From 7e578441a4a3bba2a79426ca0f709c801210d08e Mon Sep 17 00:00:00 2001 From: Josh Poimboeuf Date: Thu, 14 Apr 2016 14:52:24 -0500 Subject: 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%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 Reported-by: kbuild test robot Signed-off-by: Josh Poimboeuf Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Link: http://lkml.kernel.org/r/700fa029bbb0feff34f03ffc69d666a3c3b57a61.1460663532.git.jpoimboe@redhat.com Signed-off-by: Ingo Molnar --- tools/objtool/builtin-check.c | 53 +++++++++++++++++++++++++++++++++---------- 1 file changed, 41 insertions(+), 12 deletions(-) (limited to 'tools') 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) -- cgit v1.2.3 From b1547d3101e74e809b9790174b27f1080747b009 Mon Sep 17 00:00:00 2001 From: Josh Poimboeuf Date: Fri, 15 Apr 2016 09:17:10 -0500 Subject: objtool: Detect falling through to the next function There are several cases in compiled C code where a function may not return at the end, and may instead fall through to the next function. That may indicate a bug in the code, or a gcc bug, or even an objtool bug. But in each case, objtool reports an unhelpful warning, something like: drivers/scsi/qla2xxx/qla_attr.o: warning: objtool: qla2x00_get_fc_host_stats()+0x0: duplicate frame pointer save drivers/scsi/qla2xxx/qla_attr.o: warning: objtool: qla2x00_get_fc_host_stats()+0x0: frame pointer state mismatch Detect this situation and print a more useful error message: drivers/scsi/qla2xxx/qla_attr.o: warning: objtool: qla2x00_get_host_fabric_name() falls through to next function qla2x00_get_starget_node_name() Also add some information about this warning and its potential causes to the documentation. Reported-by: kbuild test robot Signed-off-by: Josh Poimboeuf Cc: Alexander Shishkin Cc: Andy Lutomirski Cc: Arnaldo Carvalho de Melo Cc: Borislav Petkov Cc: Brian Gerst Cc: Denys Vlasenko Cc: H. Peter Anvin Cc: Jiri Olsa Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Link: http://lkml.kernel.org/r/caa4ec6c687931db805e692d4e4bf06cd87d33e6.1460729697.git.jpoimboe@redhat.com Signed-off-by: Ingo Molnar --- tools/objtool/Documentation/stack-validation.txt | 38 +++++++++++++++----- tools/objtool/builtin-check.c | 46 ++++++++++++++++-------- 2 files changed, 61 insertions(+), 23 deletions(-) (limited to 'tools') diff --git a/tools/objtool/Documentation/stack-validation.txt b/tools/objtool/Documentation/stack-validation.txt index 5a95896105bc..55a60d331f47 100644 --- a/tools/objtool/Documentation/stack-validation.txt +++ b/tools/objtool/Documentation/stack-validation.txt @@ -299,18 +299,38 @@ they mean, and suggestions for how to fix them. Errors in .c files ------------------ -If you're getting an objtool error in a compiled .c file, chances are -the file uses an asm() statement which has a "call" instruction. An -asm() statement with a call instruction must declare the use of the -stack pointer in its output operand. For example, on x86_64: +1. c_file.o: warning: objtool: funcA() falls through to next function funcB() - register void *__sp asm("rsp"); - asm volatile("call func" : "+r" (__sp)); + This means that funcA() doesn't end with a return instruction or an + unconditional jump, and that objtool has determined that the function + can fall through into the next function. There could be different + reasons for this: -Otherwise the stack frame may not get created before the call. + 1) funcA()'s last instruction is a call to a "noreturn" function like + panic(). In this case the noreturn function needs to be added to + objtool's hard-coded global_noreturns array. Feel free to bug the + objtool maintainer, or you can submit a patch. -Another possible cause for errors in C code is if the Makefile removes --fno-omit-frame-pointer or adds -fomit-frame-pointer to the gcc options. + 2) funcA() uses the unreachable() annotation in a section of code + that is actually reachable. + + 