linux-kernel - [PATCH] objtool: Prevent GCC from merging annotate

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Message-ID: <20171103221941.cajpwszir7ujxyc4@treble>
Date:   Fri, 3 Nov 2017 17:19:41 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     x86@...nel.org
Cc:     Andy Lutomirski <luto@...nel.org>, kbuild-all@...org,
        linux-kernel@...r.kernel.org, tipbuild@...or.com,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...e.de>,
        kbuild test robot <fengguang.wu@...el.com>
Subject: [PATCH] objtool: Prevent GCC from merging annotate_unreachable(),
 take 2


This fixes the following warning with GCC 4.6:

  mm/migrate.o: warning: objtool: migrate_misplaced_transhuge_page()+0x71: unreachable instruction

The problem is that the compiler merged identical annotate_unreachable()
inline asm blocks, resulting in a missing 'unreachable' annotation.

This problem happened before, and was partially fixed with:

  3d1e236022cc ("objtool: Prevent GCC from merging annotate_unreachable()")

That commit tried to ensure that each instance of the
annotate_unreachable() inline asm statement has a unique label.  It used
the __LINE__ macro to generate the label number.  However, even the line
number isn't necessarily unique when used in an inline function with
multiple callers (in this case, __alloc_pages_node()'s use of
VM_BUG_ON).

Reported-by: kbuild test robot <fengguang.wu@...el.com>
Fixes: 3d1e236022cc ("objtool: Prevent GCC from merging annotate_unreachable()")
Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
---
 include/linux/compiler.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index e95a2631e545..e9fe95b138cf 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -190,13 +190,13 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 	asm("%c0:\n\t"							\
 	    ".pushsection .discard.reachable\n\t"			\
 	    ".long %c0b - .\n\t"					\
-	    ".popsection\n\t" : : "i" (__LINE__));			\
+	    ".popsection\n\t" : : "i" (__COUNTER__));			\
 })
 #define annotate_unreachable() ({					\
 	asm("%c0:\n\t"							\
 	    ".pushsection .discard.unreachable\n\t"			\
 	    ".long %c0b - .\n\t"					\
-	    ".popsection\n\t" : : "i" (__LINE__));			\
+	    ".popsection\n\t" : : "i" (__COUNTER__));			\
 })
 #define ASM_UNREACHABLE							\
 	"999:\n\t"							\
-- 
2.13.6