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

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Message-ID: <alpine.DEB.2.20.1611291520070.4358@nanos>
Date:   Tue, 29 Nov 2016 15:22:17 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     John Stultz <john.stultz@...aro.org>
cc:     lkml <linux-kernel@...r.kernel.org>,
        Liav Rehana <liavr@...lanox.com>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Richard Cochran <richardcochran@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        Prarit Bhargava <prarit@...hat.com>,
        Laurent Vivier <lvivier@...hat.com>,
        David Gibson <david@...son.dropbear.id.au>,
        "Christopher S . Hall" <christopher.s.hall@...el.com>,
        "4.6+" <stable@...r.kernel.org>
Subject: Re: [PATCH] timekeeping: Change type of nsec variable to unsigned
 in its calculation.

On Fri, 18 Nov 2016, John Stultz wrote:
> From: Liav Rehana <liavr@...lanox.com>
> 
> 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.
>
> Thomas/Ingo: This is for tip:timers/urgent.

Certainly not! My objections against this still stand. See:

http://lkml.kernel.org/r/alpine.DEB.2.20.1609261956160.4915@nanos

http://lkml.kernel.org/r/alpine.DEB.2.20.1609270929170.4891@nanos

If we have legitimate use cases with a negative delta, then this patch
breaks them no matter what. See the basic C course section in the second
link.

Thanks,

	tglx