 91	alt->jump_or_nop = entry->jump_or_nop;
 92
 93	if (alt->group) {
 94		alt->orig_len = *(unsigned char *)(sec->data->d_buf + offset +
 95						   entry->orig_len);
 96		alt->new_len = *(unsigned char *)(sec->data->d_buf + offset +
 97						  entry->new_len);
 98	}
 99
100	if (entry->feature) {
101		unsigned short feature;
102
103		feature = *(unsigned short *)(sec->data->d_buf + offset +
104					      entry->feature);
105
106		/*
107		 * It has been requested that we don't validate the !POPCNT
108		 * feature path which is a "very very small percentage of
109		 * machines".
110		 */
111		if (feature == X86_FEATURE_POPCNT)
112			alt->skip_orig = true;
113	}
114
115	orig_rela = find_rela_by_dest(sec, offset + entry->orig);
116	if (!orig_rela) {
117		WARN_FUNC("can't find orig rela", sec, offset + entry->orig);
118		return -1;
119	}
120	if (orig_rela->sym->type != STT_SECTION) {
121		WARN_FUNC("don't know how to handle non-section rela symbol %s",
122			   sec, offset + entry->orig, orig_rela->sym->name);
123		return -1;
124	}
125
126	alt->orig_sec = orig_rela->sym->sec;
127	alt->orig_off = orig_rela->addend;
128
129	if (!entry->group || alt->new_len) {
130		new_rela = find_rela_by_dest(sec, offset + entry->new);
131		if (!new_rela) {
132			WARN_FUNC("can't find new rela",
133				  sec, offset + entry->new);
134			return -1;
135		}
136
137		alt->new_sec = new_rela->sym->sec;
138		alt->new_off = (unsigned int)new_rela->addend;
139
140		/* _ASM_EXTABLE_EX hack */
141		if (alt->new_off >= 0x7ffffff0)
142			alt->new_off -= 0x7ffffff0;
143	}
144
145	return 0;
146}
147
148/*
149 * Read all the special sections and create a list of special_alt structs which
150 * describe all the alternate instructions which can be patched in or
151 * redirected to at runtime.
152 */
153int special_get_alts(struct elf *elf, struct list_head *alts)
154{
155	struct special_entry *entry;
156	struct section *sec;
157	unsigned int nr_entries;
158	struct special_alt *alt;
159	int idx, ret;
160
161	INIT_LIST_HEAD(alts);
162
163	for (entry = entries; entry->sec; entry++) {
164		sec = find_section_by_name(elf, entry->sec);
165		if (!sec)
166			continue;
167
168		if (sec->len % entry->size != 0) {
169			WARN("%s size not a multiple of %d",
170			     sec->name, entry->size);
171			return -1;
172		}
173
174		nr_entries = sec->len / entry->size;
175
176		for (idx = 0; idx < nr_entries; idx++) {
177			alt = malloc(sizeof(*alt));
178			if (!alt) {
179				WARN("malloc failed");
180				return -1;
181			}
182			memset(alt, 0, sizeof(*alt));
183
184			ret = get_alt_entry(elf, entry, sec, idx, alt);
185			if (ret)
186				return ret;
187
188			list_add_tail(&alt->list, alts);
189		}
190	}
191
192	return 0;
193}