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

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Date:   Sat, 19 Sep 2020 09:41:18 +0300
From:   Ilie Halip <ilie.halip@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     Ilie Halip <ilie.halip@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Rong Chen <rong.a.chen@...el.com>,
        Marco Elver <elver@...gle.com>,
        Philip Li <philip.li@...el.com>,
        Borislav Petkov <bp@...en8.de>, kasan-dev@...glegroups.com,
        x86@...nel.org, clang-built-linux@...glegroups.com,
        kbuild test robot <lkp@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Nathan Chancellor <natechancellor@...il.com>
Subject: [PATCH v2] 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@...gle.com>
Cc: Rong Chen <rong.a.chen@...el.com>
Cc: Marco Elver <elver@...gle.com>
Cc: Philip Li <philip.li@...el.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: kasan-dev@...glegroups.com
Cc: x86@...nel.org
Cc: clang-built-linux@...glegroups.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@...gle.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
Tested-by: Nick Desaulniers <ndesaulniers@...gle.com>
Reported-by: kbuild test robot <lkp@...el.com>
Signed-off-by: Ilie Halip <ilie.halip@...il.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