linux-kernel - Re: [PATCH] poll/select: avoid arithmetic overflow in __estimate

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Date:	Mon, 17 Aug 2009 17:11:16 +0800
From:	Amerigo Wang <xiyou.wangcong@...il.com>
To:	Guillaume Knispel <gknispel@...formatique.com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH] poll/select: avoid arithmetic overflow in
	__estimate_accuracy()

On Sun, Aug 16, 2009 at 10:29:21PM +0200, Guillaume Knispel wrote:
>__estimate_accuracy() was prone to integer overflow, for example
>if *tv == {2147, 483648000} on a 32 bit computer (or even for delays
>as small as {429, 500000000} if the task is niced).
>
>Because the result was already forced between 0 and 100ms, the effect
>of the overflow was not too problematic, but the use of the hrtimer
>range feature was not optimal in overflow cases.
>
>This patch ensures that there can not be an integer overflow in this
>function.
>
>Signed-off-by: Guillaume Knispel <gknispel@...formatique.com>
>---
> fs/select.c |   14 ++++++++++----
> 1 files changed, 10 insertions(+), 4 deletions(-)
>
>diff --git a/fs/select.c b/fs/select.c
>index 8084834..a201fc3 100644
>--- a/fs/select.c
>+++ b/fs/select.c
>@@ -41,22 +41,28 @@
>  * better solutions..
>  */
> 
>+#define MAX_SLACK	(100 * NSEC_PER_MSEC)
>+
> static long __estimate_accuracy(struct timespec *tv)
> {
> 	long slack;
> 	int divfactor = 1000;
> 
>+	if (tv->tv_sec < 0)
>+		return 0;
>+
> 	if (task_nice(current) > 0)
> 		divfactor = divfactor / 5;
> 
>+	if (tv->tv_sec > MAX_SLACK / (NSEC_PER_SEC/divfactor))
>+		return MAX_SLACK;
>+


Yes? Isn't MAX_SLACK also too big for 'long' on 32bit machine?

Try:

% printf "%d\n" "100*1000000000 > 0x7fffffff"
1

Am I missing something here?


Thanks.


> 	slack = tv->tv_nsec / divfactor;
> 	slack += tv->tv_sec * (NSEC_PER_SEC/divfactor);
> 
>-	if (slack > 100 * NSEC_PER_MSEC)
>-		slack =  100 * NSEC_PER_MSEC;
>+	if (slack > MAX_SLACK)
>+		return MAX_SLACK;
> 
>-	if (slack < 0)
>-		slack = 0;
> 	return slack;
> }
> 
>--
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