kernfs: fix ino wrap-around detection

commit e23f568aa63f64cd6b355094224cc9356c0f696b upstream.

When the 32bit ino wraps around, kernfs increments the generation
number to distinguish reused ino instances.  The wrap-around detection
tests whether the allocated ino is lower than what the cursor but the
cursor is pointing to the next ino to allocate so the condition never
triggers.

Fix it by remembering the last ino and comparing against that.
Signed-off-by: NTejun Heo <tj@kernel.org>
Reviewed-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Fixes: 4a3ef68a ("kernfs: implement i_generation")
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: stable@vger.kernel.org # v4.14+
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/kernfs/dir.c

@@ -624,7 +624,6 @@ static struct kernfs_node *__kernfs_new_node(struct kernfs_root *root,
 {
 	struct kernfs_node *kn;
 	u32 gen;
-	int cursor;
 	int ret;
 
 	name = kstrdup_const(name, GFP_KERNEL);
@@ -637,11 +636,11 @@ static struct kernfs_node *__kernfs_new_node(struct kernfs_root *root,
 
 	idr_preload(GFP_KERNEL);
 	spin_lock(&kernfs_idr_lock);
-	cursor = idr_get_cursor(&root->ino_idr);
 	ret = idr_alloc_cyclic(&root->ino_idr, kn, 1, 0, GFP_ATOMIC);
-	if (ret >= 0 && ret < cursor)
+	if (ret >= 0 && ret < root->last_ino)
 		root->next_generation++;
 	gen = root->next_generation;
+	root->last_ino = ret;
 	spin_unlock(&kernfs_idr_lock);
 	idr_preload_end();
 	if (ret < 0)

include/linux/kernfs.h

@@ -186,6 +186,7 @@ struct kernfs_root {
 
 	/* private fields, do not use outside kernfs proper */
 	struct idr		ino_idr;
+	u32			last_ino;
 	u32			next_generation;
 	struct kernfs_syscall_ops *syscall_ops;