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 <ndesaulni...@google.com> Cc: Rong Chen <rong.a.c...@intel.com> Cc: Marco Elver <el...@google.com> Cc: Philip Li <philip...@intel.com> Cc: Borislav Petkov <b...@alien8.de> Cc: kasan-...@googlegroups.com Cc: x...@kernel.org Cc: clang-built-li...@googlegroups.com BugLink: https://github.com/ClangBuiltLinux/linux/issues/1148 Link: https://lore.kernel.org/lkml/CAKwvOdmptEpi8fiOyWUo=aizjix+z+vhjom2bulprwsmtwl...@mail.gmail.com Suggested-by: Nick Desaulniers <ndesaulni...@google.com> Reviewed-by: Nick Desaulniers <ndesaulni...@google.com> Tested-by: Nick Desaulniers <ndesaulni...@google.com> Reported-by: kbuild test robot <l...@intel.com> Signed-off-by: Ilie Halip <ilie.ha...@gmail.com> --- 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) "__ubsan_handle_builtin_unreachable")); } -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->type == INSN_JUMP_UNCONDITIONAL && 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)) continue; WARN_FUNC("unreachable instruction", insn->sec, insn->offset); -- 2.25.1
