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From: Thomas Gleixner <tglx@linutronix.de>
To: Miroslav Lichvar <mlichvar@redhat.com>
Cc: Justin Stitt <justinstitt@google.com>, John Stultz <jstultz@google.com>,
 Stephen Boyd <sboyd@kernel.org>, Nathan Chancellor <nathan@kernel.org>,
 Bill Wendling <morbo@google.com>, Nick Desaulniers
 <ndesaulniers@google.com>, linux-kernel@vger.kernel.org,
 llvm@lists.linux.dev, linux-hardening@vger.kernel.org
Subject: Re: [PATCH v2] ntp: remove accidental integer wrap-around
In-Reply-To: <ZlRDoyJk2Erzje_z@localhost>
References: <20240517-b4-sio-ntp-usec-v2-1-d539180f2b79@google.com>
 <87ed9re7i4.ffs@tglx> <87bk4ve5wc.ffs@tglx> <ZlRDoyJk2Erzje_z@localhost>
Date: Wed, 29 May 2024 10:18:04 +0200
Message-ID: <8734q19glf.ffs@tglx>
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On Mon, May 27 2024 at 10:26, Miroslav Lichvar wrote:
> On Fri, May 24, 2024 at 02:44:19PM +0200, Thomas Gleixner wrote:
>> On Fri, May 24 2024 at 14:09, Thomas Gleixner wrote:
>> > So instead of turning the clock back, we might be better off to actually
>> > put the normalization in place at the assignment:
>> >
>> >     time_maxerror = min(max(0, txc->maxerror), NTP_PHASE_LIMIT);
>> >
>> > or something like that.
>
> Yes, I think that's a better approach. Failing the system call could
> break existing applications, e.g. ntpd can be configured to accept a
> large root distance and it doesn't clamp the maxerror value, while
> updating the PLL offset in the same adjtimex() call.

Thanks for confirming. I suspected that, but the original change logs
are pretty useless in that regard.

>> So that commit also removed the sanity check for time_esterror, but
>> that's not doing anything in the kernel other than being reset in
>> clear_ntp() and being handed back to user space. No idea what this is
>> actually used for.
>
> It's a lower-bound estimate of the clock error, which applications can
> check if it's acceptable for them. I think it should be clamped too.
> It doesn't make much sense for it to be larger than the maximum error.

Ok.

> Another possible improvement of adjtimex() would be to set the UNSYNC
> flag immediately in the call if maxerror >= 16s to avoid the delay of
> up to 1 second for applications which check only that flag instead of
> the maxerror value.

That needs to be a seperate change.

Thanks,

        tglx