[PATCH v2] objtool: ignore unreachable trap after call to noreturn functions

From: Ilie Halip
Date: Sat Sep 19 2020 - 02:41:50 EST

With CONFIG_UBSAN_TRAP enabled, the compiler may insert a trap instruction
after a call to a noreturn function. In this case, objtool warns that the
ud2 instruction is unreachable.

This is a behavior seen with clang, from the oldest version capable of
building the mainline x64_64 kernel (9.0), to the latest experimental
version (12.0).

objtool silences similar warnings (trap after dead end instructions), so
so expand that check to include dead end functions.

Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Cc: Rong Chen <rong.a.chen@xxxxxxxxx>
Cc: Marco Elver <elver@xxxxxxxxxx>
Cc: Philip Li <philip.li@xxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: kasan-dev@xxxxxxxxxxxxxxxx
Cc: x86@xxxxxxxxxx
Cc: clang-built-linux@xxxxxxxxxxxxxxxx
BugLink: https://github.com/ClangBuiltLinux/linux/issues/1148
Link: https://lore.kernel.org/lkml/CAKwvOdmptEpi8fiOyWUo=AiZJiX+Z+VHJOM2buLPrWsMTwLnyw@xxxxxxxxxxxxxx
Suggested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Tested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Reported-by: kbuild test robot <lkp@xxxxxxxxx>
Signed-off-by: Ilie Halip <ilie.halip@xxxxxxxxx>

Changed in v2:
- added a mention that this is a clang issue across all versions
- added Nick's Reviewed-by, Tested-by
- added Reported-by

tools/objtool/check.c | 10 +++++++---
1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index e034a8f24f46..eddf8bf16b05 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -2612,9 +2612,10 @@ static bool is_ubsan_insn(struct instruction *insn)

-static bool ignore_unreachable_insn(struct instruction *insn)
+static bool ignore_unreachable_insn(struct objtool_file *file, struct instruction *insn)
int i;
+ struct instruction *prev_insn;

if (insn->ignore || insn->type == INSN_NOP)
return true;
@@ -2639,8 +2640,11 @@ static bool ignore_unreachable_insn(struct instruction *insn)
* __builtin_unreachable(). The BUG() macro has an unreachable() after
* the UD2, which causes GCC's undefined trap logic to emit another UD2
* (or occasionally a JMP to UD2).
+ * CONFIG_UBSAN_TRAP may also insert a UD2 after calling a __noreturn
+ * function.
- if (list_prev_entry(insn, list)->dead_end &&
+ prev_insn = list_prev_entry(insn, list);
+ if ((prev_insn->dead_end || dead_end_function(file, prev_insn->call_dest)) &&
(insn->type == INSN_BUG ||
insn->jump_dest && insn->jump_dest->type == INSN_BUG)))
@@ -2767,7 +2771,7 @@ static int validate_reachable_instructions(struct objtool_file *file)
return 0;

for_each_insn(file, insn) {
- if (insn->visited || ignore_unreachable_insn(insn))
+ if (insn->visited || ignore_unreachable_insn(file, insn))

WARN_FUNC("unreachable instruction", insn->sec, insn->offset);