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SubjectRe: possible bug in tcp_input.c
On Oct-24 2003, Fri, 19:30 -0700
David S. Miller <davem@redhat.com> wrote:

> > The passed NULL (and yes, this is where we are getting one) is dereferenced
> > immediately in:
> >
> > /* tcp_input.c, line 1133 */
> > static inline int tcp_skb_timedout(struct tcp_opt *tp, struct sk_buff *skb)
> > {
> > return (tcp_time_stamp - TCP_SKB_CB(skb)->when > tp->rto);
> > }
>
> If tp->packets_out is non-zero (which by definition it is
> in your case else the right hand side of the "&&" would not be
> evaluated) then we _MUST_ have some packets in sk->write_queue.
>
> Something is being fiercely corrupted. Probably some piece of
> netfilter is freeing up an SKB one too many times thus corrupting
> the TCP write queue list pointers.

Dave, we've been thinking about this some more and have concluded
that since the problem is a relatively non-fatal one, the kernel
should just print out an "assertion failed" error similar to the
one in tcp_input.c, line 1323 [BUG_TRAP(cnt <= tp->packets_out);]
and maybe fix things up a little rather than oops on a NULL pointer
dereference; The state in question, although invalid, is possible
and should IMHO be checked for as in all the other "if (skb != NULL)
..." places).
What do you think? We keep on trying to locate which code is causing
the corruption, meanwhile the affected system has been running crash-lessly
with the attached patch.

Thanks,
--
Tomas Szepe <szepe@pinerecords.com>

diff -urN a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
--- a/net/ipv4/tcp_input.c 2003-06-13 16:51:39 +0200
+++ b/net/ipv4/tcp_input.c 2003-10-26 07:29:11 +0100
@@ -1138,7 +1138,19 @@

static inline int tcp_head_timedout(struct sock *sk, struct tcp_opt *tp)
{
- return tp->packets_out && tcp_skb_timedout(tp, skb_peek(&sk->write_queue));
+ struct sk_buff *skb = skb_peek(&sk->write_queue);
+
+ if (skb == NULL) {
+ if (tp->packets_out) {
+ printk("KERNEL: assertion (%s) failed at %s(%d)\n",
+ "skb == NULL && tp->packets_out",
+ __FILE__, __LINE__);
+ tp->packets_out = 0;
+ }
+ return 0;
+ } else {
+ return tp->packets_out && tcp_skb_timedout(tp, skb);
+ }
}

/* Linux NewReno/SACK/FACK/ECN state machine.
-
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