Re: a problem tcp_v4_err()

From: Eric Dumazet <eric.dumazet@gmail.com>
To: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
Cc: Patrick McHardy <kaber@trash.net>,
	David Lamparter <equinox@diac24.net>,
	Eric Paris <eparis@redhat.com>, Hua Zhong <hzhong@gmail.com>,
	netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
	davem@davemloft.net, pekkas@netcore.fi, jmorris@namei.org,
	yoshfuji@linux-ipv6.org, paul.moore@hp.com
Subject: Re: a problem tcp_v4_err()
Date: Fri, 12 Nov 2010 19:21:54 +0100	[thread overview]
Message-ID: <1289586114.3185.271.camel@edumazet-laptop> (raw)
In-Reply-To: <1289585578.3185.268.camel@edumazet-laptop>

Le vendredi 12 novembre 2010 à 19:12 +0100, Eric Dumazet a écrit :
> Le vendredi 12 novembre 2010 à 20:57 +0300, Alexey Kuznetsov a écrit :
> > Hello!
> > 
> > I looked at tcp_v4_err() and found something strange. Quite non-trivial operations
> > are performed on unlocked sockets. It looks like at least this BUG_ON():
> > 
> >                 skb = tcp_write_queue_head(sk);
> >                 BUG_ON(!skb);
> > 
> > can be easily triggered.
> > 
> > Do I miss something?
> > 
> 
> Hi Alexey !
> 
> I see socket is locked around line 368,
> 
>         bh_lock_sock(sk);
>         /* If too many ICMPs get dropped on busy
>          * servers this needs to be solved differently.
>          */
>         if (sock_owned_by_user(sk))
>                 NET_INC_STATS_BH(net, LINUX_MIB_LOCKDROPPEDICMPS);
> 
> 
> Hmm, maybe some goto is missing ;)
> 

Well, goto is not missing.

Why do you think BUG_ON(!skb) can be triggered ?

We test before :

	if (seq != tp->snd_una  || !icsk->icsk_retransmits ||
		!icsk->icsk_backoff)
		break;

So a concurrent user only can add new skb(s) in the (non empty) queue ?