[PATCH] clean overflow checks in count_mounts() a bit

[Al Viro <viro@zeniv.linux.org.uk>]

On Mon, Feb 14, 2022 at 03:04:27AM +0000, Al Viro wrote:

> 	I don't believe it's worth the trouble.  Sure, you run that loop
> only once, instead of once per copy.  And if that's more than noise,
> compared to allocating the same mounts we'd been counting, connecting
> them into tree, hashing, etc., I would be *very* surprised.
> 
> NAKed-by: Al Viro <viro@zeniv.linux.org.uk>

BTW, speaking of count_mounts(), the wraparound checks there are somewhat
confused: x + y wraparound will lead to both x + y < x and x + y < y - no
need to check both (the value of x + y is either their sum as natural
numbers, in which case there's no wraparound and both checks are false,
or the sum minus 2^32, in which case both checks are true since both x and
y are below 2^32).

IMO more straightforward code would be better here.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
diff --git a/fs/namespace.c b/fs/namespace.c
index 13d025a9ecf5d..42d4fc21263b2 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2069,22 +2069,23 @@ static int invent_group_ids(struct mount *mnt, bool recurse)
 int count_mounts(struct mnt_namespace *ns, struct mount *mnt)
 {
 	unsigned int max = READ_ONCE(sysctl_mount_max);
-	unsigned int mounts = 0, old, pending, sum;
+	unsigned int mounts = 0;
 	struct mount *p;
 
+	if (ns->mounts >= max)
+		return -ENOSPC;
+	max -= ns->mounts;
+	if (ns->pending_mounts >= max)
+		return -ENOSPC;
+	max -= ns->pending_mounts;
+
 	for (p = mnt; p; p = next_mnt(p, mnt))
 		mounts++;
 
-	old = ns->mounts;
-	pending = ns->pending_mounts;
-	sum = old + pending;
-	if ((old > sum) ||
-	    (pending > sum) ||
-	    (max < sum) ||
-	    (mounts > (max - sum)))
+	if (mounts > max)
 		return -ENOSPC;
 
-	ns->pending_mounts = pending + mounts;
+	ns->pending_mounts += mounts;
 	return 0;
 }