Re: [PATCH] ktime_add_ns() may overflow on 32bit architectures

From: David Engraf
Date: Tue Mar 19 2013 - 08:54:01 EST

Am 19.03.2013 13:38, schrieb Eric Dumazet:
On Tue, 2013-03-19 at 13:29 +0100, David Engraf wrote:

I've triggered an overflow when using ktime_add_ns() on a 32bit
architecture not supporting CONFIG_KTIME_SCALAR.

When passing a very high value for u64 nsec, e.g. 7881299347898368000
the do_div() function converts this value to seconds (7881299347) which
is still to high to pass to the ktime_set() function as long. The result
in my case is a negative value.

The problem on my system occurs in the tick-sched.c,
tick_nohz_stop_sched_tick() when time_delta is set to
timekeeping_max_deferment(). The check for time_delta < KTIME_MAX is
valid, thus ktime_add_ns() is called with a too large value resulting in
a negative expire value. This leads to an endless loop in the ticker code:

time_delta: 7881299347898368000
expires = ktime_add_ns(last_update, time_delta)
expires: negative value

This error doesn't occurs on 64bit or architectures supporting

Best regards
- David

Signed-off-by: David Engraf <david.engraf@xxxxxxxxx>

But check already exists for 64bit arches in ktime_set()

Yes, but not for 32bit arches. 64-bit arches doesn't run into this problem because ktime_add_ns() can directly calculate the result without calling do_div() and ktime_set().

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