Re: [PATCH] timekeeping: Change type of nsec variable to unsigned in its calculation.

From: John Stultz
Date: Mon Nov 28 2016 - 17:51:11 EST

On Fri, Nov 18, 2016 at 8:53 PM, John Stultz <john.stultz@xxxxxxxxxx> wrote:
> From: Liav Rehana <liavr@xxxxxxxxxxxx>
> During the calculation of the nsec variable in the inline function
> timekeeping_delta_to_ns, it may undergo a sign extension if its msb
> is set just before the shift. The sign extension may, in some cases,
> gain it a value near the maximum value of the 64-bit range. This is
> bad when it is later used in a division function, such as
> __iter_div_u64_rem, where the amount of loops it will go through to
> calculate the division will be too large. One can encounter such a
> problem, for example, when trying to connect through ftp from an
> outside host to the operation system. When the OS is too overloaded,
> delta will get a high enough value for the msb of the sum
> delta * tkr->mult + tkr->xtime_nsec to be set, and so after the
> shift the nsec variable will gain a value similar to
> 0xffffffffff000000. Using a variable with such a value in the
> inline function __iter_div_u64_rem will take too long, making the
> ftp connection attempt seem to get stuck.
> The following commit fixes that chance of sign extension, while
> maintaining the type of the nsec variable as signed for other
> functions that use this variable, for possible legit negative
> time intervals.
> Cc: Chris Metcalf <cmetcalf@xxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Richard Cochran <richardcochran@xxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Prarit Bhargava <prarit@xxxxxxxxxx>
> Cc: Laurent Vivier <lvivier@xxxxxxxxxx>
> Cc: David Gibson <david@xxxxxxxxxxxxxxxxxxxxx>
> Cc: "Christopher S . Hall" <christopher.s.hall@xxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx (4.6+)
> Fixes: 6bd58f09e1d8 ("time: Add cycles to nanoseconds translation")
> Also-Reported-by: Chris Metcalf <cmetcalf@xxxxxxxxxxxx>
> Signed-off-by: Liav Rehana <liavr@xxxxxxxxxxxx>
> Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx>
> ---
> Thomas/Ingo: This is for tip:timers/urgent.

Hey Thomas, Ingo,
I just wanted to follow up to make sure this wasn't missed last
time. Should be applied against tip/timers/urgent.