linux-kernel - Re: [RFC PATCH] poll() in 32-bit applications does not handle timeout of -1 properly on 64-bit kernels

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Date:	Tue, 07 Feb 2012 11:51:56 -0600
From:	Josh Hunt <johunt@...mai.com>
To:	Al Viro <viro@...IV.linux.org.uk>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	x86@...nel.org, arnd@...db.de, linux-arch@...r.kernel.org
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] poll() in 32-bit applications does not handle timeout
 of -1 properly on 64-bit kernels

On 02/06/2012 06:38 PM, Al Viro wrote:
> On Mon, Feb 06, 2012 at 06:05:30PM -0600, Josh Hunt wrote:
>> We've hit an issue where our 32-bit applications, when running on a
>> 64-bit kernel, using poll() and passing in a value of -1 for the timeout
>> return after ~49 days (2^32 msec). Instead of waiting indefinitely as it
>> is stated they should. Reproducing the issue is trivial. I've
>> instrumented the kernel and found we are hitting the case where poll()
>> believes we've passed in a positive number and thus creates a timespec,
>> etc. Currently poll() is defined in userspace as:
>>
>> int poll(struct pollfd *ufds, nfds_t nfds, int timeout);
>>
>> but in the kernel timeout is of type long.
>>
>> I can think of a few ways to solve this. One, which is the patch I've
>> attached, is to change the type of timeout to int in the kernel. I'm not
>> certain the ramifications this may have since it's changing a syscall's
>> arguments which may be a big no-no :) Another way I am proposing is by
>> bounds checking. Currently we do the following:
>>
>> if (timeout_msecs >= 0) {
>>         to = &end_time;
>>         poll_select_set_timeout(to, timeout_msecs / MSEC_PER_SEC,
>>                         NSEC_PER_MSEC * (timeout_msecs % MSEC_PER_SEC));
>> }
>>
>> We could add an upper bound on timeout_msecs to say < 0xffffffff. I'm
>> not sure if either is acceptable though.
> 
> Or just add compat_sys_poll() with that argument being int and have it call
> sys_poll().  The value will be sign-extended...

Al

I've implemented what you suggested by adding compat_sys_poll() with
an int argument for timeout allowing it to do the sign extension. I
wanted to point out there was an almost identical patch submitted last
year, which appears to have gotten lost in the wash:
https://lkml.org/lkml/2011/9/18/19

I am guessing there are other architectures affected by this bug. This
patch only fixes x86.

Josh


View attachment "compat-sys-poll.patch" of type "text/x-patch" (2340 bytes)