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 <[email protected]>
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 <[email protected]>
>
But check already exists for 64bit arches in ktime_set()
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 <[email protected]>
>>
>
> 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().
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
Am 08.04.2013 22:20, schrieb John Stultz:
> 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?
Yes, please tag it for -stable as well because I had the problem with
kernel 3.0 and it can overflow on all current version.
Best regards
- David
> thanks
> -john
>