linux-kernel - Re: commit 64ff3b938ec6782e6585a83d5459b98b0c3f6eb8 breaks rlogin

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Date:	Mon, 9 Feb 2009 18:28:06 +1100
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
Cc:	David Miller <davem@...emloft.net>, jeff.chua.linux@...il.com,
	rjw@...k.pl, torvalds@...ux-foundation.org,
	LKML <linux-kernel@...r.kernel.org>,
	Netdev <netdev@...r.kernel.org>
Subject: Re: commit 64ff3b938ec6782e6585a83d5459b98b0c3f6eb8 breaks rlogin

On Mon, Feb 09, 2009 at 09:13:00AM +0200, Ilpo Järvinen wrote:
> 
> Hmm, what happens if a opposite dir ACK gets sent (it's zero sized seqno
> range will be above urgent sequence when urg is not yet acknowledged)? So
> we'll take the later branch after your change and send bogus 0xffff urg?
> I think you're change might have actually broken bidir tcp completely (and
> probably window probing as well under some conditions). Congraz, two flies
> with a single stroke :-). You would have needed an additional check that
> the tcb->seq is below urg...
> 
> ...Below is a patch but Jeff might need to revert the revert first if
> he tests with the latest Linus' tree.

Good point! That would definitely explain this.
 
> [PATCH] tcp: check that we're still < urg instead of sending a bogus one
> 
> Opposite dir ACK with zero sized seqno range can go into the
> later condition once urg is in the outstanding window not
> yet acknowledge (we're still in urg mode but seqno we sent in
> the ack won't be below snd_up). Need to check that we never
> advertize bogus urg for a segment which is after the snd_up.
> 
> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
> ---
>  net/ipv4/tcp_output.c |    3 ++-
>  1 files changed, 2 insertions(+), 1 deletions(-)
> 
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index 557fe16..c808d73 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -667,7 +667,8 @@ static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
>  		if (between(tp->snd_up, tcb->seq + 1, tcb->seq + 0xFFFF)) {
>  			th->urg_ptr = htons(tp->snd_up - tcb->seq);
>  			th->urg = 1;
> -		} else if (after(tcb->seq + 0xFFFF, tp->snd_nxt)) {
> +		} else if (after(tcb->seq + 0xFFFF, tp->snd_nxt) &&
> +			   before(tcb->seq, tcp->snd_up)) {
>  			th->urg_ptr = 0xFFFF;
>  			th->urg = 1;
>  		}

We could rewrite this as

	if (unlikely(tcp_urg_mode(tp) && before(tcb->seq, tcp->snd_up))) {
		if (before(tp->sndup, tcb->seq + 0x10000)) {
			th->urg_ptr = htons(tp->snd_up - tcb->seq);
			th->urg = 1;
		} else if (after(tcb->seq + 0xFFFF, tp->snd_nxt)) {
			th->urg_ptr = 0xFFFF;
			th->urg = 1;
		}
	}

Thanks,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
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