NFSv4/pNFS: Fix another issue with a list iterator pointing to the head

In nfs4_callback_devicenotify(), if we don't find a matching entry for
the deviceid, we're left with a pointer to 'struct nfs_server' that
actually points to the list of super blocks associated with our struct
nfs_client.
Furthermore, even if we have a valid pointer, nothing pins the super
block, and so the struct nfs_server could end up getting freed while
we're using it.

Since all we want is a pointer to the struct pnfs_layoutdriver_type,
let's skip all the iteration over super blocks, and just use APIs to
find the layout driver directly.

Reported-by: Xiaomeng Tong <xiam0nd.tong@gmail.com>
Fixes: 1be5683 ("pnfs: CB_NOTIFY_DEVICEID")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/callback_proc.c

@@ -358,36 +358,27 @@ __be32 nfs4_callback_devicenotify(void *argp, void *resp,
 				  struct cb_process_state *cps)
 {
 	struct cb_devicenotifyargs *args = argp;
+	const struct pnfs_layoutdriver_type *ld = NULL;
 	uint32_t i;
 	__be32 res = 0;
-	struct nfs_client *clp = cps->clp;
-	struct nfs_server *server = NULL;
 
-	if (!clp) {
+	if (!cps->clp) {
 		res = cpu_to_be32(NFS4ERR_OP_NOT_IN_SESSION);
 		goto out;
 	}
 
 	for (i = 0; i < args->ndevs; i++) {
 		struct cb_devicenotifyitem *dev = &args->devs[i];
 
-		if (!server ||
-		    server->pnfs_curr_ld->id != dev->cbd_layout_type) {
-			rcu_read_lock();
-			list_for_each_entry_rcu(server, &clp->cl_superblocks, client_link)
-				if (server->pnfs_curr_ld &&
-				    server->pnfs_curr_ld->id == dev->cbd_layout_type) {
-					rcu_read_unlock();
-					goto found;
-				}
-			rcu_read_unlock();
-			continue;
+		if (!ld || ld->id != dev->cbd_layout_type) {
+			pnfs_put_layoutdriver(ld);
+			ld = pnfs_find_layoutdriver(dev->cbd_layout_type);
+			if (!ld)
+				continue;
 		}
-
-	found:
-		nfs4_delete_deviceid(server->pnfs_curr_ld, clp, &dev->cbd_dev_id);
+		nfs4_delete_deviceid(ld, cps->clp, &dev->cbd_dev_id);
 	}
-
+	pnfs_put_layoutdriver(ld);
 out:
 	kfree(args->devs);
 	return res;

fs/nfs/pnfs.c

@@ -92,6 +92,17 @@ find_pnfs_driver(u32 id)
 	return local;
 }
 
+const struct pnfs_layoutdriver_type *pnfs_find_layoutdriver(u32 id)
+{
+	return find_pnfs_driver(id);
+}
+
+void pnfs_put_layoutdriver(const struct pnfs_layoutdriver_type *ld)
+{
+	if (ld)
+		module_put(ld->owner);
+}
+
 void
 unset_pnfs_layoutdriver(struct nfs_server *nfss)
 {

fs/nfs/pnfs.h

@@ -234,6 +234,8 @@ struct pnfs_devicelist {
 
 extern int pnfs_register_layoutdriver(struct pnfs_layoutdriver_type *);
 extern void pnfs_unregister_layoutdriver(struct pnfs_layoutdriver_type *);
+extern const struct pnfs_layoutdriver_type *pnfs_find_layoutdriver(u32 id);
+extern void pnfs_put_layoutdriver(const struct pnfs_layoutdriver_type *ld);
 
 /* nfs4proc.c */
 extern size_t max_response_pages(struct nfs_server *server);