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 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 41 bool 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 */ 94 struct 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 } 146