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
10void 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
41bool arch_support_alt_relocation(struct special_alt *special_alt,
42				 struct instruction *insn,
43				 struct reloc *reloc)
44{
45	return true;
46}
47
48/*
49 * There are 3 basic jump table patterns:
50 *
51 * 1. jmpq *[rodata addr](,%reg,8)
52 *
53 *    This is the most common case by far.  It jumps to an address in a simple
54 *    jump table which is stored in .rodata.
55 *
56 * 2. jmpq *[rodata addr](%rip)
57 *
58 *    This is caused by a rare GCC quirk, currently only seen in three driver
59 *    functions in the kernel, only with certain obscure non-distro configs.
60 *
61 *    As part of an optimization, GCC makes a copy of an existing switch jump
62 *    table, modifies it, and then hard-codes the jump (albeit with an indirect
63 *    jump) to use a single entry in the table.  The rest of the jump table and
64 *    some of its jump targets remain as dead code.
65 *
66 *    In such a case we can just crudely ignore all unreachable instruction
67 *    warnings for the entire object file.  Ideally we would just ignore them
68 *    for the function, but that would require redesigning the code quite a
69 *    bit.  And honestly that's just not worth doing: unreachable instruction
70 *    warnings are of questionable value anyway, and this is such a rare issue.
71 *
72 * 3. mov [rodata addr],%reg1
73 *    ... some instructions ...
74 *    jmpq *(%reg1,%reg2,8)
75 *
76 *    This is a fairly uncommon pattern which is new for GCC 6.  As of this
77 *    writing, there are 11 occurrences of it in the allmodconfig kernel.
78 *
79 *    As of GCC 7 there are quite a few more of these and the 'in between' code
80 *    is significant. Esp. with KASAN enabled some of the code between the mov
81 *    and jmpq uses .rodata itself, which can confuse things.
82 *
83 *    TODO: Once we have DWARF CFI and smarter instruction decoding logic,
84 *    ensure the same register is used in the mov and jump instructions.
85 *
86 *    NOTE: MITIGATION_RETPOLINE made it harder still to decode dynamic jumps.
87 */
88struct reloc *arch_find_switch_table(struct objtool_file *file,
89				    struct instruction *insn)
90{
91	struct reloc  *text_reloc, *rodata_reloc;
92	struct section *table_sec;
93	unsigned long table_offset;
94
95	/* look for a relocation which references .rodata */
96	text_reloc = find_reloc_by_dest_range(file->elf, insn->sec,
97					      insn->offset, insn->len);
98	if (!text_reloc || text_reloc->sym->type != STT_SECTION ||
99	    !text_reloc->sym->sec->rodata)
100		return NULL;
101
102	table_offset = reloc_addend(text_reloc);
103	table_sec = text_reloc->sym->sec;
104
105	if (reloc_type(text_reloc) == R_X86_64_PC32)
106		table_offset += 4;
107
108	/*
109	 * Make sure the .rodata address isn't associated with a
110	 * symbol.  GCC jump tables are anonymous data.
111	 *
112	 * Also support C jump tables which are in the same format as
113	 * switch jump tables.  For objtool to recognize them, they
114	 * need to be placed in the C_JUMP_TABLE_SECTION section.  They
115	 * have symbols associated with them.
116	 */
117	if (find_symbol_containing(table_sec, table_offset) &&
118	    strcmp(table_sec->name, C_JUMP_TABLE_SECTION))
119		return NULL;
120
121	/*
122	 * Each table entry has a rela associated with it.  The rela
123	 * should reference text in the same function as the original
124	 * instruction.
125	 */
126	rodata_reloc = find_reloc_by_dest(file->elf, table_sec, table_offset);
127	if (!rodata_reloc)
128		return NULL;
129
130	/*
131	 * Use of RIP-relative switch jumps is quite rare, and
132	 * indicates a rare GCC quirk/bug which can leave dead
133	 * code behind.
134	 */
135	if (reloc_type(text_reloc) == R_X86_64_PC32)
136		file->ignore_unreachables = true;
137
138	return rodata_reloc;
139}
140