linux-kernel - Re: Why does timerfd() only support CLOCK_REALTIME and CLOCK

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Message-ID: <cfd18e0f0902081412x67f97e9eq7df94148807107c8@mail.gmail.com>
Date:	Mon, 9 Feb 2009 11:12:21 +1300
From:	Michael Kerrisk <mtk.manpages@...glemail.com>
To:	Davide Libenzi <davidel@...ilserver.org>
Cc:	"linux-man@...r.kernel.org" <linux-man@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>, stable@...nel.org,
	Greg KH <gregkh@...e.de>
Subject: Re: Why does timerfd() only support CLOCK_REALTIME and 
	CLOCK_MONOTONIC?

On Mon, Feb 9, 2009 at 11:06 AM, Davide Libenzi <davidel@...ilserver.org> wrote:
> On Mon, 9 Feb 2009, Michael Kerrisk wrote:
>
>> On Mon, Feb 9, 2009 at 10:50 AM, Davide Libenzi <davidel@...ilserver.org> wrote:
>> > On Mon, 9 Feb 2009, Michael Kerrisk wrote:
>> >
>> >> > @@ -186,12 +187,9 @@ SYSCALL_DEFINE2(timerfd_create, int, clo
>> >> >        BUILD_BUG_ON(TFD_CLOEXEC != O_CLOEXEC);
>> >> >        BUILD_BUG_ON(TFD_NONBLOCK != O_NONBLOCK);
>> >> >
>> >> > -       if (flags & ~(TFD_CLOEXEC | TFD_NONBLOCK))
>> >> > +       if ((flags & ~TFD_FLAGS_SET) ||
>> >> > +           invalid_clockid(clockid))
>> >> >                return -EINVAL;
>> >>
>> >> Oh!  Does this mean that in 2.6.2[789] it wasn't possible to use
>> >> TFD_TIMER_ABSTIME?
>> >
>> > No, sorry, my fault. Patch is wrong. In "create" ATM we accept only
>> > FCNTL-like flags. In "settime" we get TFD_TIMER_ABSTIME (that needs a
>> > check too for EINVAL).
>>
>> That last piece should be a separate patch, that also gets pushed back
>> into -stable.  Do you agree?
>
> Hmm, it's a check for extra bits that do not cause any harm. Dunno if it
> fits -stable requirements. You should ask Greg.

I agree that it's not a critical fix.  The point is of course that if
other flags are evntually added to timerfd_settime(), an application
needs a way of checking for kernel support / lack of support (a la the
recent discussion of eventfd "[patch/rfc] eventfd semaphore-like
behavior").  It seems to me it would be good to have that check work
as far back as possible in older kernels. If that check is to be added
to 2.6.29, it seems reasonable to push it back to the current -stable
kernels.

Greg?


-- 
Michael Kerrisk
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