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1// SPDX-License-Identifier: GPL-2.0-or-later2#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 the42 * alternative code flow at the same time, in that case we can43 * find paths that see the STAC but take the NOP instead of44 * CLAC and the other way around.45 */46 if (opts.uaccess)47 alt->skip_orig = true;48 else49 alt->skip_alt = true;50 break;51 case X86_FEATURE_POPCNT:52 /*53 * It has been requested that we don't validate the !POPCNT54 * feature path which is a "very very small percentage of55 * 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 simple77 * 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 driver82 * 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 jump85 * table, modifies it, and then hard-codes the jump (albeit with an indirect86 * jump) to use a single entry in the table. The rest of the jump table and87 * some of its jump targets remain as dead code.88 *89 * In such a case we can just crudely ignore all unreachable instruction90 * warnings for the entire object file. Ideally we would just ignore them91 * for the function, but that would require redesigning the code quite a92 * bit. And honestly that's just not worth doing: unreachable instruction93 * warnings are of questionable value anyway, and this is such a rare issue.94 *95 * 3. mov [rodata addr],%reg196 * ... some instructions ...97 * jmpq *(%reg1,%reg2,8)98 *99 * This is a fairly uncommon pattern which is new for GCC 6. As of this100 * 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' code103 * is significant. Esp. with KASAN enabled some of the code between the mov104 * 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 a133 * symbol. GCC jump tables are anonymous data.134 *135 * Also support C jump tables which are in the same format as136 * switch jump tables. For objtool to recognize them, they137 * need to be placed in the C_JUMP_TABLE_SECTION section. They138 * 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 rela146 * should reference text in the same function as the original147 * 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, and155 * indicates a rare GCC quirk/bug which can leave dead156 * code behind.157 */158 if (reloc_type(text_reloc) == R_X86_64_PC32)159 file->ignore_unreachables = true;160 161 return rodata_reloc;162}163