linux-kernel - [PATCH] objtool: Fix noreturn detection for ignored functions

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Message-Id: <5b1e2536cdbaa5246b60d7791b76130a74082c62.1599751464.git.jpoimboe@redhat.com>
Date:   Thu, 10 Sep 2020 10:24:57 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     x86@...nel.org
Cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH] objtool: Fix noreturn detection for ignored functions

When a function is annotated with STACK_FRAME_NON_STANDARD, objtool
doesn't validate its code paths.  It also skips sibling call detection
within the function.

But sibling call detection is actually needed for the case where the
ignored function doesn't have any return instructions.  Otherwise
objtool naively marks the function as implicit static noreturn, which
affects the reachability of its callers, resulting in "unreachable
instruction" warnings.

Fix it by just enabling sibling call detection for ignored functions.
The 'insn->ignore' check in add_jump_destinations() is no longer needed
after commit e6da9567959e ("objtool: Don't use ignore flag for fake
jumps").

Fixes the following warning:

  arch/x86/kvm/vmx/vmx.o: warning: objtool: vmx_handle_exit_irqoff()+0x142: unreachable instruction

Reported-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
---
 tools/objtool/check.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index e034a8f24f46..90a66891441a 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -619,7 +619,7 @@ static int add_jump_destinations(struct objtool_file *file)
 		if (!is_static_jump(insn))
 			continue;

-		if (insn->ignore || insn->offset == FAKE_JUMP_OFFSET)
+		if (insn->offset == FAKE_JUMP_OFFSET)
 			continue;

 		reloc = find_reloc_by_dest_range(file->elf, insn->sec,
-- 
2.25.4