sunrpc: Enable the kernel to specify the hostname part of service principals

A multi-homed NFS server may have more than one "nfs" key in its
keytab. Enable the kernel to pick the key it wants as a machine
credential when establishing a GSS context.

This is useful for GSS-protected NFSv4.0 callbacks, which are
required by RFC 7530 S3.3.3 to use the same principal as the service
principal the client used when establishing its lease.

A complementary modification to rpc.gssd is required to fully enable
this feature.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

net/sunrpc/auth_gss/auth_gss.c

@@ -284,7 +284,12 @@ err:
 	return p;
 }
 
-#define UPCALL_BUF_LEN 128
+/* XXX: Need some documentation about why UPCALL_BUF_LEN is so small.
+ *	Is user space expecting no more than UPCALL_BUF_LEN bytes?
+ *	Note that there are now _two_ NI_MAXHOST sized data items
+ *	being passed in this string.
+ */
+#define UPCALL_BUF_LEN	256
 
 struct gss_upcall_msg {
 	refcount_t count;
@@ -462,8 +467,17 @@ static int gss_encode_v1_msg(struct gss_upcall_msg *gss_msg,
 		p += len;
 		gss_msg->msg.len += len;
 	}
-	if (service_name != NULL) {
-		len = scnprintf(p, buflen, "service=%s ", service_name);
+	if (service_name) {
+		char *c = strchr(service_name, '@');
+
+		if (!c)
+			len = scnprintf(p, buflen, "service=%s ",
+					service_name);
+		else
+			len = scnprintf(p, buflen,
+					"service=%.*s srchost=%s ",
+					(int)(c - service_name),
+					service_name, c + 1);
 		buflen -= len;
 		p += len;
 		gss_msg->msg.len += len;