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>

2 files changed, 7 insertions, 3 deletions

diff --git a/include/linux/timex.h b/include/linux/timex.h
index 37ac3ff90faf..24c6a2b59511 100644
--- a/include/linux/timex.h
+++ b/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
diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c
index 14a2ecf2b318..e64efaf957e8 100644
--- a/kernel/time/ntp.c
+++ b/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)) *