sctp: don’t compare hb_timer expire date before starting it [Linux 5.0]

sctp: don’t compare hb_timer expire date before starting it [Linux 5.0]

This Linux kernel change "sctp: don’t compare hb_timer expire date before starting it" is included in the Linux 5.0 release. This change is authored by Maciej Kwiecien <maciej.kwiecien [at] nokia.com> on Fri Feb 22 09:45:26 2019 +0100. The commit for this change in Linux stable tree is d1f20c0 (patch).

sctp: don't compare hb_timer expire date before starting it

hb_timer might not start at all for a particular transport because its
start is conditional. In a result a node is not sending heartbeats.

Function sctp_transport_reset_hb_timer has two roles:
    - initial start of hb_timer for a given transport,
    - update expire date of hb_timer for a given transport.
The function is optimized to update timer's expire only if it is before
a new calculated one but this comparison is invalid for a timer which
has not yet started. Such a timer has expire == 0 and if a new expire
value is bigger than (MAX_JIFFIES / 2 + 2) then "time_before" macro will
fail and timer will not start resulting in no heartbeat packets send by
the node.

This was found when association was initialized within first 5 mins
after system boot due to jiffies init value which is near to MAX_JIFFIES.

Test kernel version: 4.9.154 (ARCH=arm)
hb_timer.expire = 0;                //initialized, not started timer
new_expire = MAX_JIFFIES / 2 + 2;   //or more
time_before(hb_timer.expire, new_expire) == false

Fixes: ba6f5e33bdbb ("sctp: avoid refreshing heartbeat timer too often")
Reported-by: Marcin Stojek <[email protected]>
Tested-by: Marcin Stojek <[email protected]>
Signed-off-by: Maciej Kwiecien <[email protected]>
Reviewed-by: Alexander Sverdlin <[email protected]>
Acked-by: Marcelo Ricardo Leitner <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

There are 3 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 net/sctp/transport.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/sctp/transport.c b/net/sctp/transport.c
index 033696e..ad158d3 100644
--- a/net/sctp/transport.c
+++ b/net/sctp/transport.c
@@ -207,7 +207,8 @@ void sctp_transport_reset_hb_timer(struct sctp_transport *transport)

    /* When a data chunk is sent, reset the heartbeat interval.  */
    expires = jiffies + sctp_transport_timeout(transport);
-   if (time_before(transport->hb_timer.expires, expires) &&
+   if ((time_before(transport->hb_timer.expires, expires) ||
+        !timer_pending(&transport->hb_timer)) &&
        !mod_timer(&transport->hb_timer,
               expires + prandom_u32_max(transport->rto)))
        sctp_transport_hold(transport);

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