Re: [Qemu-devel] [PATCH] Proposed fix broken RST response to a slirp redirect socket

[Edgar E. Iglesias]

On Wed, Jun 11, 2008 at 12:21:45PM -0500, Jason Wessel wrote:
> 
> When using slirp networking with a redirected tcp socket, the qemu guest
> os does not receive RST packets when a redirected, accepted socket goes
> into the FIN_WAIT_2 status.  Presently slirp sends ACKs instead of RST
> packets, which means the guest os application socket writes do not fail
> event after the client has terminated the socket.
> 
> Here is a simple way to demonstrate the problem.
> 
> * Start qemu with user mode networking plus:
>      -redir tcp:4441::4441
> 
> * Assuming you booted a linux guest os you could run:
>      cat /dev/zero | nc -p 4441 -l
> 
> * On the host run the following command and you
>   must hit control-c after about 1 second
>      nc localhost 4441

Hello Jason,

IIRC connections in FIN_WAIT_2 can continue to receive data.

If I might take a wild guess at whats going on:
The host closed the receiving socket when you ctrl-c nc. That socket still has
data in it's rcvbuf so the stack aborts the connection and sends a RST. The
slirp code should now see a -1 on it's next write to that socket and an errno
ECONNRESET but it's not correctly taking care of that case, instead it's
incorrectly setting the TCP state to FIN_WAIT_2. It should have set it to
CLOSED and sent a RST to the guest.

Best regards

> 
> 
> If you were to TCP dump the connection in the guest OS you would see
> after killing the "nc" on the host computer that slirp keep acking the
> packets, even though no client application is there.
> 
> 14:55:38.385310 IP 10.0.2.15.4441 > 10.0.2.2.37227: P
> 8884509:8885077(568) ack 2 win 5840
> 14:55:38.385310 IP 10.0.2.2.37227 > 10.0.2.15.4441: . ack 8885077 win 0
> 14:55:38.589613 IP 10.0.2.15.4441 > 10.0.2.2.37227: . ack 2 win 5840
> 14:55:38.589613 IP 10.0.2.2.37227 > 10.0.2.15.4441: . ack 8885077 win 0
> 14:55:38.997437 IP 10.0.2.15.4441 > 10.0.2.2.37227: . ack 2 win 5840
> 14:55:38.997653 IP 10.0.2.2.37227 > 10.0.2.15.4441: . ack 8885077 win 0
> 14:55:39.813522 IP 10.0.2.15.4441 > 10.0.2.2.37227: . ack 2 win 5840
> 14:55:39.813758 IP 10.0.2.2.37227 > 10.0.2.15.4441: . ack 8885077 win 0
> 14:55:41.445562 IP 10.0.2.15.4441 > 10.0.2.2.37227: . ack 2 win 5840
> 14:55:41.445769 IP 10.0.2.2.37227 > 10.0.2.15.4441: . ack 8885077 win 0
> 
> 
> The correct behavior should be to send an RST and not an ACK.  There
> might be several ways to correct this problem.  The attached patch is
> one possible way to implement the RFC compliant behavior.  With the
> patch, the tcp dump starts to look like:
> 
> 15:04:34.567350 IP 10.0.2.15.4441 > 10.0.2.2.58510: P
> 2101533:2102993(1460) ack 1 win 5840
> 15:04:34.567350 IP 10.0.2.2.58510 > 10.0.2.15.4441: . ack 2102993 win 5840
> 15:04:34.570718 IP 10.0.2.2.58510 > 10.0.2.15.4441: F 1:1(0) ack 2102993
> win 5840
> 15:04:34.571383 IP 10.0.2.15.4441 > 10.0.2.2.58510: .
> 2102993:2104453(1460) ack 1 win 5840
> 15:04:34.571383 IP 10.0.2.2.58510 > 10.0.2.15.4441: F 1:1(0) ack 2104453
> win 4380
> 15:04:34.571383 IP 10.0.2.15.4441 > 10.0.2.2.58510: P
> 2104453:2105345(892) ack 1 win 5840
> 15:04:34.571383 IP 10.0.2.2.58510 > 10.0.2.15.4441: F 1:1(0) ack 2105345
> win 3488
> 15:04:34.571383 IP 10.0.2.15.4441 > 10.0.2.2.58510: . ack 2 win 5840
> 15:04:34.571383 IP 10.0.2.15.4441 > 10.0.2.2.58510: . ack 2 win 5840
> 15:04:34.571383 IP 10.0.2.2.58510 > 10.0.2.15.4441: R
> 12032003:12032003(0) win 3488
> 
> Also with the patch, the SIG_PIPE handlers start to work correctly in
> the guest OS.
> 
> Thanks,
> Jason.

> From: Jason Wessel <address@hidden>
> Subject: [PATCH] slirp: Fix broken RST response to a slirp redirect socket
> 
> When using slirp networking with a redirected tcp socket, the qemu
> guest os does not receive RST packets when a redirected, accepted
> socket goes into the FIN_WAIT_2 status.  Presently slirp sends ACKs
> instead of RST packets, which means the guest os application socket
> writes do not fail event after the client has terminated the socket.
> 
> This patch changes the behavior to correctly send RST packets instead
> of ACKS.
> 
> Signed-off-by: Jason Wessel <address@hidden>
> 
> ---
>  slirp/tcp_input.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> --- a/slirp/tcp_input.c
> +++ b/slirp/tcp_input.c
> @@ -432,7 +432,7 @@ findso:
>       tp = sototcpcb(so);
>  
>       /* XXX Should never fail */
> -     if (tp == 0)
> +     if (tp == 0 || tp->t_state == TCPS_FIN_WAIT_2)
>               goto dropwithreset;
>       if (tp->t_state == TCPS_CLOSED)
>               goto drop;


-- 
Edgar E. Iglesias
Axis Communications AB