ipc/mqueue: correct mq_attr_ok test

While working on the other parts of the mqueue stuff, I noticed that the
calculation for overflow in mq_attr_ok didn't actually match reality (this
is especially true since my last patch which changed how we account memory
slightly).

In particular, we used to test for overflow using:
  msgs * msgsize + msgs * sizeof(struct msg_msg *)

That was never really correct because each message we allocate via
load_msg() is actually a struct msg_msg followed by the data for the
message (and if struct msg_msg + data exceeds PAGE_SIZE we end up
allocating struct msg_msgseg structs too, but accounting for them would
get really tedious, so let's ignore those...they're only a pointer in size
anyway).  This patch updates the calculation to be more accurate in
regards to maximum possible memory consumption by the mqueue.

[[email protected]: add a local to simplify overflow-checking expression]
Signed-off-by: Doug Ledford <[email protected]>
Cc: Stephen Rothwell <[email protected]>
Cc: Manfred Spraul <[email protected]>
Acked-by: KOSAKI Motohiro <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

ipc/mqueue.c

@@ -676,6 +676,9 @@ static void remove_notification(struct mqueue_inode_info *info)
 
 static int mq_attr_ok(struct ipc_namespace *ipc_ns, struct mq_attr *attr)
 {
+	int mq_treesize;
+	unsigned long total_size;
+
 	if (attr->mq_maxmsg <= 0 || attr->mq_msgsize <= 0)
 		return 0;
 	if (capable(CAP_SYS_RESOURCE)) {
@@ -690,9 +693,11 @@ static int mq_attr_ok(struct ipc_namespace *ipc_ns, struct mq_attr *attr)
 	/* check for overflow */
 	if (attr->mq_msgsize > ULONG_MAX/attr->mq_maxmsg)
 		return 0;
-	if ((unsigned long)(attr->mq_maxmsg * (attr->mq_msgsize
-	    + sizeof (struct msg_msg *))) <
-	    (unsigned long)(attr->mq_maxmsg * attr->mq_msgsize))
+	mq_treesize = attr->mq_maxmsg * sizeof(struct msg_msg) +
+		min_t(unsigned int, attr->mq_maxmsg, MQ_PRIO_MAX) *
+		sizeof(struct posix_msg_tree_node);
+	total_size = attr->mq_maxmsg * attr->mq_msgsize;
+	if (total_size + mq_treesize < total_size)
 		return 0;
 	return 1;
 }