time: add ADJ_OFFSET_SS_READ

Michael Kerrisk reported that a long standing bug in the adjtimex()
system call causes glibc's adjtime(3) function to deliver the wrong
results if 'delta' is NULL.

add the ADJ_OFFSET_SS_READ API detail, which will be used by glibc
to fix this API compatibility bug.

Also see: http://bugzilla.kernel.org/show_bug.cgi?id=6761

[ mingo@elte.hu: added patch description and made it backwards compatible ]

NOTE: the new flag is defined 0xa001 so that it returns -EINVAL on
older kernels - this way glibc can use it safely. Suggested by Ulrich
Drepper.
Acked-by: Ulrich Drepper <drepper@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

include/linux/timex.h

@@ -137,6 +137,7 @@ struct timex {
 #define ADJ_TIMECONST		0x0020	/* pll time constant */
 #define ADJ_TICK		0x4000	/* tick value */
 #define ADJ_OFFSET_SINGLESHOT	0x8001	/* old-fashioned adjtime */
+#define ADJ_OFFSET_SS_READ	0xa001  /* read-only adjtime */
 
 /* xntp 3.4 compatibility names */
 #define MOD_OFFSET	ADJ_OFFSET

kernel/time/ntp.c

@@ -249,10 +249,12 @@ int do_adjtimex(struct timex *txc)
 
 	/* Now we validate the data before disabling interrupts */
 
-	if ((txc->modes & ADJ_OFFSET_SINGLESHOT) == ADJ_OFFSET_SINGLESHOT)
+	if ((txc->modes & ADJ_OFFSET_SINGLESHOT) == ADJ_OFFSET_SINGLESHOT) {
 	  /* singleshot must not be used with any other mode bits */
-		if (txc->modes != ADJ_OFFSET_SINGLESHOT)
+		if (txc->modes != ADJ_OFFSET_SINGLESHOT &&
+					txc->modes != ADJ_OFFSET_SS_READ)
 			return -EINVAL;
+	}
 
 	if (txc->modes != ADJ_OFFSET_SINGLESHOT && (txc->modes & ADJ_OFFSET))
 	  /* adjustment Offset limited to +- .512 seconds */
@@ -372,7 +374,8 @@ int do_adjtimex(struct timex *txc)
 leave:	if ((time_status & (STA_UNSYNC|STA_CLOCKERR)) != 0)
 		result = TIME_ERROR;
 
-	if ((txc->modes & ADJ_OFFSET_SINGLESHOT) == ADJ_OFFSET_SINGLESHOT)
+	if ((txc->modes == ADJ_OFFSET_SINGLESHOT) ||
+			(txc->modes == ADJ_OFFSET_SS_READ))
 		txc->offset = save_adjust;
 	else
 		txc->offset = ((long)shift_right(time_offset, SHIFT_UPDATE)) *