linux-kernel - Re: ADJ_OFFSET_SS

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Date:	Tue, 1 Jul 2008 03:30:24 +0200
From:	"Michael Kerrisk" <mtk.manpages@...glemail.com>
To:	"john stultz" <johnstul@...ibm.com>
Cc:	"Michael Kerrisk" <mtk.manpages@...il.com>,
	"Roman Zippel" <zippel@...ux-m68k.org>,
	lkml <linux-kernel@...r.kernel.org>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"Ingo Molnar" <mingo@...e.hu>
Subject: Re: ADJ_OFFSET_SS_READ bug?

On Tue, Jul 1, 2008 at 12:07 AM, john stultz <johnstul@...ibm.com> wrote:
>
> On Sun, 2008-06-22 at 09:32 +0200, Michael Kerrisk wrote:
>> Roman, John
>>
>> John, thanks for ADJ_OFFSET_SS_READ, which fixed my bug report
>> (http://sourceware.org/bugzilla/show_bug?id=2449,
>> http://bugzilla.kernel.org/show_bug.cgi?id=6761)
>>
>> Roman, thanks for fixing John's fix ;-)
>>
>> However, I'm wondering if there is a potential bug in the
>> implementation of this flag.  Note the following definitions
>> from include/linux/timex.h:
>>
>> #define ADJ_OFFSET              0x0001  /* time offset */
>> [...]
>> #define ADJ_OFFSET_SINGLESHOT   0x8001  /* old-fashioned adjtime */
>> #define ADJ_OFFSET_SS_READ      0xa001  /* read-only adjtime */
>>
>>
>> Using the the above value for ADJ_OFFSET_SS_READ, where the bits match those
>> in ADJ_OFFSET and ADJ_OFFSET_SINGLESHOT, seems unnecessary as far as I can
>> see.  Why was that done?
>
> Hrm. My original fix was to use 0x2000, but from the commit Ingo changed
> it at Ulrich's suggestion. Had something to do with old glibc's doing
> the right thing?
>
>> More to the point, it looks like it creates a bug, since the "read-only
>> adjtime" triggers the code path for ADJ_OFFSET:
>>
>>          if (txc->modes) {
>>                  ...
>>                  if (txc->modes & ADJ_OFFSET) {
>>                          if (txc->modes == ADJ_OFFSET_SINGLESHOT)
>>                                  /* adjtime() is independent from ntp_adjtime() */
>>                                  time_adjust = txc->offset;
>>                          else
>>                                  ntp_update_offset(txc->offset); /*XXX*/
>>                  }
>>                  if (txc->modes & ADJ_TICK)
>>                          tick_usec = txc->tick;
>>
>>                  if (txc->modes & (ADJ_TICK|ADJ_FREQUENCY|ADJ_OFFSET))
>>                          ntp_update_frequency(); /*XXX*/
>>          }
>>
>> Unless I misunderstood something, ADJ_OFFSET_SS_READ causes the code marked
>> XXX to be executed, but I don't think that is what is desired.  Is that true?
>
> Yea. That does look like an issue. Thanks for the close inspection and
> review!

You're welcome -- thanks for getting back to me (I was beginning to
wonder if my mail got dropped somewhere)/

> Sort of a quick off the cuff patch, but does the following look like the
> right fix to you?

I haven't tested this, but given your statement about maintaining old
glibc behavior, this looks like the riht fix, so:

Acked-by: Michael Kerrisk <mtk.manpages@...il.com>

> Roman: your thoughts?
>
>
> Signed-off-by: John Stultz <johnstul@...ibm.com>
>
> diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c
> index 5125ddd..7842a8d 100644
> --- a/kernel/time/ntp.c
> +++ b/kernel/time/ntp.c
> @@ -379,13 +379,14 @@ int do_adjtimex(struct timex *txc)
>                        if (txc->modes == ADJ_OFFSET_SINGLESHOT)
>                                /* adjtime() is independent from ntp_adjtime() */
>                                time_adjust = txc->offset;
> -                       else
> +                       else if (txc->modes != ADJ_OFFSET_SS_READ)
>                                ntp_update_offset(txc->offset);
>                }
>                if (txc->modes & ADJ_TICK)
>                        tick_usec = txc->tick;
>
> -               if (txc->modes & (ADJ_TICK|ADJ_FREQUENCY|ADJ_OFFSET))
> +               if ((txc->modes & (ADJ_TICK|ADJ_FREQUENCY|ADJ_OFFSET)) &&
> +                               (txc->modes != ADJ_OFFSET_SS_READ))
>                        ntp_update_frequency();
>        }
>
>
>
>



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
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