netdev - Re: [PATCH 0/3] net: time stamping fixes

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Date:	Wed, 19 Oct 2011 07:15:36 +0200
From:	Johannes Berg <johannes@...solutions.net>
To:	David Miller <davem@...emloft.net>
Cc:	richardcochran@...il.com, netdev@...r.kernel.org,
	eric.dumazet@...il.com
Subject: Re: [PATCH 0/3] net: time stamping fixes

On Wed, 2011-10-19 at 00:16 -0400, David Miller wrote:
> From: Richard Cochran <richardcochran@...il.com>
> Date: Thu, 13 Oct 2011 11:46:26 +0200
> 
> > The first patch fixes a bug in the time stamping code introduced in
> > v2.6.36 of the kernel.
> > 
> > The other two patches depend on the first patch and fix two bugs in a
> > PTP Hardware Clock driver. This driver was first introduced in Linux
> > version 3.0.
> > 
> > Richard Cochran (3):
> >   net: hold sock reference while processing tx timestamps
> >   dp83640: use proper function to free transmit time stamping packets
> >   dp83640: free packet queues on remove
> 
> Johannes and Eric, please help review Richard's fixes here.

The only thing I'm not completely sure about is whether or not it is
permissible to sock_hold() at that point. I'm probably just missing
something, but: if sk_free() was called before hard_start_xmit() which
will call skb_clone_tx_timestamp(), can we really call sock_hold()?

The reason I ask is that sock_wfree() doesn't check sk_refcnt, so if it
is possible for sk_free() to have been called before hard_start_xmit(),
maybe because the packet was stuck on the qdisc for a while, the socket
won't be released (sk_free checks sk_wmem_alloc) but the sk_wfree() when
the original skb is freed will actually free the socket, invalidating
the clone's sk pointer *even though* we called sock_hold() right after
making the clone.

So what guarantees that sk_refcnt is still non-zero when we make the
clone? Alternatively, should sock_wfree() check sk_refcnt?

johannes

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