Messages in this thread | | | Date | Tue, 19 Mar 2013 13:53:55 +0100 | From | David Engraf <> | Subject | Re: [PATCH] ktime_add_ns() may overflow on 32bit architectures |
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Am 19.03.2013 13:38, schrieb Eric Dumazet: > On Tue, 2013-03-19 at 13:29 +0100, 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). >> >> Best regards >> - David >> >> Signed-off-by: David Engraf <david.engraf@sysgo.com> >> > > 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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