[PATCH] arm64: futex: Restore oldval initialization to work around buggy compilers

[PATCH] arm64: futex: Restore oldval initialization to work around buggy compilers

[Nathan Chancellor]

Commit 045afc24124d ("arm64: futex: Fix FUTEX_WAKE_OP atomic ops with
non-zero result value") removed oldval's zero initialization in
arch_futex_atomic_op_inuser because it is not necessary. Unfortunately,
Android's arm64 GCC 4.9.4 [1] does not agree:

../kernel/futex.c: In function 'do_futex':
../kernel/futex.c:1658:17: warning: 'oldval' may be used uninitialized
in this function [-Wmaybe-uninitialized]
   return oldval == cmparg;
                 ^
In file included from ../kernel/futex.c:73:0:
../arch/arm64/include/asm/futex.h:53:6: note: 'oldval' was declared here
  int oldval, ret, tmp;
      ^

GCC fails to follow that when ret is non-zero, futex_atomic_op_inuser
returns right away, avoiding the uninitialized use that it claims.
Restoring the zero initialization works around this issue.

[1]: https://android.googlesource.com/platform/prebuilts/gcc/linux-x86/aarch64/aarch64-linux-android-4.9/

Cc: stable@vger.kernel.org
Fixes: 045afc24124d ("arm64: futex: Fix FUTEX_WAKE_OP atomic ops with non-zero result value")
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
---
 arch/arm64/include/asm/futex.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/futex.h b/arch/arm64/include/asm/futex.h
index e1d95f08f8e1..c7e1a7837706 100644
--- a/arch/arm64/include/asm/futex.h
+++ b/arch/arm64/include/asm/futex.h
@@ -50,7 +50,7 @@ do {									\
 static inline int
 arch_futex_atomic_op_inuser(int op, int oparg, int *oval, u32 __user *_uaddr)
 {
-	int oldval, ret, tmp;
+	int oldval = 0, ret, tmp;
 	u32 __user *uaddr = __uaccess_mask_ptr(_uaddr);
 
 	pagefault_disable();
-- 
2.21.0


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Re: [PATCH] arm64: futex: Restore oldval initialization to work around buggy compilers

[Greg Kroah-Hartman]

On Wed, Apr 17, 2019 at 12:21:21AM -0700, Nathan Chancellor wrote:
> Commit 045afc24124d ("arm64: futex: Fix FUTEX_WAKE_OP atomic ops with
> non-zero result value") removed oldval's zero initialization in
> arch_futex_atomic_op_inuser because it is not necessary. Unfortunately,
> Android's arm64 GCC 4.9.4 [1] does not agree:
> 
> ../kernel/futex.c: In function 'do_futex':
> ../kernel/futex.c:1658:17: warning: 'oldval' may be used uninitialized
> in this function [-Wmaybe-uninitialized]
>    return oldval == cmparg;
>                  ^
> In file included from ../kernel/futex.c:73:0:
> ../arch/arm64/include/asm/futex.h:53:6: note: 'oldval' was declared here
>   int oldval, ret, tmp;
>       ^
> 
> GCC fails to follow that when ret is non-zero, futex_atomic_op_inuser
> returns right away, avoiding the uninitialized use that it claims.
> Restoring the zero initialization works around this issue.
> 
> [1]: https://android.googlesource.com/platform/prebuilts/gcc/linux-x86/aarch64/aarch64-linux-android-4.9/
> 
> Cc: stable@vger.kernel.org
> Fixes: 045afc24124d ("arm64: futex: Fix FUTEX_WAKE_OP atomic ops with non-zero result value")
> Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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Re: [PATCH] arm64: futex: Restore oldval initialization to work around buggy compilers

[Catalin Marinas]

On Wed, Apr 17, 2019 at 12:21:21AM -0700, Nathan Chancellor wrote:
> Commit 045afc24124d ("arm64: futex: Fix FUTEX_WAKE_OP atomic ops with
> non-zero result value") removed oldval's zero initialization in
> arch_futex_atomic_op_inuser because it is not necessary. Unfortunately,
> Android's arm64 GCC 4.9.4 [1] does not agree:
> 
> ../kernel/futex.c: In function 'do_futex':
> ../kernel/futex.c:1658:17: warning: 'oldval' may be used uninitialized
> in this function [-Wmaybe-uninitialized]
>    return oldval == cmparg;
>                  ^
> In file included from ../kernel/futex.c:73:0:
> ../arch/arm64/include/asm/futex.h:53:6: note: 'oldval' was declared here
>   int oldval, ret, tmp;
>       ^
> 
> GCC fails to follow that when ret is non-zero, futex_atomic_op_inuser
> returns right away, avoiding the uninitialized use that it claims.
> Restoring the zero initialization works around this issue.
> 
> [1]: https://android.googlesource.com/platform/prebuilts/gcc/linux-x86/aarch64/aarch64-linux-android-4.9/
> 
> Cc: stable@vger.kernel.org
> Fixes: 045afc24124d ("arm64: futex: Fix FUTEX_WAKE_OP atomic ops with non-zero result value")
> Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>

Thanks. That compiler version doesn't seem very smart. I'll queue this
for -rc6.

-- 
Catalin

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