3) If funcA() calls an inline function, the object code for funcA() + might be corrupt due to a gcc bug. For more details, see: + https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70646 + +2. If you're getting any other objtool error in a compiled .c file, it + may be because the file uses an asm() statement which has a "call" + instruction. An asm() statement with a call instruction must declare + the use of the stack pointer in its output operand. For example, on + x86_64: + + register void *__sp asm("rsp"); + asm volatile("call func" : "+r" (__sp)); + + Otherwise the stack frame may not get created before the call. + +3. Another possible cause for errors in C code is if the Makefile removes + -fno-omit-frame-pointer or adds -fomit-frame-pointer to the gcc options. Also see the above section for .S file errors for more information what the individual error messages mean. diff --git a/tools/objtool/builtin-check.c b/tools/objtool/builtin-check.c index 157a0f96d64d..e8a1e69eb92c 100644 --- a/tools/objtool/builtin-check.c +++ b/tools/objtool/builtin-check.c @@ -54,6 +54,7 @@ struct instruction { struct symbol *call_dest; struct instruction *jump_dest; struct list_head alts; + struct symbol *func; }; struct alternative { @@ -66,7 +67,7 @@ struct objtool_file { struct list_head insn_list; DECLARE_HASHTABLE(insn_hash, 16); struct section *rodata, *whitelist; - bool ignore_unreachables; + bool ignore_unreachables, c_file; }; const char *objname; @@ -229,7 +230,7 @@ static int __dead_end_function(struct objtool_file *file, struct symbol *func, } } - if (insn->type == INSN_JUMP_DYNAMIC) + if (insn->type == INSN_JUMP_DYNAMIC && list_empty(&insn->alts)) /* sibling call */ return 0; } @@ -249,6 +250,7 @@ static int dead_end_function(struct objtool_file *file, struct symbol *func) static int decode_instructions(struct objtool_file *file) { struct section *sec; + struct symbol *func; unsigned long offset; struct instruction *insn; int ret; @@ -282,6 +284,21 @@ static int decode_instructions(struct objtool_file *file) hash_add(file->insn_hash, &insn->hash, insn->offset); list_add_tail(&insn->list, &file->insn_list); } + + list_for_each_entry(func, &sec->symbol_list, list) { + if (func->type != STT_FUNC) + continue; + + if (!find_insn(file, sec, func->offset)) { + WARN("%s(): can't find starting instruction", + func->name); + return -1; + } + + func_for_each_insn(file, func, insn) + if (!insn->func) + insn->func = func; + } } return 0; @@ -824,6 +841,7 @@ static int validate_branch(struct objtool_file *file, struct alternative *alt; struct instruction *insn; struct section *sec; + struct symbol *func = NULL; unsigned char state; int ret; @@ -838,6 +856,16 @@ static int validate_branch(struct objtool_file *file, } while (1) { + if (file->c_file && insn->func) { + if (func && func != insn->func) { + WARN("%s() falls through to next function %s()", + func->name, insn->func->name); + return 1; + } + + func = insn->func; + } + if (insn->visited) { if (frame_state(insn->state) != frame_state(state)) { WARN_FUNC("frame pointer state mismatch", @@ -848,13 +876,6 @@ static int validate_branch(struct objtool_file *file, return 0; } - /* - * Catch a rare case where a noreturn function falls through to - * the next function. - */ - if (is_fentry_call(insn) && (state & STATE_FENTRY)) - return 0; - insn->visited = true; insn->state = state; @@ -1060,12 +1081,8 @@ static int validate_functions(struct objtool_file *file) continue; insn = find_insn(file, sec, func->offset); - if (!insn) { - WARN("%s(): can't find starting instruction", - func->name); - warnings++; + if (!insn) continue; - } ret = validate_branch(file, insn, 0); warnings += ret; @@ -1162,6 +1179,7 @@ int cmd_check(int argc, const char **argv) 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; + file.c_file = find_section_by_name(file.elf, ".comment"); ret = decode_sections(&file); if (ret < 0) -- cgit v1.2.3