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

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Date:	Wed, 19 Oct 2011 15:25:38 +0200
From:	Johannes Berg <johannes@...solutions.net>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Richard Cochran <richardcochran@...il.com>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH 0/3] net: time stamping fixes

On Wed, 2011-10-19 at 15:21 +0200, Eric Dumazet wrote:

> > Not disputing this either. But you said sk_refcnt can be 0, so why can't
> > the following happen:
> > 
> > /* skb; skb->sk = sk; skb->destructor = sock_wfree; */
> > 
> > /* skb is on qdisc, some time passes */
> > 
> > sk_free(sk); /* user closed socket,
> >                 sk->sk_refcnt reaches 0,
> > 		sk->sk_wmem_alloc == skb->truesize,
> > 		__sk_free not called, socket still lives,
> > 		but no more +1 in sk_wmem_alloc */
> > 
> > /* some more time passes */
> > 
> > /* ethernet hard_start_xmit calls skb_clone_tx_timestamp() */
> > skb2 = skb_clone(skb);
> > skb2->sk = skb->sk;
> > sock_hold(skb->sk);
> > 
> > /* ethernet TX completion calls skb_free(skb) */
> > skb_free(skb):
> >   sock_wfree(skb); /* sk_wmem_alloc reaches 0,
> >                       __sk_free called DESPITE sk_refcnt > 0 */
> > 
> > /* later, in skb_complete_tx_timestamp() */
> > sock_put(sk);	/* KABOOM */
> > 
> > 
> > I just want to understand why this can't happen :-)
> 
> Since you answer your own question :)

Did I?

> Hmm, oh well, sk_refcnt is/should not be changed if a xmit packet is
> duped, but sk_wmem_alloc should be, exactly paired with skb->truesize

Well, yeah, ok, but Richard's patches do modify sk_refcnt, and can't
modify sk_wmem_alloc as discussed upthread.

I'll admit that I don't really trust the whole thing anyway -- it seems
rather error prone to forbid you from doing sock_hold() on a socket you
obtained from a TX packet.

johannes

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