linux-kernel - Re: kernel/time/ntp.c: Possible off-by-one error in TAI range check?

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Message-ID: <20190415085641.GI28203@localhost>
Date:   Mon, 15 Apr 2019 10:56:41 +0200
From:   Miroslav Lichvar <mlichvar@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Ondrej Mosnacek <omosnace@...hat.com>,
        Roman Zippel <zippel@...ux-m68k.org>,
        John Stultz <john.stultz@...aro.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux kernel mailing list <linux-kernel@...r.kernel.org>
Subject: Re: kernel/time/ntp.c: Possible off-by-one error in TAI range check?

On Mon, Apr 15, 2019 at 10:09:43AM +0200, Thomas Gleixner wrote:
> > On Mon, Apr 8, 2019 at 10:47 AM Ondrej Mosnacek <omosnace@...hat.com> wrote:
> > > Commit 153b5d054ac2 ("ntp: support for TAI") added a possibility to
> > > change the TAI offset from userspace via adjtimex(2). The code checks
> > > if the input value (txc->constant) is greater than 0 and if it is not,
> > > then it doesn't modify the value. Ignoring the fact that this check
> > > should probably be in timekeeping_validate_timex() and cause -EINVAL
> > > to be returned when false, I find it strange that the check doesn't
> > > allow to set the value to 0, which seems to be the default value...
> > >
> > > Was this behavior intended or should the code actually check for
> > > txc->constant >= 0 instead of txc->constant > 0?

I guess zero here means "unknown" and maybe the intention was to not
allow setting the offset to an unknown value once it has been set to a
valid value. The trouble is that after inserting a leap second the
offset may change from zero to one.

I think it should be changed to allow setting the offset to zero.

-- 
Miroslav Lichvar