Re: ADJ_OFFSET_SS_READ bug?

[john stultz <johnstul@us.ibm.com>]

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!

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

Roman: your thoughts?


Signed-off-by: John Stultz <johnstul@us.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();
 	}