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Avoid unneeded call to arc4random() in syncache_add()

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Don't call arc4random() unconditionally to initialize sc_iss, and
then when syncookies are enabled, just overwrite it with the
return value from from syncookie_generate(). Instead, only call
arc4random() to initialize sc_iss when syncookies are not
enabled.

Note that on a system under a syn flood attack, arc4random()
becomes quite expensive, and the chacha_poly crypto that it calls
is one of the more expensive things happening on the
system. Removing this unneeded arc4random() call reduces CPU from
about 40% to about 35% in my test scenario (Broadwell Xeon, 6Mpps
syn flood attack).

Reviewed by:	rrs, tuxen, bz
Sponsored by:	Netflix
Differential Revision:	https://reviews.freebsd.org/D21591
This commit is contained in:
Andrew Gallatin 2019-09-11 18:48:26 +00:00
parent dfcf2d87dd
commit d2e6258258
1 changed files with 2 additions and 1 deletions
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3
sys/netinet/tcp_syncache.c
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@ -1543,7 +1543,6 @@ syncache_add(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
sc->sc_todctx = todctx;
#endif
sc->sc_irs = th->th_seq;
sc->sc_iss = arc4random();
sc->sc_flags = 0;
sc->sc_flowlabel = 0;
@ -1617,6 +1616,8 @@ syncache_add(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
if (V_tcp_syncookies)
sc->sc_iss = syncookie_generate(sch, sc);
else
sc->sc_iss = arc4random();
#ifdef INET6
if (autoflowlabel) {
if (V_tcp_syncookies)