filelock: default to returning -EINVAL when ->setlease operation is NULL

Now that most filesystems where we expect to need lease support have
their ->setlease() operations explicitly set, change kernel_setlease()
to return -EINVAL when the setlease is a NULL pointer.

Also update the Documentation/ with info about this change.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260108-setlease-6-20-v1-23-ea4dec9b67fa@kernel.org
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: jtlayton

Documentation/filesystems/porting.rst

@@ -1334,3 +1334,12 @@ end_creating() and the parent will be unlocked precisely when necessary.
 
 kill_litter_super() is gone; convert to DCACHE_PERSISTENT use (as all
 in-tree filesystems have done).
+
+---
+
+**mandatory**
+
+The ->setlease() file_operation must now be explicitly set in order to provide
+support for leases. When set to NULL, the kernel will now return -EINVAL to
+attempts to set a lease. Filesystems that wish to use the kernel-internal lease
+implementation should set it to generic_setlease().

Documentation/filesystems/vfs.rst

@@ -1180,9 +1180,12 @@ otherwise noted.
 	method is used by the splice(2) system call
 
 ``setlease``
-	called by the VFS to set or release a file lock lease.  setlease
-	implementations should call generic_setlease to record or remove
-	the lease in the inode after setting it.
+	called by the VFS to set or release a file lock lease.  Local
+	filesystems that wish to use the kernel-internal lease implementation
+	should set this to generic_setlease(). Other setlease implementations
+	should call generic_setlease() to record or remove the lease in the inode
+	after setting it. When set to NULL, attempts to set or remove a lease will
+	return -EINVAL.
 
 ``fallocate``
 	called by the VFS to preallocate blocks or punch a hole.

fs/locks.c

@@ -2019,8 +2019,7 @@ kernel_setlease(struct file *filp, int arg, struct file_lease **lease, void **pr
 		setlease_notifier(arg, *lease);
 	if (filp->f_op->setlease)
 		return filp->f_op->setlease(filp, arg, lease, priv);
-	else
-		return generic_setlease(filp, arg, lease, priv);
+	return -EINVAL;
 }
 EXPORT_SYMBOL_GPL(kernel_setlease);