selftests/bpf: Add backlog for network_helper_opts

Some callers expect __start_server() helper to pass their own "backlog"
value to listen() instead of the default of 1. So this patch adds struct
member "backlog" for network_helper_opts to allow callers to set "backlog"
value via start_server_str() helper.

listen(fd, 0 /* backlog */) can be used to enforce syncookie. Meaning
backlog 0 is a legit value.

Using 0 as a default and changing it to 1 here is fine. It makes the test
program easier to write for the common case. Enforcing syncookie mode by
using backlog 0 is a niche use case but it should at least have a way for
the caller to do that. Thus, -ve backlog value is used here for the
syncookie use case. Please see the comment in network_helpers.h for
the details.
Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Link: https://lore.kernel.org/r/1660229659b66eaad07aa2126e9c9fe217eba0dd.1720515893.git.tanggeliang@kylinos.cnSigned-off-by: Martin KaFai Lau <martin.lau@kernel.org>

tools/testing/selftests/bpf/network_helpers.c

@@ -106,7 +106,7 @@ static int __start_server(int type, const struct sockaddr *addr, socklen_t addrl
 	}
 
 	if (type == SOCK_STREAM) {
-		if (listen(fd, 1) < 0) {
+		if (listen(fd, opts->backlog ? MAX(opts->backlog, 0) : 1) < 0) {
 			log_err("Failed to listed on socket");
 			goto error_close;
 		}

tools/testing/selftests/bpf/network_helpers.h

@@ -25,6 +25,16 @@ struct network_helper_opts {
 	int timeout_ms;
 	bool must_fail;
 	int proto;
+	/* +ve: Passed to listen() as-is.
+	 *   0: Default when the test does not set
+	 *      a particular value during the struct init.
+	 *      It is changed to 1 before passing to listen().
+	 *      Most tests only have one on-going connection.
+	 * -ve: It is changed to 0 before passing to listen().
+	 *      It is useful to force syncookie without
+	 *	changing the "tcp_syncookies" sysctl from 1 to 2.
+	 */
+	int backlog;
 	int (*post_socket_cb)(int fd, void *opts);
 	void *cb_opts;
 };