1// SPDX-License-Identifier: GPL-2.0-or-later
2#include <string.h>
3
4#include <objtool/special.h>
5#include <objtool/builtin.h>
6#include <objtool/warn.h>
7#include <asm/cpufeatures.h>
8
9/* cpu feature name array generated from cpufeatures.h */
10#include "cpu-feature-names.c"
11
12void arch_handle_alternative(struct special_alt *alt)
13{
14 static struct special_alt *group, *prev;
15
16 /*
17 * Recompute orig_len for nested ALTERNATIVE()s.
18 */
19 if (group && group->orig_sec == alt->orig_sec &&
20 group->orig_off == alt->orig_off) {
21
22 struct special_alt *iter = group;
23 for (;;) {
24 unsigned int len = max(iter->orig_len, alt->orig_len);
25 iter->orig_len = alt->orig_len = len;
26
27 if (iter == prev)
28 break;
29
30 iter = list_next_entry(iter, list);
31 }
32
33 } else group = alt;
34
35 prev = alt;
36}
37
38bool arch_support_alt_relocation(struct special_alt *special_alt,
39 struct instruction *insn,
40 struct reloc *reloc)
41{
42 return true;
43}
44
45/*
46 * There are 3 basic jump table patterns:
47 *
48 * 1. jmpq *[rodata addr](,%reg,8)
49 *
50 * This is the most common case by far. It jumps to an address in a simple
51 * jump table which is stored in .rodata.
52 *
53 * 2. jmpq *[rodata addr](%rip)
54 *
55 * This is caused by a rare GCC quirk, currently only seen in three driver
56 * functions in the kernel, only with certain obscure non-distro configs.
57 *
58 * As part of an optimization, GCC makes a copy of an existing switch jump
59 * table, modifies it, and then hard-codes the jump (albeit with an indirect
60 * jump) to use a single entry in the table. The rest of the jump table and
61 * some of its jump targets remain as dead code.
62 *
63 * In such a case we can just crudely ignore all unreachable instruction
64 * warnings for the entire object file. Ideally we would just ignore them
65 * for the function, but that would require redesigning the code quite a
66 * bit. And honestly that's just not worth doing: unreachable instruction
67 * warnings are of questionable value anyway, and this is such a rare issue.
68 *
69 * 3. mov [rodata addr],%reg1
70 * ... some instructions ...
71 * jmpq *(%reg1,%reg2,8)
72 *
73 * This is a fairly uncommon pattern which is new for GCC 6. As of this
74 * writing, there are 11 occurrences of it in the allmodconfig kernel.
75 *
76 * As of GCC 7 there are quite a few more of these and the 'in between' code
77 * is significant. Esp. with KASAN enabled some of the code between the mov
78 * and jmpq uses .rodata itself, which can confuse things.
79 *
80 * TODO: Once we have DWARF CFI and smarter instruction decoding logic,
81 * ensure the same register is used in the mov and jump instructions.
82 *
83 * NOTE: MITIGATION_RETPOLINE made it harder still to decode dynamic jumps.
84 */
85struct reloc *arch_find_switch_table(struct objtool_file *file,
86 struct instruction *insn,
87 unsigned long *table_size)
88{
89 struct reloc *text_reloc, *rodata_reloc;
90 struct section *table_sec;
91 unsigned long table_offset;
92
93 /* look for a relocation which references .rodata */
94 text_reloc = find_reloc_by_dest_range(elf: file->elf, sec: insn->sec,
95 offset: insn->offset, len: insn->len);
96 if (!text_reloc || !is_sec_sym(sym: text_reloc->sym) ||
97 !text_reloc->sym->sec->rodata)
98 return NULL;
99
100 table_offset = reloc_addend(reloc: text_reloc);
101 table_sec = text_reloc->sym->sec;
102
103 if (reloc_type(reloc: text_reloc) == R_X86_64_PC32)
104 table_offset += 4;
105
106 /*
107 * Make sure the .rodata address isn't associated with a
108 * symbol. GCC jump tables are anonymous data.
109 *
110 * Also support C jump tables which are in the same format as
111 * switch jump tables. For objtool to recognize them, they
112 * need to be placed in the C_JUMP_TABLE_SECTION section. They
113 * have symbols associated with them.
114 */
115 if (find_symbol_containing(sec: table_sec, offset: table_offset) &&
116 strcmp(s1: table_sec->name, C_JUMP_TABLE_SECTION))
117 return NULL;
118
119 /*
120 * Each table entry has a rela associated with it. The rela
121 * should reference text in the same function as the original
122 * instruction.
123 */
124 rodata_reloc = find_reloc_by_dest(elf: file->elf, sec: table_sec, offset: table_offset);
125 if (!rodata_reloc)
126 return NULL;
127
128 /*
129 * Use of RIP-relative switch jumps is quite rare, and
130 * indicates a rare GCC quirk/bug which can leave dead
131 * code behind.
132 */
133 if (!file->ignore_unreachables && reloc_type(reloc: text_reloc) == R_X86_64_PC32) {
134 WARN_INSN(insn, "ignoring unreachables due to jump table quirk");
135 file->ignore_unreachables = true;
136 }
137
138 *table_size = 0;
139 return rodata_reloc;
140}
141
142const char *arch_cpu_feature_name(int feature_number)
143{
144 return (feature_number < ARRAY_SIZE(cpu_feature_names)) ?
145 cpu_feature_names[feature_number] : NULL;
146}
147

source code of linux/tools/objtool/arch/x86/special.c