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