nfsd: Ensure nfsv4 calls the underlying filesystem on LOCKT

Since nfsv4 allows LOCKT without an open, but the ->lock() method is a
file method, we fake up a struct file in the nfsv4 code with just the
fields we need initialized.  But we forgot to initialize the file
operations, with the result that LOCKT never results in a call to the
filesystem's ->lock() method (if it exists).

We could just add that one more initialization.  But this hack of faking
up a struct file with only some fields initialized seems the kind of
thing that might cause more problems in the future.  We should either do
an open and get a real struct file, or make lock-testing an inode (not a
file) method.

This patch does the former.
Reported-by: NMarc Eshel <eshel@almaden.ibm.com>
Tested-by: NMarc Eshel <eshel@almaden.ibm.com>
Signed-off-by: NJ. Bruce Fields <bfields@citi.umich.edu>

fs/nfsd/nfs4state.c

@@ -2781,6 +2781,25 @@ nfsd4_lock(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
 	return status;
 }
 
+/*
+ * The NFSv4 spec allows a client to do a LOCKT without holding an OPEN,
+ * so we do a temporary open here just to get an open file to pass to
+ * vfs_test_lock.  (Arguably perhaps test_lock should be done with an
+ * inode operation.)
+ */
+static int nfsd_test_lock(struct svc_rqst *rqstp, struct svc_fh *fhp, struct file_lock *lock)
+{
+	struct file *file;
+	int err;
+
+	err = nfsd_open(rqstp, fhp, S_IFREG, NFSD_MAY_READ, &file);
+	if (err)
+		return err;
+	err = vfs_test_lock(file, lock);
+	nfsd_close(file);
+	return err;
+}
+
 /*
  * LOCKT operation
  */
@@ -2789,7 +2808,6 @@ nfsd4_lockt(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
 	    struct nfsd4_lockt *lockt)
 {
 	struct inode *inode;
-	struct file file;
 	struct file_lock file_lock;
 	int error;
 	__be32 status;
@@ -2847,16 +2865,8 @@ nfsd4_lockt(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
 
 	nfs4_transform_lock_offset(&file_lock);
 
-	/* vfs_test_lock uses the struct file _only_ to resolve the inode.
-	 * since LOCKT doesn't require an OPEN, and therefore a struct
-	 * file may not exist, pass vfs_test_lock a struct file with
-	 * only the dentry:inode set.
-	 */
-	memset(&file, 0, sizeof (struct file));
-	file.f_path.dentry = cstate->current_fh.fh_dentry;
-
 	status = nfs_ok;
-	error = vfs_test_lock(&file, &file_lock);
+	error = nfsd_test_lock(rqstp, &cstate->current_fh, &file_lock);
 	if (error) {
 		status = nfserrno(error);
 		goto out;