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 <fengguang.wu@intel.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/700fa029bbb0feff34f03ffc69d666a3c3b57a61.1460663532.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
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 list_head insn_list;
DECLARE_HASHTABLE(insn_hash, 16);
struct section *rodata, *whitelist;
+ bool ignore_unreachables;
};
const char *objname;
@@ -664,13 +665,40 @@
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 @@
{
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 @@
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 @@
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)