NFSD: Support write delegations in LAYOUTGET

I noticed LAYOUTGET(LAYOUTIOMODE4_RW) returning NFS4ERR_ACCESS
unexpectedly. The NFS client had created a file with mode 0444, and
the server had returned a write delegation on the OPEN(CREATE). The
client was requesting a RW layout using the write delegation stateid
so that it could flush file modifications.

Creating a read-only file does not seem to be problematic for
NFSv4.1 without pNFS, so I began looking at NFSD's implementation of
LAYOUTGET.

The failure was because fh_verify() was doing a permission check as
part of verifying the FH presented during the LAYOUTGET. It uses the
loga_iomode value to specify the @accmode argument to fh_verify().
fh_verify(MAY_WRITE) on a file whose mode is 0444 fails with -EACCES.

To permit LAYOUT* operations in this case, add OWNER_OVERRIDE when
checking the access permission of the incoming file handle for
LAYOUTGET and LAYOUTCOMMIT.

Cc: Christoph Hellwig <[e-mail]>
Cc: [e-mail] # v6.6+
Message-Id: [e-mail]
Reviewed-by: Jeff Layton <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>
Signed-off-by: Chuck Lever <[e-mail]>

fs/nfsd/nfs4proc.c

···
 	const struct nfsd4_layout_ops *ops;
 	struct nfs4_layout_stateid *ls;
 	__be32 nfserr;
-	int accmode = NFSD_MAY_READ_IF_EXEC;
+	int accmode = NFSD_MAY_READ_IF_EXEC | NFSD_MAY_OWNER_OVERRIDE;
 
 	switch (lgp->lg_seg.iomode) {
 	case IOMODE_READ:
···
 	struct nfs4_layout_stateid *ls;
 	__be32 nfserr;
 
-	nfserr = fh_verify(rqstp, current_fh, 0, NFSD_MAY_WRITE);
+	nfserr = fh_verify(rqstp, current_fh, 0,
+			   NFSD_MAY_WRITE | NFSD_MAY_OWNER_OVERRIDE);
 	if (nfserr)
 		goto out;