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	/*
 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 */
 94struct 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}