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From:   Ilie Halip <ilie.halip@gmail.com>
To:     linux-kernel@vger.kernel.org
Cc:     Ilie Halip <ilie.halip@gmail.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Rong Chen <rong.a.chen@intel.com>,
        Marco Elver <elver@google.com>,
        Philip Li <philip.li@intel.com>,
        Borislav Petkov <bp@alien8.de>, kasan-dev@googlegroups.com,
        x86@kernel.org, clang-built-linux@googlegroups.com,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Nathan Chancellor <natechancellor@gmail.com>
Subject: [PATCH] objtool: ignore unreachable trap after call to noreturn functions
Date:   Thu, 17 Sep 2020 11:49:04 +0300
Message-Id: <20200917084905.1647262-1-ilie.halip@gmail.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

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.

objtool silences similar warnings (trap after dead end instructions), 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>
Signed-off-by: Ilie Halip <ilie.halip@gmail.com>
---
 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