linux-kernel - Re: strange crashes in tcp_poll() via epoll

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Message-ID: <20130719235008.GA4518@dcvr.yhbt.net>
Date:	Fri, 19 Jul 2013 23:50:08 +0000
From:	Eric Wong <normalperson@...t.net>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Al Viro <viro@...IV.linux.org.uk>, netdev <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: strange crashes in tcp_poll() via epoll_wait

Eric Dumazet <eric.dumazet@...il.com> wrote:
> Hi Al
> 
> I tried to debug strange crashes in tcp_poll() called from
> sys_epoll_wait() -> sock_poll()
> 
> The symptom is that sock->sk is NULL and we therefore dereference a NULL
> pointer.
> 
> It's really rare crashes but still, it would be nice to understand where
> is the bug. Presumably latest kernels would crash in sock_poll() because
> of the sk_can_busy_loop(sock->sk) call.
> 
> We do test sock->sk being NULL in sock_fasync(), but epoll should be
> safe because of existing synchronization (epmutex) ?

It should be safe because of ep->mtx, actually, as epmutex is not taken
in sys_epoll_wait.

I took a look at this but have not found anything.  I've yet to see this
this on my machines.

When did you start noticing this?
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