xref: /openbmc/linux/tools/objtool/arch/x86/special.c (revision 0696b6e3)
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 	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: RETPOLINE made it harder still to decode dynamic jumps.
87  */
arch_find_switch_table(struct objtool_file * file,struct instruction * insn)88 struct 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