tree ea7a64dd1bf5b00e5c6c1bb4c709ac51aa3f7675
parent 729abd2ba7007cc2be9e77718ba52d0866d3f60f
author Petr Mladek <pmladek@suse.cz> 1390871220 -0800
committer Linus Torvalds <torvalds@linux-foundation.org> 1390885359 -0800

ipc/sem.c: avoid overflow of semop undo (semadj) value

When trying to understand semop code, I found a small mistake in the check
for semadj (undo) value overflow.  The new undo value is not stored
immediately and next potential checks are done against the old value.

The failing scenario is not much practical.  One semop call has to do more
operations on the same semaphore.  Also semval and semadj must have
different values, so there has to be some operations without SEM_UNDO
flag.  For example:

	struct sembuf depositor_op[1];
	struct sembuf collector_op[2];

	depositor_op[0].sem_num = 0;
	depositor_op[0].sem_op = 20000;
	depositor_op[0].sem_flg = 0;

	collector_op[0].sem_num = 0;
	collector_op[0].sem_op = -10000;
	collector_op[0].sem_flg = SEM_UNDO;
	collector_op[1].sem_num = 0;
	collector_op[1].sem_op = -10000;
	collector_op[1].sem_flg = SEM_UNDO;

	if (semop(semid, depositor_op, 1) == -1)
		{ perror("Failed to do 1st deposit"); return 1; }

	if (semop(semid, collector_op, 2) == -1)
		{ perror("Failed to do 1st collect"); return 1; }

	if (semop(semid, depositor_op, 1) == -1)
		{ perror("Failed to do 2nd deposit"); return 1; }

	if (semop(semid, collector_op, 2) == -1)
		{ perror("Failed to do 2nd collect"); return 1; }

	return 0;

It passes without error now but the semadj value has overflown in the 2nd
collector operation.

[akpm@linux-foundation.org: restore lessened scope of local `undo']
[davidlohr@hp.com: correct header comment for perform_atomic_semop]
Signed-off-by: Petr Mladek <pmladek@suse.cz>
Acked-by: Davidlohr Bueso <davidlohr@hp.com>
Acked-by: Manfred Spraul <manfred@colorfullife.com>
Cc: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>