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

From: John Stultz
Date: Mon Apr 08 2013 - 16:20:13 EST


On 03/19/2013 05:29 AM, David Engraf wrote:
Hello,

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 CONFIG_KTIME_SCALAR (e.g. ARM, x86-32).

Sorry, this fell through the cracks. I see Andrew caught it, but I've queued for 3.10 in my tree as well.

This should be tagged for -stable as well, no?

thanks
-john

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