AX.25 unaccepted socket makes problems

From: Tihomir Heidelberg (gzxv@kerailya.tunkki.fi)
Date: Tue May 27 2003 - 19:24:50 EEST

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    Hi linux-hams,

    finaly I found a way how to produce AX.25 kernel get mad,
    I mean, when "cat /proc/net/ax25" produce segmentation fault,
    and everything got unstable.

    If your program bind ax.25 socket for listening, other station
    connects to that listening socket and if listening socket dies
    without accepting new connection from you program, after short
    period the things got mad...

    In ax25_destroy_socket function (af_ax25.c), there is a while
    loop for dequeuing those unaccepted connections and preparing
    them to die when heardbeat catch them. Also, there is code in
    ax25_std_heartbeat_expiry function (ax25_std_timer.c) that
    should destroy those connections, unfortunately those connection
    are NOT destroyed here ! A condition:
            (ax25->sk->state == TCP_LISTEN && ax25->sk->dead)
    is not true because state of those connections are not
    TCP_LISTEN. State is TCP_ESTABLISHED or TCP_CLOSE, depending if
    other end disconnected or not. Cannot get the reason why
    segmentation fault is happening, but hope someone else can find
    a reason when problem is reproducable.

    To avoid this problem I added to ax25_destroy_socket function
    (af_ax25.c) following:
            if (skb->sk != ax25->sk) {
    + skb->sk->state = TCP_LISTEN;
    + if (!skb->sk->dead) {
    + skb->sk->state_change(skb->sk);
    + }
                    skb->sk->dead = 1;
                    ax25_start_heartbeat(skb->sk->protinfo.ax25);
                    skb->sk->protinfo.ax25->state = AX25_STATE_0;
            }

    I am pretty sure that this is not the way how problem should be
    fixed, there should be a different way how to mark socket for
    destroying, but at least, I got rid of this problem.

    Also, some months ago I mention here that regulary I get this
    AX.25 kernel behavior after few days of running 9A0TCP gateway
    machine. I noticed that very often ax25d died or had to restart
    ax25d because it was not handling connections. Think this
    bind/non-accept kernel problem is very probably the reason.

    Anyone agree with me ? Can anyone make a resonable patch/fix
    for this problem ? Maybe setting skb->sk->destroy to 1 is right
    way ?

    73 de Tihomir Heidelberg, terhi.victor@logonet.com
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