linux-kernel - Re: ADJ_OFFSET_SS

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Date:	Mon, 21 Jul 2008 12:36:56 +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 3:30 AM, Michael Kerrisk
<mtk.manpages@...glemail.com> wrote:
> 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>


John,

Are you pushing this into 2.6.27-rc1?

Cheers,

Michael

>> 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/
> man-pages online: http://www.kernel.org/doc/man-pages/online_pages.html
> Found a bug? http://www.kernel.org/doc/man-pages/reporting_bugs.html
>



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
man-pages online: http://www.kernel.org/doc/man-pages/online_pages.html
Found a bug? http://www.kernel.org/doc/man-pages/reporting_bugs.html
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