[Dnsmasq-discuss] Hang when system time is changed backwards

[Simon Kelley]

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Using monotonic time seems to be a bit of a portability nightmare, as
an alternative, I've added code to count the number of zero returns
from select(). The standard wait is three seconds, so once select()
has returned zero 12 times, the loop ends even if the result of
gettimeofday() doesn't end it. Unless the net is unfeasibly busy
(ending every select call with a received packet) then  that should
put a sensible limit of the waiting time.

Code in the git repo now, if you could test it.


Cheers,

Simon.

On 29/06/15 14:00, Jacob Siverskog wrote:
> Hi! It seems like dnsmasq may occasionally hang (=become
> unresponsive, unkillable with "killall dnsmasq" (-9 works), doesn't
> seem to answer on DHCP requests etc) if the system time is changed
> backwards.
> 
> I ran strace on dnsmasq once when this happened: root at OD-11:~#
> strace -p 1645 -f -tt -s 1024 Process 1645 attached 12:12:08.502148
> select(18, [6 7 8 9 10 11 15 16 17], [], NULL, {0, 46327}) = 0
> (Timeout) 12:12:08.552953 gettimeofday({1435572728, 553933}, NULL)
> = 0 12:12:08.555642 select(18, [6 7 8 9 10 11 15 16 17], [], NULL,
> {0, 250000}) = 0 (Timeout) 12:12:08.808748
> gettimeofday({1435572728, 809379}, NULL) = 0 12:12:08.810668
> select(18, [6 7 8 9 10 11 15 16 17], [], NULL, {0, 250000}) = 0
> (Timeout) 12:12:09.063807 gettimeofday({1435572729, 65256}, NULL) =
> 0 12:12:09.066343 select(18, [6 7 8 9 10 11 15 16 17], [], NULL,
> {0, 250000}) = 0 (Timeout) 12:12:09.319034
> gettimeofday({1435572729, 320590}, NULL) = 0 12:12:09.321021
> select(18, [6 7 8 9 10 11 15 16 17], [], NULL, {0, 250000}) = 0
> (Timeout)
> 
> I grep:ed for select:s in the source code and my guess is that it's
> the icmp_ping() select loop in dnsmasq.c that gets confused.
> Perhaps monotonic time should be used in this for loop: for (now =
> start = dnsmasq_time(); difftime(now, start) < (float)PING_WAIT;)
> 
> I manually set the time forward a few hours and dnsmasq released 
> immediately: root at SYSTEM:~# date -s "2015-06-29 14:14:56" Mon Jun
> 29 14:14:56 CEST 2015 root at SYSTEM:~# Mon Jun 29 14:14:56 2015
> daemon.info dnsmasq-dhcp[1645]: DHCPDISCOVER(wlan0-work)
> 8a:33:14:0f:3b:d8 Mon Jun 29 14:14:56 2015 daemon.info
> dnsmasq-dhcp[1645]: DHCPOFFER(wlan0-work) 192.168.7.146
> 8a:33:14:0f:3b:d8
> 
> I'm using dnsmasq 2.72.
> 
> Thanks, Jacob
> 
> 
> 
> _______________________________________________ Dnsmasq-discuss
> mailing list Dnsmasq-discuss at lists.thekelleys.org.uk 
> http://lists.thekelleys.org.uk/mailman/listinfo/dnsmasq-discuss
> 
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