path: root/kernel
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authorDavid Gibson <david@gibson.dropbear.id.au>2015-11-30 12:30:30 +1100
committerSasha Levin <sasha.levin@oracle.com>2016-02-02 13:53:36 -0500
commit9282d8b196725d802a57a757e78a3c74cccb9708 (patch)
treeb4ff6fd172e6ead94b3ed85f80ad973f22c4357a /kernel
parent8a610e047525a9b882dad09419f038fbf09f705d (diff)
time: Avoid signed overflow in timekeeping_get_ns()
[ Upstream commit 35a4933a895927990772ae96fdcfd2f806929ee2 ] 1e75fa8 "time: Condense timekeeper.xtime into xtime_sec" replaced a call to clocksource_cyc2ns() from timekeeping_get_ns() with an open-coded version of the same logic to avoid keeping a semi-redundant struct timespec in struct timekeeper. However, the commit also introduced a subtle semantic change - where clocksource_cyc2ns() uses purely unsigned math, the new version introduces a signed temporary, meaning that if (delta * tk->mult) has a 63-bit overflow the following shift will still give a negative result. The choice of 'maxsec' in __clocksource_updatefreq_scale() means this will generally happen if there's a ~10 minute pause in examining the clocksource. This can be triggered on a powerpc KVM guest by stopping it from qemu for a bit over 10 minutes. After resuming time has jumped backwards several minutes causing numerous problems (jiffies does not advance, msleep()s can be extended by minutes..). It doesn't happen on x86 KVM guests, because the guest TSC is effectively frozen while the guest is stopped, which is not the case for the powerpc timebase. Obviously an unsigned (64 bit) overflow will only take twice as long as a signed, 63-bit overflow. I don't know the time code well enough to know if that will still cause incorrect calculations, or if a 64-bit overflow is avoided elsewhere. Still, an incorrect forwards clock adjustment will cause less trouble than time going backwards. So, this patch removes the potential for intermediate signed overflow. Cc: stable@vger.kernel.org (3.7+) Suggested-by: Laurent Vivier <lvivier@redhat.com> Tested-by: Laurent Vivier <lvivier@redhat.com> Signed-off-by: David Gibson <david@gibson.dropbear.id.au> Signed-off-by: John Stultz <john.stultz@linaro.org> Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Diffstat (limited to 'kernel')
1 files changed, 1 insertions, 2 deletions
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index a4038c57e25d..c596236607ae 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -202,8 +202,7 @@ static inline s64 timekeeping_get_ns(struct tk_read_base *tkr)
/* calculate the delta since the last update_wall_time: */
delta = clocksource_delta(cycle_now, tkr->cycle_last, tkr->mask);
- nsec = delta * tkr->mult + tkr->xtime_nsec;
- nsec >>= tkr->shift;
+ nsec = (delta * tkr->mult + tkr->xtime_nsec) >> tkr->shift;
/* If arch requires, add in get_arch_timeoffset() */
return nsec + arch_gettimeoffset();