linux-kernel - Re: [PATCH 1/6] [2.6.29] epoll: fix for epoll_wait sometimes returning events on closed fds

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Date:	Tue, 24 Feb 2009 19:12:13 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Tony Battersby <tonyb@...ernetics.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Davide Libenzi <davidel@...ilserver.org>,
	Jonathan Corbet <corbet@....net>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/6] [2.6.29] epoll: fix for epoll_wait sometimes returning
 events on closed fds

Tony Battersby a écrit :
>>>From the epoll manpage:
> Q: Will closing a file descriptor cause it to be removed from all
>    epoll sets automatically?
> A: Yes
> 
> sys_close() calls filp_close(), which calls fput().  If no one else
> holds a reference to the file, then fput() calls __fput(), and __fput()
> calls eventpoll_release(), which prevents epoll_wait() from returning
> events on the fd to userspace.  In the rare case that sys_close()
> doesn't call __fput() because someone else has a reference to the file,
> a subsequent epoll_wait() may still unexpectedly return events on the
> fd after it has been closed.  This can end up confusing or crashing
> a userspace program that doesn't do epoll_ctl(EPOLL_CTL_DEL) before
> closing the fd.  I have reports of this actually happening under
> heavy load with a program using epoll with network sockets on 2.6.27.
> 
> This patch fixes the problem by calling eventpoll_release_file()
> from filp_close() instead of from __fput().
> 
> The locking in eventpoll_release() and eventpoll_release_file() needs
> to be changed because previously it relied on the fact that no one
> else could have a reference to the file when called from __fput(),
> and this is no longer true.  The new locking is admittedly ugly,
> but I believe it works.
> 
> ep_insert() now also needs to check if the file has been closed
> to avoid races in multi-threaded programs where one thread is doing
> epoll_ctl(EPOLL_CTL_ADD) and another thread is closing the fd.  This is
> done by checking that fget_light still returns the same struct file *
> as before.
> 
> Note that the list_del_init(&epi->fllink) previously done in
> eventpoll_release_file() was unnecessary because it is also done
> by ep_remove().
> 
> Userspace programs that might run on kernels with this bug can work
> around the problem by doing epoll_ctl(EPOLL_CTL_DEL) before close().
> 
> Signed-off-by: Tony Battersby <tonyb@...ernetics.com>
> CC: <stable@...nel.org>

Your patch may solve part of the problem. In your programs, maybe you
have one thread doing all epoll_wait() and close() syscalls, but what
of other programs ?

What prevents a thread doing close(fd) right after an other thread
got this fd from epoll_wait() ?
Nothing, and application may strangely react.

The moment you have several threads doing read()/write()/close() syscalls
on the same fd at the same time, you obviously get problems, not
only with epoll.

In a typical epoll driven application, with a pool of N worker threads all doing :

while (1) {
	fd = epoll_wait(epoll_fd);
	work_on_fd(fd); /* possibly calling close(fd); */
}

Then, you must be prepared to get a *false* event, ie an fd that another worker
already closed (and eventually reopened)


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