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

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Date:	Mon, 17 Aug 2009 13:20:50 +0200 (CEST)
From:	"Guillaume Knispel" <gknispel@...formatique.com>
To:	"Amerigo Wang" <xiyou.wangcong@...il.com>
Cc:	"Guillaume Knispel" <gknispel@...formatique.com>,
	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?

100 * NSEC_PER_MSEC  =>  100 * 1000000
               -

The exact same multiplication was already in the existing code,
and this is not the one that can overflow.


Cheers,
Guillaume Knispel

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