linux-kernel - Re: [PATCH] a multiplication overflow in drivers/pps/pps.c

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Date:	Thu, 19 Dec 2013 16:37:36 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	xqx12 <s2exqx@...il.com>
Cc:	giometti@...eenne.com, linux-kernel@...r.kernel.org,
	chyyuu@...il.com, xuyongjiande@...il.com,
	xqx12 <xiaoqixue_1@....com>
Subject: Re: [PATCH] a multiplication overflow in drivers/pps/pps.c

On Wed, 18 Dec 2013 15:36:11 +0800 xqx12 <s2exqx@...il.com> wrote:

> there is an overflow in the following code :
> 
> ticks = fdata.timeout.sec * HZ;
> 
> while ticks is a signed 64-bit, but the result of fdata.timeout.sec *
> HZ will be converted be 32-bit first. So ticks will be a wrong value
> after multiplication overflow.

When fixing a bug, please always include a full description of the
user-visible effects of that bug.  This helps others decide which
kernel version(s) should be fixed.

> ...
>
> --- a/drivers/pps/pps.c
> +++ b/drivers/pps/pps.c
> @@ -164,7 +164,7 @@ static long pps_cdev_ioctl(struct file *file,
>  			dev_dbg(pps->dev, "timeout %lld.%09d\n",
>  					(long long) fdata.timeout.sec,
>  					fdata.timeout.nsec);
> -			ticks = fdata.timeout.sec * HZ;
> +			ticks = (s64)(fdata.timeout.sec) * HZ;
>  			ticks += fdata.timeout.nsec / (NSEC_PER_SEC / HZ);
>  
>  			if (ticks != 0) {

pps_fdata.timeout.sec has type __s64, so the patch should be a no-op?

Did you really observe a runtime problem from this?  If so, I suspect
your compiler is busted.


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