special.c

  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#include <asm/cpufeatures.h>
  8
  9/* cpu feature name array generated from cpufeatures.h */
 10#include "cpu-feature-names.c"
 11
 12void arch_handle_alternative(struct special_alt *alt)
 13{
 14	static struct special_alt *group, *prev;
 15
 16	/*
 17	 * Recompute orig_len for nested ALTERNATIVE()s.
 18	 */
 19	if (group && group->orig_sec == alt->orig_sec &&
 20	             group->orig_off == alt->orig_off) {
 21
 22		struct special_alt *iter = group;
 23		for (;;) {
 24			unsigned int len = max(iter->orig_len, alt->orig_len);
 25			iter->orig_len = alt->orig_len = len;
 26
 27			if (iter == prev)
 28				break;
 29
 30			iter = list_next_entry(iter, list);
 31		}
 32
 33	} else group = alt;
 34
 35	prev = alt;
 36}
 37
 38bool arch_support_alt_relocation(struct special_alt *special_alt,
 39				 struct instruction *insn,
 40				 struct reloc *reloc)
 41{
 42	return true;
 43}
 44
 45/*
 46 * There are 3 basic jump table patterns:
 47 *
 48 * 1. jmpq *[rodata addr](,%reg,8)
 49 *
 50 *    This is the most common case by far.  It jumps to an address in a simple
 51 *    jump table which is stored in .rodata.
 52 *
 53 * 2. jmpq *[rodata addr](%rip)
 54 *
 55 *    This is caused by a rare GCC quirk, currently only seen in three driver
 56 *    functions in the kernel, only with certain obscure non-distro configs.
 57 *
 58 *    As part of an optimization, GCC makes a copy of an existing switch jump
 59 *    table, modifies it, and then hard-codes the jump (albeit with an indirect
 60 *    jump) to use a single entry in the table.  The rest of the jump table and
 61 *    some of its jump targets remain as dead code.
 62 *
 63 *    In such a case we can just crudely ignore all unreachable instruction
 64 *    warnings for the entire object file.  Ideally we would just ignore them
 65 *    for the function, but that would require redesigning the code quite a
 66 *    bit.  And honestly that's just not worth doing: unreachable instruction
 67 *    warnings are of questionable value anyway, and this is such a rare issue.
 68 *
 69 * 3. mov [rodata addr],%reg1
 70 *    ... some instructions ...
 71 *    jmpq *(%reg1,%reg2,8)
 72 *
 73 *    This is a fairly uncommon pattern which is new for GCC 6.  As of this
 74 *    writing, there are 11 occurrences of it in the allmodconfig kernel.
 75 *
 76 *    As of GCC 7 there are quite a few more of these and the 'in between' code
 77 *    is significant. Esp. with KASAN enabled some of the code between the mov
 78 *    and jmpq uses .rodata itself, which can confuse things.
 79 *
 80 *    TODO: Once we have DWARF CFI and smarter instruction decoding logic,
 81 *    ensure the same register is used in the mov and jump instructions.
 82 *
 83 *    NOTE: MITIGATION_RETPOLINE made it harder still to decode dynamic jumps.
 84 */
 85struct reloc *arch_find_switch_table(struct objtool_file *file,
 86				     struct instruction *insn,
 87				     unsigned long *table_size)
 88{
 89	struct reloc  *text_reloc, *rodata_reloc;
 90	struct section *table_sec;
 91	unsigned long table_offset;
 92
 93	/* look for a relocation which references .rodata */
 94	text_reloc = find_reloc_by_dest_range(file->elf, insn->sec,
 95					      insn->offset, insn->len);
 96	if (!text_reloc || !is_sec_sym(text_reloc->sym) ||
 97	    !text_reloc->sym->sec->rodata)
 98		return NULL;
 99
100	table_offset = reloc_addend(text_reloc);
101	table_sec = text_reloc->sym->sec;
102
103	if (reloc_type(text_reloc) == R_X86_64_PC32)
104		table_offset += 4;
105
106	/*
107	 * Make sure the .rodata address isn't associated with a
108	 * symbol.  GCC jump tables are anonymous data.
109	 *
110	 * Also support C jump tables which are in the same format as
111	 * switch jump tables.  For objtool to recognize them, they
112	 * need to be placed in the C_JUMP_TABLE_SECTION section.  They
113	 * have symbols associated with them.
114	 */
115	if (find_symbol_containing(table_sec, table_offset) &&
116	    strcmp(table_sec->name, C_JUMP_TABLE_SECTION))
117		return NULL;
118
119	/*
120	 * Each table entry has a rela associated with it.  The rela
121	 * should reference text in the same function as the original
122	 * instruction.
123	 */
124	rodata_reloc = find_reloc_by_dest(file->elf, table_sec, table_offset);
125	if (!rodata_reloc)
126		return NULL;
127
128	/*
129	 * Use of RIP-relative switch jumps is quite rare, and
130	 * indicates a rare GCC quirk/bug which can leave dead
131	 * code behind.
132	 */
133	if (!file->ignore_unreachables && reloc_type(text_reloc) == R_X86_64_PC32) {
134		WARN_INSN(insn, "ignoring unreachables due to jump table quirk");
135		file->ignore_unreachables = true;
136	}
137
138	*table_size = 0;
139	return rodata_reloc;
140}
141
142const char *arch_cpu_feature_name(int feature_number)
143{
144	return (feature_number < ARRAY_SIZE(cpu_feature_names)) ?
145		cpu_feature_names[feature_number] : NULL;
146}