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
arch_handle_alternative(unsigned short feature,struct special_alt * alt)10 void arch_handle_alternative(unsigned short feature, struct special_alt *alt)
11 {
12 switch (feature) {
13 case X86_FEATURE_SMAP:
14 /*
15 * If UACCESS validation is enabled; force that alternative;
16 * otherwise force it the other way.
17 *
18 * What we want to avoid is having both the original and the
19 * alternative code flow at the same time, in that case we can
20 * find paths that see the STAC but take the NOP instead of
21 * CLAC and the other way around.
22 */
23 if (opts.uaccess)
24 alt->skip_orig = true;
25 else
26 alt->skip_alt = true;
27 break;
28 case X86_FEATURE_POPCNT:
29 /*
30 * It has been requested that we don't validate the !POPCNT
31 * feature path which is a "very very small percentage of
32 * machines".
33 */
34 alt->skip_orig = true;
35 break;
36 default:
37 break;
38 }
39 }
40
arch_support_alt_relocation(struct special_alt * special_alt,struct instruction * insn,struct reloc * reloc)41 bool arch_support_alt_relocation(struct special_alt *special_alt,
42 struct instruction *insn,
43 struct reloc *reloc)
44 {
45 /*
46 * The x86 alternatives code adjusts the offsets only when it
47 * encounters a branch instruction at the very beginning of the
48 * replacement group.
49 */
50 return insn->offset == special_alt->new_off &&
51 (insn->type == INSN_CALL || is_jump(insn));
52 }
53
54 /*
55 * There are 3 basic jump table patterns:
56 *
57 * 1. jmpq *[rodata addr](,%reg,8)
58 *
59 * This is the most common case by far. It jumps to an address in a simple
60 * jump table which is stored in .rodata.
61 *
62 * 2. jmpq *[rodata addr](%rip)
63 *
64 * This is caused by a rare GCC quirk, currently only seen in three driver
65 * functions in the kernel, only with certain obscure non-distro configs.
66 *
67 * As part of an optimization, GCC makes a copy of an existing switch jump
68 * table, modifies it, and then hard-codes the jump (albeit with an indirect
69 * jump) to use a single entry in the table. The rest of the jump table and
70 * some of its jump targets remain as dead code.
71 *
72 * In such a case we can just crudely ignore all unreachable instruction
73 * warnings for the entire object file. Ideally we would just ignore them
74 * for the function, but that would require redesigning the code quite a
75 * bit. And honestly that's just not worth doing: unreachable instruction
76 * warnings are of questionable value anyway, and this is such a rare issue.
77 *
78 * 3. mov [rodata addr],%reg1
79 * ... some instructions ...
80 * jmpq *(%reg1,%reg2,8)
81 *
82 * This is a fairly uncommon pattern which is new for GCC 6. As of this
83 * writing, there are 11 occurrences of it in the allmodconfig kernel.
84 *
85 * As of GCC 7 there are quite a few more of these and the 'in between' code
86 * is significant. Esp. with KASAN enabled some of the code between the mov
87 * and jmpq uses .rodata itself, which can confuse things.
88 *
89 * TODO: Once we have DWARF CFI and smarter instruction decoding logic,
90 * ensure the same register is used in the mov and jump instructions.
91 *
92 * NOTE: RETPOLINE made it harder still to decode dynamic jumps.
93 */
arch_find_switch_table(struct objtool_file * file,struct instruction * insn)94 struct reloc *arch_find_switch_table(struct objtool_file *file,
95 struct instruction *insn)
96 {
97 struct reloc *text_reloc, *rodata_reloc;
98 struct section *table_sec;
99 unsigned long table_offset;
100
101 /* look for a relocation which references .rodata */
102 text_reloc = find_reloc_by_dest_range(file->elf, insn->sec,
103 insn->offset, insn->len);
104 if (!text_reloc || text_reloc->sym->type != STT_SECTION ||
105 !text_reloc->sym->sec->rodata)
106 return NULL;
107
108 table_offset = text_reloc->addend;
109 table_sec = text_reloc->sym->sec;
110
111 if (text_reloc->type == R_X86_64_PC32)
112 table_offset += 4;
113
114 /*
115 * Make sure the .rodata address isn't associated with a
116 * symbol. GCC jump tables are anonymous data.
117 *
118 * Also support C jump tables which are in the same format as
119 * switch jump tables. For objtool to recognize them, they
120 * need to be placed in the C_JUMP_TABLE_SECTION section. They
121 * have symbols associated with them.
122 */
123 if (find_symbol_containing(table_sec, table_offset) &&
124 strcmp(table_sec->name, C_JUMP_TABLE_SECTION))
125 return NULL;
126
127 /*
128 * Each table entry has a rela associated with it. The rela
129 * should reference text in the same function as the original
130 * instruction.
131 */
132 rodata_reloc = find_reloc_by_dest(file->elf, table_sec, table_offset);
133 if (!rodata_reloc)
134 return NULL;
135
136 /*
137 * Use of RIP-relative switch jumps is quite rare, and
138 * indicates a rare GCC quirk/bug which can leave dead
139 * code behind.
140 */
141 if (text_reloc->type == R_X86_64_PC32)
142 file->ignore_unreachables = true;
143
144 return rodata_reloc;
145 }
146