Commit-ID: 7e578441a4a3bba2a79426ca0f709c801210d08e Gitweb: http://git.kernel.org/tip/7e578441a4a3bba2a79426ca0f709c801210d08e Author: Josh Poimboeuf <[email protected]> AuthorDate: Thu, 14 Apr 2016 14:52:24 -0500 Committer: Ingo Molnar <[email protected]> CommitDate: Fri, 15 Apr 2016 11:42:13 +0200
objtool: Add workaround for GCC switch jump table bug GCC has a rare quirk, currently only seen in three driver functions in the kernel, and only with certain obscure non-distro configs, which can cause objtool to produce "unreachable instruction" false positive warnings. 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 a very rare issue. kbuild reports: https://lkml.kernel.org/r/201603231906.lwcvupxm%[email protected] https://lkml.kernel.org/r/201603271114.k9i45biy%[email protected] https://lkml.kernel.org/r/201603291058.zuj6ben1%[email protected] GCC bug: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70604 Reported-by: kbuild test robot <[email protected]> Signed-off-by: Josh Poimboeuf <[email protected]> Cc: Linus Torvalds <[email protected]> Cc: Peter Zijlstra <[email protected]> Cc: Thomas Gleixner <[email protected]> Link: http://lkml.kernel.org/r/700fa029bbb0feff34f03ffc69d666a3c3b57a61.1460663532.git.jpoim...@redhat.com Signed-off-by: Ingo Molnar <[email protected]> --- tools/objtool/builtin-check.c | 53 +++++++++++++++++++++++++++++++++---------- 1 file changed, 41 insertions(+), 12 deletions(-) diff --git a/tools/objtool/builtin-check.c b/tools/objtool/builtin-check.c index 7515cb2..157a0f9 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%[email protected] + * - https://lkml.kernel.org/r/201603271114.k9i45biy%[email protected] + * - https://lkml.kernel.org/r/201603291058.zuj6ben1%[email protected] + * + * 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)
