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 #include <objtool/warn.h> 7 8 void arch_handle_alternative(struct special_alt *alt) 9 { 10 static struct special_alt *group, *prev; 11 12 /* 13 * Recompute orig_len for nested ALTERNATIVE()s. 14 */ 15 if (group && group->orig_sec == alt->orig_sec && 16 group->orig_off == alt->orig_off) { 17 18 struct special_alt *iter = group; 19 for (;;) { 20 unsigned int len = max(iter->orig_len, alt->orig_len); 21 iter->orig_len = alt->orig_len = len; 22 23 if (iter == prev) 24 break; 25 26 iter = list_next_entry(iter, list); 27 } 28 29 } else group = alt; 30 31 prev = alt; 32 } 33 34 bool arch_support_alt_relocation(struct special_alt *special_alt, 35 struct instruction *insn, 36 struct reloc *reloc) 37 { 38 return true; 39 } 40 41 /* 42 * There are 3 basic jump table patterns: 43 * 44 * 1. jmpq *[rodata addr](,%reg,8) 45 * 46 * This is the most common case by far. It jumps to an address in a simple 47 * jump table which is stored in .rodata. 48 * 49 * 2. jmpq *[rodata addr](%rip) 50 * 51 * This is caused by a rare GCC quirk, currently only seen in three driver 52 * functions in the kernel, only with certain obscure non-distro configs. 53 * 54 * As part of an optimization, GCC makes a copy of an existing switch jump 55 * table, modifies it, and then hard-codes the jump (albeit with an indirect 56 * jump) to use a single entry in the table. The rest of the jump table and 57 * some of its jump targets remain as dead code. 58 * 59 * In such a case we can just crudely ignore all unreachable instruction 60 * warnings for the entire object file. Ideally we would just ignore them 61 * for the function, but that would require redesigning the code quite a 62 * bit. And honestly that's just not worth doing: unreachable instruction 63 * warnings are of questionable value anyway, and this is such a rare issue. 64 * 65 * 3. mov [rodata addr],%reg1 66 * ... some instructions ... 67 * jmpq *(%reg1,%reg2,8) 68 * 69 * This is a fairly uncommon pattern which is new for GCC 6. As of this 70 * writing, there are 11 occurrences of it in the allmodconfig kernel. 71 * 72 * As of GCC 7 there are quite a few more of these and the 'in between' code 73 * is significant. Esp. with KASAN enabled some of the code between the mov 74 * and jmpq uses .rodata itself, which can confuse things. 75 * 76 * TODO: Once we have DWARF CFI and smarter instruction decoding logic, 77 * ensure the same register is used in the mov and jump instructions. 78 * 79 * NOTE: MITIGATION_RETPOLINE made it harder still to decode dynamic jumps. 80 */ 81 struct reloc *arch_find_switch_table(struct objtool_file *file, 82 struct instruction *insn, 83 unsigned long *table_size) 84 { 85 struct reloc *text_reloc, *rodata_reloc; 86 struct section *table_sec; 87 unsigned long table_offset; 88 89 /* look for a relocation which references .rodata */ 90 text_reloc = find_reloc_by_dest_range(file->elf, insn->sec, 91 insn->offset, insn->len); 92 if (!text_reloc || text_reloc->sym->type != STT_SECTION || 93 !text_reloc->sym->sec->rodata) 94 return NULL; 95 96 table_offset = reloc_addend(text_reloc); 97 table_sec = text_reloc->sym->sec; 98 99 if (reloc_type(text_reloc) == R_X86_64_PC32) 100 table_offset += 4; 101 102 /* 103 * Make sure the .rodata address isn't associated with a 104 * symbol. GCC jump tables are anonymous data. 105 * 106 * Also support C jump tables which are in the same format as 107 * switch jump tables. For objtool to recognize them, they 108 * need to be placed in the C_JUMP_TABLE_SECTION section. They 109 * have symbols associated with them. 110 */ 111 if (find_symbol_containing(table_sec, table_offset) && 112 strcmp(table_sec->name, C_JUMP_TABLE_SECTION)) 113 return NULL; 114 115 /* 116 * Each table entry has a rela associated with it. The rela 117 * should reference text in the same function as the original 118 * instruction. 119 */ 120 rodata_reloc = find_reloc_by_dest(file->elf, table_sec, table_offset); 121 if (!rodata_reloc) 122 return NULL; 123 124 /* 125 * Use of RIP-relative switch jumps is quite rare, and 126 * indicates a rare GCC quirk/bug which can leave dead 127 * code behind. 128 */ 129 if (!file->ignore_unreachables && reloc_type(text_reloc) == R_X86_64_PC32) { 130 WARN_INSN(insn, "ignoring unreachables due to jump table quirk"); 131 file->ignore_unreachables = true; 132 } 133 134 *table_size = 0; 135 return rodata_reloc; 136 } 137