I edited the original change to retain the use of arc4random() as a seed for
the hashing as a very basic defense against intentional lagg port selection.
The author's original commit message (edited slightly):
sys/net/ieee8023ad_lacp.c
sys/net/if_lagg.c
In lagg_hashmbuf, use the FNV hash instead of the old
hash32_buf. The hash32 family of functions operate one octet
at a time, and when run on a string s of length n, their output
is equivalent to :
----- i=n-1
\
n \ (n-i-1) 32
( seed^ + / 33^ * s[i] ) % 2^
/
----- i=0
The problem is that the last five bytes of input don't get
multiplied by sufficiently many powers of 33 to rollover 2^32.
That means that changing the last few bytes (but obviously not
the very last) of input will always change the value of the
hash by a multiple of 33. In the case of lagg_hashmbuf() with
ipv4 input, the last four bytes are the TCP or UDP port
numbers. Since the output of lagg_hashmbuf is always taken
modulo the port count, and 3 is a common port count for a lagg,
that's bad. It means that the UDP or TCP source port will
never affect which lagg member is selected on a 3-port lagg.
At 10Gbps, I was not able to measure any difference in CPU
consumption between the old and new hash.
Submitted by: asomers (original commit)
Reviewed by: emaste, glebius
MFC after: 1 week
Sponsored by: Spectra Logic
MFSpectraBSD: 1001723 on 2013/08/28 (original)
1114258 on 2015/01/22 (edit)
0e5f55bb95