selftests/bpf: Support af_unix SOCK_DGRAM socket pair creation

Handle af_unix in init_addr_loopback(). For pair creation, bind() the peer
socket to make SOCK_DGRAM connect() happy.

Signed-off-by: Michal Luczaj <mhal@rbox.co>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/r/20250515-selftests-sockmap-redir-v3-1-a1ea723f7e7e@rbox.co

Author: mmhal

tools/testing/selftests/bpf/prog_tests/socket_helpers.h

@@ -3,6 +3,7 @@
 #ifndef __SOCKET_HELPERS__
 #define __SOCKET_HELPERS__
 
+#include <sys/un.h>
 #include <linux/vm_sockets.h>
 
 /* include/linux/net.h */
@@ -169,6 +170,15 @@ static inline void init_addr_loopback6(struct sockaddr_storage *ss,
 	*len = sizeof(*addr6);
 }
 
+static inline void init_addr_loopback_unix(struct sockaddr_storage *ss,
+					   socklen_t *len)
+{
+	struct sockaddr_un *addr = memset(ss, 0, sizeof(*ss));
+
+	addr->sun_family = AF_UNIX;
+	*len = sizeof(sa_family_t);
+}
+
 static inline void init_addr_loopback_vsock(struct sockaddr_storage *ss,
 					    socklen_t *len)
 {
@@ -190,6 +200,9 @@ static inline void init_addr_loopback(int family, struct sockaddr_storage *ss,
 	case AF_INET6:
 		init_addr_loopback6(ss, len);
 		return;
+	case AF_UNIX:
+		init_addr_loopback_unix(ss, len);
+		return;
 	case AF_VSOCK:
 		init_addr_loopback_vsock(ss, len);
 		return;
@@ -315,21 +328,27 @@ static inline int create_pair(int family, int sotype, int *p0, int *p1)
 {
 	__close_fd int s, c = -1, p = -1;
 	struct sockaddr_storage addr;
-	socklen_t len = sizeof(addr);
+	socklen_t len;
 	int err;
 
 	s = socket_loopback(family, sotype);
 	if (s < 0)
 		return s;
 
-	err = xgetsockname(s, sockaddr(&addr), &len);
-	if (err)
-		return err;
-
 	c = xsocket(family, sotype, 0);
 	if (c < 0)
 		return c;
 
+	init_addr_loopback(family, &addr, &len);
+	err = xbind(c, sockaddr(&addr), len);
+	if (err)
+		return err;
+
+	len = sizeof(addr);
+	err = xgetsockname(s, sockaddr(&addr), &len);
+	if (err)
+		return err;
+
 	err = connect(c, sockaddr(&addr), len);
 	if (err) {
 		if (errno != EINPROGRESS) {