Probable Bug in tcp.h

[Bruce Evans]

On Fri, 6 Jun 2008, Marc [iso-8859-1] Lörner wrote:

> On Friday 06 June 2008 09:52, Peter Jeremy wrote:
>> I gather from this comment that you have some code using struct tcphdr
>> that is getting alignment errors.  struct tcphdr is extensively used
>> in the TCP stack within the kernel so it's likely that any layout or
>> alignment problem with it would show up there.  I suspect you are
>> dereferencing a mis-aligned struct tcphdr.
>
> The funny thing is that the dereferencing occurs in
> "/usr/src/sys/netinet/tcp_input.c" in function tcp_input in line 550:
>
> 	/*
> 	 * Check that TCP offset makes sense,
> 	 * pull out TCP options and adjust length.		XXX
> 	 */
> 	off = th->th_off << 2;								<----- here
> 	if (off < sizeof (struct tcphdr) || off > tlen) {
> 		tcpstat.tcps_rcvbadoff++;
> 		goto drop;
> 	}
>
> So the misalignment may probably lie in TCP stack?

Quite likely.  th is normally at offset off0 in ip, where ip is required
to be 32-bit aligned (see my previous reply).  You can see off0 in a
stack trace.

Bruce