Commit 14db1f0a authored Sep 19, 2020 by Ilie Halip Committed by Josh Poimboeuf Sep 21, 2020

objtool: Ignore unreachable trap after call to noreturn functions

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@google.com>
Cc: Rong Chen <rong.a.chen@intel.com>
Cc: Marco Elver <elver@google.com>
Cc: Philip Li <philip.li@intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: kasan-dev@googlegroups.com
Cc: x86@kernel.org
Cc: clang-built-linux@googlegroups.com
BugLink: https://github.com/ClangBuiltLinux/linux/issues/1148
Link: https://lore.kernel.org/lkml/CAKwvOdmptEpi8fiOyWUo=AiZJiX+Z+VHJOM2buLPrWsMTwLnyw@mail.gmail.com


Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Ilie Halip <ilie.halip@gmail.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>

parent 2b232a22

tools/objtool/check.c

+7 −3

Original line number	Diff line number	Diff line
		@@ -2638,9 +2638,10 @@ 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;
		@@ -2668,8 +2669,11 @@ 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).
		*
		* It 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)))
		@@ -2796,7 +2800,7 @@ 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);

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