freebsd5: replace ucred with xucred

FreeBSD 5 introduced the `xucred` struct for external users in commit
c0511d3b58c985e17c0c23585ea7d16b6f52fe61 to avoid breaking user space
applications when the `ucred` struct changes.

oidentd happened to still work after this commit because the EUID it
reads was stored at the same location in both structures. This is no
longer the case with FreeBSD 13.

This commit updates the freebsd5 module to use `xucred` instead of
`ucred` as the expected return type for getcred calls.

src/kernel/freebsd5.c

@@ -17,9 +17,6 @@
 ** Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 */
 
-#define _GNU_SOURCE
-#define _WANT_UCRED
-
 #include <config.h>
 
 #include <unistd.h>
@@ -72,12 +69,12 @@ uid_t get_user4(	in_port_t lport,
 				struct sockaddr_storage *laddr,
 				struct sockaddr_storage *faddr)
 {
-	struct ucred ucred;
+	struct xucred xuc;
 	struct sockaddr_in sin4[2];
 	size_t len;
 	int ret;
 
-	len = sizeof(struct ucred);
+	len = sizeof(xuc);
 
 	memset(sin4, 0, sizeof(sin4));
 
@@ -94,14 +91,20 @@ uid_t get_user4(	in_port_t lport,
 		sin4[1].sin_addr.s_addr = SIN4(faddr)->sin_addr.s_addr;
 
 	ret = sysctlbyname("net.inet.tcp.getcred",
-		&ucred, &len, sin4, sizeof(sin4));
+		&xuc, &len, sin4, sizeof(sin4));
 
 	if (ret == -1) {
 		debug("sysctlbyname: %s", strerror(errno));
 		return MISSING_UID;
 	}
 
-	return ucred.cr_uid;
+	if (xuc.cr_version != XUCRED_VERSION) {
+		debug("kernel is using xucred version %u, expected %u",
+			xuc.cr_version, XUCRED_VERSION);
+		return MISSING_UID;
+	}
+
+	return xuc.cr_uid;
 }
 
 #if WANT_IPV6
@@ -116,12 +119,12 @@ uid_t get_user6(	in_port_t lport,
 				struct sockaddr_storage *laddr,
 				struct sockaddr_storage *faddr)
 {
-	struct ucred ucred;
+	struct xucred xuc;
 	struct sockaddr_in6 sin6[2];
 	size_t len;
 	int ret;
 
-	len = sizeof(struct ucred);
+	len = sizeof(xuc);
 
 	memset(sin6, 0, sizeof(sin6));
 
@@ -138,14 +141,20 @@ uid_t get_user6(	in_port_t lport,
 		sizeof(sin6[1].sin6_addr));
 
 	ret = sysctlbyname("net.inet6.tcp6.getcred",
-			&ucred, &len, sin6, sizeof(sin6));
+		&xuc, &len, sin6, sizeof(sin6));
 
 	if (ret == -1) {
 		debug("sysctlbyname: %s", strerror(errno));
 		return MISSING_UID;
 	}
 
-	return ucred.cr_uid;
+	if (xuc.cr_version != XUCRED_VERSION) {
+		debug("kernel is using xucred version %u, expected %u",
+			xuc.cr_version, XUCRED_VERSION);
+		return MISSING_UID;
+	}
+
+	return xuc.cr_uid;
 }
 
 #endif