diff options
| -rw-r--r-- | tools/objtool/builtin-check.c | 53 |
1 files changed, 41 insertions, 12 deletions
diff --git a/tools/objtool/builtin-check.c b/tools/objtool/builtin-check.c index 7515cb2e879a..157a0f96d64d 100644 --- a/tools/objtool/builtin-check.c +++ b/tools/objtool/builtin-check.c @@ -66,6 +66,7 @@ struct objtool_file { struct list_head insn_list; DECLARE_HASHTABLE(insn_hash, 16); struct section *rodata, *whitelist; + bool ignore_unreachables; }; const char *objname; @@ -664,13 +665,40 @@ static int add_func_switch_tables(struct objtool_file *file, text_rela->addend); /* - * TODO: Document where this is needed, or get rid of it. - * * rare case: jmpq *[addr](%rip) + * + * This check is for a rare gcc quirk, currently only seen in + * three driver functions in the kernel, only with certain + * obscure non-distro configs. + * + * As part of an optimization, gcc makes a copy of an existing + * switch jump table, modifies it, and then hard-codes the jump + * (albeit with an indirect jump) to use a single entry in the + * table. The rest of the jump table and some of its jump + * targets remain as dead code. + * + * In such a case we can just crudely ignore all unreachable + * instruction warnings for the entire object file. Ideally we + * would just ignore them for the function, but that would + * require redesigning the code quite a bit. And honestly + * that's just not worth doing: unreachable instruction + * warnings are of questionable value anyway, and this is such + * a rare issue. + * + * kbuild reports: + * - https://lkml.kernel.org/r/201603231906.LWcVUpxm%25fengguang.wu@intel.com + * - https://lkml.kernel.org/r/201603271114.K9i45biy%25fengguang.wu@intel.com + * - https://lkml.kernel.org/r/201603291058.zuJ6ben1%25fengguang.wu@intel.com + * + * gcc bug: + * - https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70604 */ - if (!rodata_rela) + if (!rodata_rela) { rodata_rela = find_rela_by_dest(file->rodata, text_rela->addend + 4); + if (rodata_rela) + file->ignore_unreachables = true; + } if (!rodata_rela) continue; @@ -732,9 +760,6 @@ static int decode_sections(struct objtool_file *file) { int ret; - file->whitelist = find_section_by_name(file->elf, "__func_stack_frame_non_standard"); - file->rodata = find_section_by_name(file->elf, ".rodata"); - ret = decode_instructions(file); if (ret) return ret; @@ -1056,13 +1081,14 @@ static int validate_functions(struct objtool_file *file) if (insn->visited) continue; - if (!ignore_unreachable_insn(func, insn) && - !warnings) { - WARN_FUNC("function has unreachable instruction", insn->sec, insn->offset); - warnings++; - } - insn->visited = true; + + if (file->ignore_unreachables || warnings || + ignore_unreachable_insn(func, insn)) + continue; + + WARN_FUNC("function has unreachable instruction", insn->sec, insn->offset); + warnings++; } } } @@ -1133,6 +1159,9 @@ int cmd_check(int argc, const char **argv) INIT_LIST_HEAD(&file.insn_list); hash_init(file.insn_hash); + file.whitelist = find_section_by_name(file.elf, "__func_stack_frame_non_standard"); + file.rodata = find_section_by_name(file.elf, ".rodata"); + file.ignore_unreachables = false; ret = decode_sections(&file); if (ret < 0) |