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From:   Michael Ellerman <mpe@ellerman.id.au>
To:     Christophe Leroy <christophe.leroy@c-s.fr>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>
Cc:     linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH] powerpc/futex: fix warning: 'oldval' may be used uninitialized in this function
In-Reply-To: <86b72f0c134367b214910b27b9a6dd3321af93bb.1565774657.git.christophe.leroy@c-s.fr>
References: <86b72f0c134367b214910b27b9a6dd3321af93bb.1565774657.git.christophe.leroy@c-s.fr>
Date:   Fri, 16 Aug 2019 22:35:31 +1000
Message-ID: <878srt70fg.fsf@concordia.ellerman.id.au>
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Christophe Leroy <christophe.leroy@c-s.fr> writes:
>   CC      kernel/futex.o
> kernel/futex.c: In function 'do_futex':
> kernel/futex.c:1676:17: warning: 'oldval' may be used uninitialized in this function [-Wmaybe-uninitialized]
>    return oldval == cmparg;
>                  ^
> kernel/futex.c:1651:6: note: 'oldval' was declared here
>   int oldval, ret;
>       ^
>
> This is because arch_futex_atomic_op_inuser() only sets *oval
> if ret is NUL and GCC doesn't see that it will use it only when

I prefer 0 to "NUL", as ret is an int. I'll reword it. But otherwise
this looks OK.

cheers

> ret is NUL.
>
> Anyway, the non-NUL ret path is an error path that won't suffer from
> setting *oval, and as *oval is a local var in futex_atomic_op_inuser()
> it will have no impact.
>
> Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
> ---
>  arch/powerpc/include/asm/futex.h | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/futex.h b/arch/powerpc/include/asm/futex.h
> index 3a6aa57b9d90..eea28ca679db 100644
> --- a/arch/powerpc/include/asm/futex.h
> +++ b/arch/powerpc/include/asm/futex.h
> @@ -60,8 +60,7 @@ static inline int arch_futex_atomic_op_inuser(int op, int oparg, int *oval,
>  
>  	pagefault_enable();
>  
> -	if (!ret)
> -		*oval = oldval;
> +	*oval = oldval;
>  
>  	prevent_write_to_user(uaddr, sizeof(*uaddr));
>  	return ret;
> -- 
> 2.13.3