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-rw-r--r--tools/objtool/builtin-check.c53
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)