linux-kernel - Re: [PATCH] futex: avoid undefined behaviour when shift exponent is negative

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Date:   Wed, 21 Jun 2017 18:40:36 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     zhong jiang <zhongjiang@...wei.com>
Cc:     akpm@...ux-foundation.org, tglx@...utronix.de, mingo@...hat.com,
        minchan@...nel.org, mhocko@...e.com, hpa@...or.com, x86@...nel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] futex: avoid undefined behaviour when shift exponent is
 negative


* zhong jiang <zhongjiang@...wei.com> wrote:

> when shift expoment is negative, left shift alway zero. therefore, we
> modify the logic to avoid the warining.
> 
> Signed-off-by: zhong jiang <zhongjiang@...wei.com>
> ---
>  arch/x86/include/asm/futex.h | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/include/asm/futex.h b/arch/x86/include/asm/futex.h
> index b4c1f54..2425fca 100644
> --- a/arch/x86/include/asm/futex.h
> +++ b/arch/x86/include/asm/futex.h
> @@ -49,8 +49,12 @@ static inline int futex_atomic_op_inuser(int encoded_op, u32 __user *uaddr)
>  	int cmparg = (encoded_op << 20) >> 20;
>  	int oldval = 0, ret, tem;
>  
> -	if (encoded_op & (FUTEX_OP_OPARG_SHIFT << 28))
> -		oparg = 1 << oparg;
> +	if (encoded_op & (FUTEX_OP_OPARG_SHIFT << 28)) {
> +		if (oparg >= 0)
> +			oparg = 1 << oparg;
> +		else
> +			oparg = 0;
> +	}

Could we avoid all these complications by using an unsigned type?

Thanks,

	Ingo