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

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Date:   Wed, 17 Apr 2019 10:53:37 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     Will Deacon <will.deacon@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH] arm64: futex: Restore oldval initialization to work
 around buggy compilers

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@...r.kernel.org
> Fixes: 045afc24124d ("arm64: futex: Fix FUTEX_WAKE_OP atomic ops with non-zero result value")
> Signed-off-by: Nathan Chancellor <natechancellor@...il.com>

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

-- 
Catalin