[patch 5/7] ipc/util.c: sysvipc_find_ipc() incorrectly updates position index

[Andrew Morton <akpm@linux-foundation.org>]

From: Vasily Averin <vvs@virtuozzo.com>
Subject: ipc/util.c: sysvipc_find_ipc() incorrectly updates position index

Commit 89163f93c6f9 ("ipc/util.c: sysvipc_find_ipc() should increase
position index") is causing this bug (seen on 5.6.8):

# ipcs -q

------ Message Queues --------
key        msqid      owner      perms      used-bytes   messages    

# ipcmk -Q
Message queue id: 0
# ipcs -q

------ Message Queues --------
key        msqid      owner      perms      used-bytes   messages    
0x82db8127 0          root       644        0            0           

# ipcmk -Q
Message queue id: 1
# ipcs -q

------ Message Queues --------
key        msqid      owner      perms      used-bytes   messages    
0x82db8127 0          root       644        0            0           
0x76d1fb2a 1          root       644        0            0           

# ipcrm -q 0
# ipcs -q

------ Message Queues --------
key        msqid      owner      perms      used-bytes   messages    
0x76d1fb2a 1          root       644        0            0           
0x76d1fb2a 1          root       644        0            0           

# ipcmk -Q
Message queue id: 2
# ipcrm -q 2
# ipcs -q

------ Message Queues --------
key        msqid      owner      perms      used-bytes   messages    
0x76d1fb2a 1          root       644        0            0           
0x76d1fb2a 1          root       644        0            0           

# ipcmk -Q
Message queue id: 3
# ipcrm -q 1
# ipcs -q

------ Message Queues --------
key        msqid      owner      perms      used-bytes   messages    
0x7c982867 3          root       644        0            0           
0x7c982867 3          root       644        0            0           
0x7c982867 3          root       644        0            0           
0x7c982867 3          root       644        0            0           


Whenever an IPC item with a low id is deleted, the items with higher ids
are duplicated, as if filling a hole.

new_pos should jump through hole of unused ids, pos can be updated inside
"for" cycle.

Link: http://lkml.kernel.org/r/4921fe9b-9385-a2b4-1dc4-1099be6d2e39@virtuozzo.com
Fixes: 89163f93c6f9 ("ipc/util.c: sysvipc_find_ipc() should increase position index")
Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Reported-by: Andreas Schwab <schwab@suse.de>
Acked-by: Waiman Long <longman@redhat.com>
Cc: NeilBrown <neilb@suse.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Oberparleiter <oberpar@linux.ibm.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Manfred Spraul <manfred@colorfullife.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 ipc/util.c |   12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

--- a/ipc/util.c~ipc-utilc-sysvipc_find_ipc-incorrectly-updates-position-index
+++ a/ipc/util.c
@@ -764,21 +764,21 @@ static struct kern_ipc_perm *sysvipc_fin
 			total++;
 	}
 
-	*new_pos = pos + 1;
+	ipc = NULL;
 	if (total >= ids->in_use)
-		return NULL;
+		goto out;
 
 	for (; pos < ipc_mni; pos++) {
 		ipc = idr_find(&ids->ipcs_idr, pos);
 		if (ipc != NULL) {
 			rcu_read_lock();
 			ipc_lock_object(ipc);
-			return ipc;
+			break;
 		}
 	}
-
-	/* Out of range - return NULL to terminate iteration */
-	return NULL;
+out:
+	*new_pos = pos + 1;
+	return ipc;
 }
 
 static void *sysvipc_proc_next(struct seq_file *s, void *it, loff_t *pos)
_