We should not create inode for disabled namespace. A disabled namespace
sets its ns->ops to NULL. Kernel could panic if we try to create a inode
for such namespace.

Here is an example oops in socket ioctl cmd SIOCGSKNS when NET_NS is
disabled. Kernel panicked wherever nsfs trys to access ns->ops since the
proc_ns_operations is not implemented in this case.

[7.670023] Unable to handle kernel NULL pointer dereference at virtual address 00000010
[7.670268] pgd = 32b54000
[7.670544] [00000010] *pgd=00000000
[7.671861] Internal error: Oops: 5 [#1] SMP ARM
[7.672315] Modules linked in:
[7.672918] CPU: 0 PID: 1 Comm: systemd Not tainted 5.13.0-rc3-00375-g6799d4f2da49 #16
[7.673309] Hardware name: Generic DT based system
[7.673642] PC is at nsfs_evict+0x24/0x30
[7.674486] LR is at clear_inode+0x20/0x9c

So let's reject such request for disabled namespace.

Signed-off-by: Changbin Du <redacted>
Cc: <redacted>
Cc: Cong Wang <redacted>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: David Laight <redacted>
---
 fs/nsfs.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/nsfs.c b/fs/nsfs.c
index 800c1d0eb0d0..6c055eb7757b 100644
--- a/fs/nsfs.c
+++ b/fs/nsfs.c

@@ -62,6 +62,10 @@ static int __ns_get_path(struct path *path, struct ns_common *ns)
 	struct inode *inode;
 	unsigned long d;
 
+	/* In case the namespace is not actually enabled. */
+	if (!ns->ops)
+		return -EOPNOTSUPP;
+
 	rcu_read_lock();
 	d = atomic_long_read(&ns->stashed);
 	if (!d)

-- 
2.30.2