nmap UDP scan against 8.0-CURRENT -> fatal trap 12

[Rick Macklem]

On Sun, 9 Aug 2009, Thomas Backman wrote:

[stuff snipped]
>> --- xdr/xdr_mbuf.c.sav	2009-08-07 15:02:35.000000000 -0400
>> +++ xdr/xdr_mbuf.c	2009-08-07 15:03:04.000000000 -0400
>> @@ -282,6 +282,8 @@
>> 	size_t available;
>> 	char *p;
>> 
>> +	if (!m)
>> +		return (0);
>> 	if (xdrs->x_op == XDR_ENCODE) {
>> 		available = M_TRAILINGSPACE(m) + (m->m_len - xdrs->x_handy);
>> 	} else {
>> 
>
> Initial results are certainly good! :-)
> Pre-patch, it panicked three times in a row, as I said within a few seconds. 
> Post-patch I've looped the simpler scan for a while (10 minutes, or about 8-9 
> runs) with no crash, and I also ran the more extensive one (which I doubt 
> makes any difference...) once.
> Just for fun, I tried actually using nfsd while looping the scan, too. No 
> problems.
>
Ok, sounds good. It's already in the re@ queue, so it should make it into
8.0. If it does crap out again, please let the list (and me) know.

Thanks for testing the patch, rick
ps: Thanks mostly goes to pho@ for his "wicked" test scripts that found
     the crash that the above patch fixes + a bunch of others.