NFSD: da_addr_body field missing in some GETDEVICEINFO replies

The XDR specification in RFC 8881 looks like this:

struct device_addr4 {
	layouttype4	da_layout_type;
	opaque		da_addr_body<>;
};

struct GETDEVICEINFO4resok {
	device_addr4	gdir_device_addr;
	bitmap4		gdir_notification;
};

union GETDEVICEINFO4res switch (nfsstat4 gdir_status) {
case NFS4_OK:
	GETDEVICEINFO4resok gdir_resok4;
case NFS4ERR_TOOSMALL:
	count4		gdir_mincount;
default:
	void;
};

Looking at nfsd4_encode_getdeviceinfo() ....

When the client provides a zero gd_maxcount, then the Linux NFS
server implementation encodes the da_layout_type field and then
skips the da_addr_body field completely, proceeding directly to
encode gdir_notification field.

There does not appear to be an option in the specification to skip
encoding da_addr_body. Moreover, Section 18.40.3 says:

> If the client wants to just update or turn off notifications, it
> MAY send a GETDEVICEINFO operation with gdia_maxcount set to zero.
> In that event, if the device ID is valid, the reply's da_addr_body
> field of the gdir_device_addr field will be of zero length.

Since the layout drivers are responsible for encoding the
da_addr_body field, put this fix inside the ->encode_getdeviceinfo
methods.

Fixes: 9cf514cc ("nfsd: implement pNFS operations")
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Tom Haynes <loghyr@gmail.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/blocklayoutxdr.c

@@ -83,6 +83,15 @@ nfsd4_block_encode_getdeviceinfo(struct xdr_stream *xdr,
 	int len = sizeof(__be32), ret, i;
 	__be32 *p;
 
+	/*
+	 * See paragraph 5 of RFC 8881 S18.40.3.
+	 */
+	if (!gdp->gd_maxcount) {
+		if (xdr_stream_encode_u32(xdr, 0) != XDR_UNIT)
+			return nfserr_resource;
+		return nfs_ok;
+	}
+
 	p = xdr_reserve_space(xdr, len + sizeof(__be32));
 	if (!p)
 		return nfserr_resource;

fs/nfsd/flexfilelayoutxdr.c

@@ -85,6 +85,15 @@ nfsd4_ff_encode_getdeviceinfo(struct xdr_stream *xdr,
 	int addr_len;
 	__be32 *p;
 
+	/*
+	 * See paragraph 5 of RFC 8881 S18.40.3.
+	 */
+	if (!gdp->gd_maxcount) {
+		if (xdr_stream_encode_u32(xdr, 0) != XDR_UNIT)
+			return nfserr_resource;
+		return nfs_ok;
+	}
+
 	/* len + padding for two strings */
 	addr_len = 16 + da->netaddr.netid_len + da->netaddr.addr_len;
 	ver_len = 20;

fs/nfsd/nfs4xdr.c

@@ -4686,20 +4686,17 @@ nfsd4_encode_getdeviceinfo(struct nfsd4_compoundres *resp, __be32 nfserr,
 
 	*p++ = cpu_to_be32(gdev->gd_layout_type);
 
-	/* If maxcount is 0 then just update notifications */
-	if (gdev->gd_maxcount != 0) {
-		ops = nfsd4_layout_ops[gdev->gd_layout_type];
-		nfserr = ops->encode_getdeviceinfo(xdr, gdev);
-		if (nfserr) {
-			/*
-			 * We don't bother to burden the layout drivers with
-			 * enforcing gd_maxcount, just tell the client to
-			 * come back with a bigger buffer if it's not enough.
-			 */
-			if (xdr->buf->len + 4 > gdev->gd_maxcount)
-				goto toosmall;
-			return nfserr;
-		}
+	ops = nfsd4_layout_ops[gdev->gd_layout_type];
+	nfserr = ops->encode_getdeviceinfo(xdr, gdev);
+	if (nfserr) {
+		/*
+		 * We don't bother to burden the layout drivers with
+		 * enforcing gd_maxcount, just tell the client to
+		 * come back with a bigger buffer if it's not enough.
+		 */
+		if (xdr->buf->len + 4 > gdev->gd_maxcount)
+			goto toosmall;
+		return nfserr;
 	}
 
 	if (gdev->gd_notify_types) {