ipc/sem: make semctl setting sempid consistent

As indicated by bug#112271, Linux sets the sempid value upon semctl, and
not only for semop calls.  However, within semctl we only do this for
SETVAL, leaving SETALL without updating the field, and therefore rather
inconsistent behavior when compared to other Unices.

There is really no documentation regarding this and therefore users
should not make assumptions.  With this patch, along with updating
semctl.2 manpages, this scenario should become less ambiguous As such,
set sempid on SETALL cmd.

Also update some in-code documentation, specifying where the sempid is
set.

Passes ltp and custom testcase where a child (fork) does SETALL to the
set.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Reported-by: Philip Semanchuk <linux_kernel.20.ick@spamgourmet.com>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: PrasannaKumar Muralidharan <prasannatsmkumar@gmail.com>
Cc: Manfred Spraul <manfred@colorfullife.com>
Cc: Herton R. Krzesinski <herton@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

ipc/sem.c

@@ -92,7 +92,14 @@
 /* One semaphore structure for each semaphore in the system. */
 struct sem {
 	int	semval;		/* current value */
-	int	sempid;		/* pid of last operation */
+	/*
+	 * PID of the process that last modified the semaphore. For
+	 * Linux, specifically these are:
+	 *  - semop
+	 *  - semctl, via SETVAL and SETALL.
+	 *  - at task exit when performing undo adjustments (see exit_sem).
+	 */
+	int	sempid;
 	spinlock_t	lock;	/* spinlock for fine-grained semtimedop */
 	struct list_head pending_alter; /* pending single-sop operations */
 					/* that alter the semaphore */
@@ -1444,8 +1451,10 @@ static int semctl_main(struct ipc_namespace *ns, int semid, int semnum,
 			goto out_unlock;
 		}
 
-		for (i = 0; i < nsems; i++)
+		for (i = 0; i < nsems; i++) {
 			sma->sem_base[i].semval = sem_io[i];
+			sma->sem_base[i].sempid = task_tgid_vnr(current);
+		}
 
 		ipc_assert_locked_object(&sma->sem_perm);
 		list_for_each_entry(un, &sma->list_id, list_id) {