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 return true; 46 } 47 48 /* 49 * There are 3 basic jump table patterns: 50 * 51 * 1. jmpq *[rodata addr](,%reg,8) 52 * 53 * This is the most common case by far. It jumps to an address in a simple 54 * jump table which is stored in .rodata. 55 * 56 * 2. jmpq *[rodata addr](%rip) 57 * 58 * This is caused by a rare GCC quirk, currently only seen in three driver 59 * functions in the kernel, only with certain obscure non-distro configs. 60 * 61 * As part of an optimization, GCC makes a copy of an existing switch jump 62 * table, modifies it, and then hard-codes the jump (albeit with an indirect 63 * jump) to use a single entry in the table. The rest of the jump table and 64 * some of its jump targets remain as dead code. 65 * 66 * In such a case we can just crudely ignore all unreachable instruction 67 * warnings for the entire object file. Ideally we would just ignore them 68 * for the function, but that would require redesigning the code quite a 69 * bit. And honestly that's just not worth doing: unreachable instruction 70 * warnings are of questionable value anyway, and this is such a rare issue. 71 * 72 * 3. mov [rodata addr],%reg1 73 * ... some instructions ... 74 * jmpq *(%reg1,%reg2,8) 75 * 76 * This is a fairly uncommon pattern which is new for GCC 6. As of this 77 * writing, there are 11 occurrences of it in the allmodconfig kernel. 78 * 79 * As of GCC 7 there are quite a few more of these and the 'in between' code 80 * is significant. Esp. with KASAN enabled some of the code between the mov 81 * and jmpq uses .rodata itself, which can confuse things. 82 * 83 * TODO: Once we have DWARF CFI and smarter instruction decoding logic, 84 * ensure the same register is used in the mov and jump instructions. 85 * 86 * NOTE: RETPOLINE made it harder still to decode dynamic jumps. 87 */ 88 struct reloc *arch_find_switch_table(struct objtool_file *file, 89 struct instruction *insn) 90 { 91 struct reloc *text_reloc, *rodata_reloc; 92 struct section *table_sec; 93 unsigned long table_offset; 94 95 /* look for a relocation which references .rodata */ 96 text_reloc = find_reloc_by_dest_range(file->elf, insn->sec, 97 insn->offset, insn->len); 98 if (!text_reloc || text_reloc->sym->type != STT_SECTION || 99 !text_reloc->sym->sec->rodata) 100 return NULL; 101 102 table_offset = reloc_addend(text_reloc); 103 table_sec = text_reloc->sym->sec; 104 105 if (reloc_type(text_reloc) == R_X86_64_PC32) 106 table_offset += 4; 107 108 /* 109 * Make sure the .rodata address isn't associated with a 110 * symbol. GCC jump tables are anonymous data. 111 * 112 * Also support C jump tables which are in the same format as 113 * switch jump tables. For objtool to recognize them, they 114 * need to be placed in the C_JUMP_TABLE_SECTION section. They 115 * have symbols associated with them. 116 */ 117 if (find_symbol_containing(table_sec, table_offset) && 118 strcmp(table_sec->name, C_JUMP_TABLE_SECTION)) 119 return NULL; 120 121 /* 122 * Each table entry has a rela associated with it. The rela 123 * should reference text in the same function as the original 124 * instruction. 125 */ 126 rodata_reloc = find_reloc_by_dest(file->elf, table_sec, table_offset); 127 if (!rodata_reloc) 128 return NULL; 129 130 /* 131 * Use of RIP-relative switch jumps is quite rare, and 132 * indicates a rare GCC quirk/bug which can leave dead 133 * code behind. 134 */ 135 if (reloc_type(text_reloc) == R_X86_64_PC32) 136 file->ignore_unreachables = true; 137 138 return rodata_reloc; 139 } 140