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
7 #define X86_FEATURE_POPCNT (4 * 32 + 23)
8 #define X86_FEATURE_SMAP (9 * 32 + 20)
9
10 void arch_handle_alternative(unsigned short feature, struct special_alt *alt)
11 {
12 static struct special_alt *group, *prev;
13
14 /*
15 * Recompute orig_len for nested ALTERNATIVE()s.
16 */
17 if (group && group->orig_sec == alt->orig_sec &&
18 group->orig_off == alt->orig_off) {
19
20 struct special_alt *iter = group;
21 for (;;) {
22 unsigned int len = max(iter->orig_len, alt->orig_len);
23 iter->orig_len = alt->orig_len = len;
24
25 if (iter == prev)
26 break;
27
28 iter = list_next_entry(iter, list);
29 }
30
31 } else group = alt;
32
33 prev = alt;
34
35 switch (feature) {
36 case X86_FEATURE_SMAP:
37 /*
38 * If UACCESS validation is enabled; force that alternative;
39 * otherwise force it the other way.
40 *
41 * What we want to avoid is having both the original and the
42 * alternative code flow at the same time, in that case we can
43 * find paths that see the STAC but take the NOP instead of
44 * CLAC and the other way around.
45 */
46 if (opts.uaccess)
47 alt->skip_orig = true;
48 else
49 alt->skip_alt = true;
50 break;
51 case X86_FEATURE_POPCNT:
52 /*
53 * It has been requested that we don't validate the !POPCNT
54 * feature path which is a "very very small percentage of
55 * machines".
56 */
57 alt->skip_orig = true;
58 break;
59 default:
60 break;
61 }
62 }
63
64 bool arch_support_alt_relocation(struct special_alt *special_alt,
65 struct instruction *insn,
66 struct reloc *reloc)
67 {
68 return true;
69 }
70
71 /*
72 * There are 3 basic jump table patterns:
73 *
74 * 1. jmpq *[rodata addr](,%reg,8)
75 *
76 * This is the most common case by far. It jumps to an address in a simple
77 * jump table which is stored in .rodata.
78 *
79 * 2. jmpq *[rodata addr](%rip)
80 *
81 * This is caused by a rare GCC quirk, currently only seen in three driver
82 * functions in the kernel, only with certain obscure non-distro configs.
83 *
84 * As part of an optimization, GCC makes a copy of an existing switch jump
85 * table, modifies it, and then hard-codes the jump (albeit with an indirect
86 * jump) to use a single entry in the table. The rest of the jump table and
87 * some of its jump targets remain as dead code.
88 *
89 * In such a case we can just crudely ignore all unreachable instruction
90 * warnings for the entire object file. Ideally we would just ignore them
91 * for the function, but that would require redesigning the code quite a
92 * bit. And honestly that's just not worth doing: unreachable instruction
93 * warnings are of questionable value anyway, and this is such a rare issue.
94 *
95 * 3. mov [rodata addr],%reg1
96 * ... some instructions ...
97 * jmpq *(%reg1,%reg2,8)
98 *
99 * This is a fairly uncommon pattern which is new for GCC 6. As of this
100 * writing, there are 11 occurrences of it in the allmodconfig kernel.
101 *
102 * As of GCC 7 there are quite a few more of these and the 'in between' code
103 * is significant. Esp. with KASAN enabled some of the code between the mov
104 * and jmpq uses .rodata itself, which can confuse things.
105 *
106 * TODO: Once we have DWARF CFI and smarter instruction decoding logic,
107 * ensure the same register is used in the mov and jump instructions.
108 *
109 * NOTE: MITIGATION_RETPOLINE made it harder still to decode dynamic jumps.
110 */
111 struct reloc *arch_find_switch_table(struct objtool_file *file,
112 struct instruction *insn)
113 {
114 struct reloc *text_reloc, *rodata_reloc;
115 struct section *table_sec;
116 unsigned long table_offset;
117
118 /* look for a relocation which references .rodata */
119 text_reloc = find_reloc_by_dest_range(file->elf, insn->sec,
120 insn->offset, insn->len);
121 if (!text_reloc || text_reloc->sym->type != STT_SECTION ||
122 !text_reloc->sym->sec->rodata)
123 return NULL;
124
125 table_offset = reloc_addend(text_reloc);
126 table_sec = text_reloc->sym->sec;
127
128 if (reloc_type(text_reloc) == R_X86_64_PC32)
129 table_offset += 4;
130
131 /*
132 * Make sure the .rodata address isn't associated with a
133 * symbol. GCC jump tables are anonymous data.
134 *
135 * Also support C jump tables which are in the same format as
136 * switch jump tables. For objtool to recognize them, they
137 * need to be placed in the C_JUMP_TABLE_SECTION section. They
138 * have symbols associated with them.
139 */
140 if (find_symbol_containing(table_sec, table_offset) &&
141 strcmp(table_sec->name, C_JUMP_TABLE_SECTION))
142 return NULL;
143
144 /*
145 * Each table entry has a rela associated with it. The rela
146 * should reference text in the same function as the original
147 * instruction.
148 */
149 rodata_reloc = find_reloc_by_dest(file->elf, table_sec, table_offset);
150 if (!rodata_reloc)
151 return NULL;
152
153 /*
154 * Use of RIP-relative switch jumps is quite rare, and
155 * indicates a rare GCC quirk/bug which can leave dead
156 * code behind.
157 */
158 if (reloc_type(text_reloc) == R_X86_64_PC32)
159 file->ignore_unreachables = true;
160
161 return rodata_reloc;
162 }
163
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