linux-kernel - ADJ_OFFSET_SS

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Date:	Sun, 22 Jun 2008 09:32:23 +0200
From:	Michael Kerrisk <mtk.manpages@...il.com>
To:	Roman Zippel <zippel@...ux-m68k.org>
CC:	lkml <linux-kernel@...r.kernel.org>,
	john stultz <johnstul@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>
Subject: ADJ_OFFSET_SS_READ bug?

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?

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?

Cheers,

Michael

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