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	static struct special_alt *group, *prev;
 13
 14	/*
 15	 * Recompute orig_len for nested ALTERNATIVE()s.
 16	 */
 17	if (group && group->orig_sec == alt->orig_sec &&
 18	             group->orig_off == alt->orig_off) {
 19
 20		struct special_alt *iter = group;
 21		for (;;) {
 22			unsigned int len = max(iter->orig_len, alt->orig_len);
 23			iter->orig_len = alt->orig_len = len;
 24
 25			if (iter == prev)
 26				break;
 27
 28			iter = list_next_entry(iter, list);
 29		}
 30
 31	} else group = alt;
 32
 33	prev = alt;
 34
 35	switch (feature) {
 36	case X86_FEATURE_SMAP:
 37		/*
 38		 * If UACCESS validation is enabled; force that alternative;
 39		 * otherwise force it the other way.
 40		 *
 41		 * What we want to avoid is having both the original and the
 42		 * alternative code flow at the same time, in that case we can
 43		 * find paths that see the STAC but take the NOP instead of
 44		 * CLAC and the other way around.
 45		 */
 46		if (opts.uaccess)
 47			alt->skip_orig = true;
 48		else
 49			alt->skip_alt = true;
 50		break;
 51	case X86_FEATURE_POPCNT:
 52		/*
 53		 * It has been requested that we don't validate the !POPCNT
 54		 * feature path which is a "very very small percentage of
 55		 * machines".
 56		 */
 57		alt->skip_orig = true;
 58		break;
 59	default:
 60		break;
 61	}
 62}
 63
 64bool arch_support_alt_relocation(struct special_alt *special_alt,
 65				 struct instruction *insn,
 66				 struct reloc *reloc)
 67{
 68	return true;
 69}
 70
 71/*
 72 * There are 3 basic jump table patterns:
 73 *
 74 * 1. jmpq *[rodata addr](,%reg,8)
 75 *
 76 *    This is the most common case by far.  It jumps to an address in a simple
 77 *    jump table which is stored in .rodata.
 78 *
 79 * 2. jmpq *[rodata addr](%rip)
 80 *
 81 *    This is caused by a rare GCC quirk, currently only seen in three driver
 82 *    functions in the kernel, only with certain obscure non-distro configs.
 83 *
 84 *    As part of an optimization, GCC makes a copy of an existing switch jump
 85 *    table, modifies it, and then hard-codes the jump (albeit with an indirect
 86 *    jump) to use a single entry in the table.  The rest of the jump table and
 87 *    some of its jump targets remain as dead code.
 88 *
 89 *    In such a case we can just crudely ignore all unreachable instruction
 90 *    warnings for the entire object file.  Ideally we would just ignore them
 91 *    for the function, but that would require redesigning the code quite a
 92 *    bit.  And honestly that's just not worth doing: unreachable instruction
 93 *    warnings are of questionable value anyway, and this is such a rare issue.
 94 *
 95 * 3. mov [rodata addr],%reg1
 96 *    ... some instructions ...
 97 *    jmpq *(%reg1,%reg2,8)
 98 *
 99 *    This is a fairly uncommon pattern which is new for GCC 6.  As of this
100 *    writing, there are 11 occurrences of it in the allmodconfig kernel.
101 *
102 *    As of GCC 7 there are quite a few more of these and the 'in between' code
103 *    is significant. Esp. with KASAN enabled some of the code between the mov
104 *    and jmpq uses .rodata itself, which can confuse things.
105 *
106 *    TODO: Once we have DWARF CFI and smarter instruction decoding logic,
107 *    ensure the same register is used in the mov and jump instructions.
108 *
109 *    NOTE: MITIGATION_RETPOLINE made it harder still to decode dynamic jumps.
110 */
111struct reloc *arch_find_switch_table(struct objtool_file *file,
112				    struct instruction *insn)
113{
114	struct reloc  *text_reloc, *rodata_reloc;
115	struct section *table_sec;
116	unsigned long table_offset;
117
118	/* look for a relocation which references .rodata */
119	text_reloc = find_reloc_by_dest_range(file->elf, insn->sec,
120					      insn->offset, insn->len);
121	if (!text_reloc || text_reloc->sym->type != STT_SECTION ||
122	    !text_reloc->sym->sec->rodata)
123		return NULL;
124
125	table_offset = reloc_addend(text_reloc);
126	table_sec = text_reloc->sym->sec;
127
128	if (reloc_type(text_reloc) == R_X86_64_PC32)
129		table_offset += 4;
130
131	/*
132	 * Make sure the .rodata address isn't associated with a
133	 * symbol.  GCC jump tables are anonymous data.
134	 *
135	 * Also support C jump tables which are in the same format as
136	 * switch jump tables.  For objtool to recognize them, they
137	 * need to be placed in the C_JUMP_TABLE_SECTION section.  They
138	 * have symbols associated with them.
139	 */
140	if (find_symbol_containing(table_sec, table_offset) &&
141	    strcmp(table_sec->name, C_JUMP_TABLE_SECTION))
142		return NULL;
143
144	/*
145	 * Each table entry has a rela associated with it.  The rela
146	 * should reference text in the same function as the original
147	 * instruction.
148	 */
149	rodata_reloc = find_reloc_by_dest(file->elf, table_sec, table_offset);
150	if (!rodata_reloc)
151		return NULL;
152
153	/*
154	 * Use of RIP-relative switch jumps is quite rare, and
155	 * indicates a rare GCC quirk/bug which can leave dead
156	 * code behind.
157	 */
158	if (reloc_type(text_reloc) == R_X86_64_PC32)
159		file->ignore_unreachables = true;
160
161	return rodata_reloc;
162}