From 8f61059a96c2a29c1cc5a39dfe23d06ef5b4b065 Mon Sep 17 00:00:00 2001 From: Daniel Borkmann Date: Mon, 30 Jun 2014 13:52:08 +0200 Subject: [PATCH 1/2] net: sctp: improve timer slack calculation for transport HBs RFC4960, section 8.3 says: On an idle destination address that is allowed to heartbeat, it is recommended that a HEARTBEAT chunk is sent once per RTO of that destination address plus the protocol parameter 'HB.interval', with jittering of +/- 50% of the RTO value, and exponential backoff of the RTO if the previous HEARTBEAT is unanswered. Currently, we calculate jitter via sctp_jitter() function first, and then add its result to the current RTO for the new timeout: TMO = RTO + (RAND() % RTO) - (RTO / 2) `------------------------^-=> sctp_jitter() Instead, we can just simplify all this by directly calculating: TMO = (RTO / 2) + (RAND() % RTO) With the help of prandom_u32_max(), we don't need to open code our own global PRNG, but can instead just make use of the per CPU implementation of prandom with better quality numbers. Also, we can now spare us the conditional for divide by zero check since no div or mod operation needs to be used. Note that prandom_u32_max() won't emit the same result as a mod operation, but we really don't care here as we only want to have a random number scaled into RTO interval. Note, exponential RTO backoff is handeled elsewhere, namely in sctp_do_8_2_transport_strike(). Signed-off-by: Daniel Borkmann Signed-off-by: David S. Miller --- include/net/sctp/sctp.h | 21 --------------------- net/sctp/transport.c | 17 +++++++++-------- 2 files changed, 9 insertions(+), 29 deletions(-) diff --git a/include/net/sctp/sctp.h b/include/net/sctp/sctp.h index 8e4de46c052ec..c2035c96a2ee1 100644 --- a/include/net/sctp/sctp.h +++ b/include/net/sctp/sctp.h @@ -388,27 +388,6 @@ static inline int sctp_list_single_entry(struct list_head *head) return (head->next != head) && (head->next == head->prev); } -/* Generate a random jitter in the range of -50% ~ +50% of input RTO. */ -static inline __s32 sctp_jitter(__u32 rto) -{ - static __u32 sctp_rand; - __s32 ret; - - /* Avoid divide by zero. */ - if (!rto) - rto = 1; - - sctp_rand += jiffies; - sctp_rand ^= (sctp_rand << 12); - sctp_rand ^= (sctp_rand >> 20); - - /* Choose random number from 0 to rto, then move to -50% ~ +50% - * of rto. - */ - ret = sctp_rand % rto - (rto >> 1); - return ret; -} - /* Break down data chunks at this point. */ static inline int sctp_frag_point(const struct sctp_association *asoc, int pmtu) { diff --git a/net/sctp/transport.c b/net/sctp/transport.c index 7dd672fa651f9..b10e047bbd155 100644 --- a/net/sctp/transport.c +++ b/net/sctp/transport.c @@ -594,15 +594,16 @@ void sctp_transport_burst_reset(struct sctp_transport *t) } /* What is the next timeout value for this transport? */ -unsigned long sctp_transport_timeout(struct sctp_transport *t) +unsigned long sctp_transport_timeout(struct sctp_transport *trans) { - unsigned long timeout; - timeout = t->rto + sctp_jitter(t->rto); - if ((t->state != SCTP_UNCONFIRMED) && - (t->state != SCTP_PF)) - timeout += t->hbinterval; - timeout += jiffies; - return timeout; + /* RTO + timer slack +/- 50% of RTO */ + unsigned long timeout = (trans->rto >> 1) + prandom_u32_max(trans->rto); + + if (trans->state != SCTP_UNCONFIRMED && + trans->state != SCTP_PF) + timeout += trans->hbinterval; + + return timeout + jiffies; } /* Reset transport variables to their initial values */ From eaea2da7286ebc56d557b40ad7e59e715a84e4a0 Mon Sep 17 00:00:00 2001 From: Daniel Borkmann Date: Mon, 30 Jun 2014 13:52:09 +0200 Subject: [PATCH 2/2] net: sctp: only warn in proc_sctp_do_alpha_beta if write Only warn if the value is written to alpha or beta. We don't care emitting a one-time warning when only reading it. Reported-by: Jiri Pirko Signed-off-by: Daniel Borkmann Reviewed-by: Jiri Pirko Signed-off-by: David S. Miller --- net/sctp/sysctl.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/net/sctp/sysctl.c b/net/sctp/sysctl.c index 12c7e01c26771..2e9ada10fd846 100644 --- a/net/sctp/sysctl.c +++ b/net/sctp/sysctl.c @@ -424,8 +424,9 @@ static int proc_sctp_do_alpha_beta(struct ctl_table *ctl, int write, void __user *buffer, size_t *lenp, loff_t *ppos) { - pr_warn_once("Changing rto_alpha or rto_beta may lead to " - "suboptimal rtt/srtt estimations!\n"); + if (write) + pr_warn_once("Changing rto_alpha or rto_beta may lead to " + "suboptimal rtt/srtt estimations!\n"); return proc_dointvec_minmax(ctl, write, buffer, lenp, ppos); }