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

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Date:	Mon, 06 Feb 2012 18:05:30 -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>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: [RFC PATCH] poll() in 32-bit applications does not handle timeout
 of -1 properly on 64-bit kernels

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.

Josh

View attachment "poll-timeout-try1.patch" of type "text/x-patch" (1684 bytes)