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@xxxxxxxxxxxxxxx
> Fixes: 045afc24124d ("arm64: futex: Fix FUTEX_WAKE_OP atomic ops with non-zero result value")
> Signed-off-by: Nathan Chancellor <natechancellor@xxxxxxxxx>

Reviewed-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>