netdev - strange crashes in tcp_poll() via epoll

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Message-ID: <1374251057.26476.17.camel@edumazet-glaptop>
Date:	Fri, 19 Jul 2013 09:24:17 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	netdev <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: strange crashes in tcp_poll() via epoll_wait

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) ?

Any idea?

Thanks !

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