selftests/bpf: Verify ca_name of struct mptcp_sock

This patch verifies another member of struct mptcp_sock, ca_name. Add a
new function get_msk_ca_name() to read the sysctl tcp_congestion_control
and verify it in verify_msk().

v3: Access the sysctl through the filesystem to avoid compatibility
issues with the busybox sysctl command.

v4: use ASSERT_* instead of CHECK_FAIL (Andrii)

v5: use ASSERT_STRNEQ() instead of strncmp() (Andrii)

Signed-off-by: Geliang Tang <[e-mail]>
Signed-off-by: Mat Martineau <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Acked-by: Matthieu Baerts <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/bpf_tcp_helpers.h

···
 #define SOL_TCP 6
 #endif
 
+#ifndef TCP_CA_NAME_MAX
+#define TCP_CA_NAME_MAX	16
+#endif
+
 #define tcp_jiffies32 ((__u32)bpf_jiffies64())
 
 struct sock_common {
···
 	struct inet_connection_sock	sk;
 
 	__u32		token;
+	char		ca_name[TCP_CA_NAME_MAX];
 } __attribute__((preserve_access_index));
 
 #endif

tools/testing/selftests/bpf/prog_tests/mptcp.c

···
 #include "network_helpers.h"
 #include "mptcp_sock.skel.h"
 
+#ifndef TCP_CA_NAME_MAX
+#define TCP_CA_NAME_MAX	16
+#endif
+
 struct mptcp_storage {
 	__u32 invoked;
 	__u32 is_mptcp;
 	__u32 token;
+	char ca_name[TCP_CA_NAME_MAX];
 };
 
 static int verify_tsk(int map_fd, int client_fd)
···
 	return err;
 }
 
+static void get_msk_ca_name(char ca_name[])
+{
+	size_t len;
+	int fd;
+
+	fd = open("/proc/sys/net/ipv4/tcp_congestion_control", O_RDONLY);
+	if (!ASSERT_GE(fd, 0, "failed to open tcp_congestion_control"))
+		return;
+
+	len = read(fd, ca_name, TCP_CA_NAME_MAX);
+	if (!ASSERT_GT(len, 0, "failed to read ca_name"))
+		goto err;
+
+	if (len > 0 && ca_name[len - 1] == '\n')
+		ca_name[len - 1] = '\0';
+
+err:
+	close(fd);
+}
+
 static int verify_msk(int map_fd, int client_fd, __u32 token)
 {
+	char ca_name[TCP_CA_NAME_MAX];
 	int err, cfd = client_fd;
 	struct mptcp_storage val;
 
 	if (!ASSERT_GT(token, 0, "invalid token"))
 		return -1;
+
+	get_msk_ca_name(ca_name);
 
 	err = bpf_map_lookup_elem(map_fd, &cfd, &val);
 	if (!ASSERT_OK(err, "bpf_map_lookup_elem"))
···
 		err++;
 
 	if (!ASSERT_EQ(val.token, token, "unexpected token"))
+		err++;
+
+	if (!ASSERT_STRNEQ(val.ca_name, ca_name, TCP_CA_NAME_MAX, "unexpected ca_name"))
 		err++;
 
 	return err;

tools/testing/selftests/bpf/progs/mptcp_sock.c

···
 	__u32 invoked;
 	__u32 is_mptcp;
 	__u32 token;
+	char ca_name[TCP_CA_NAME_MAX];
 };
 
 struct {
···
 			return 1;
 
 		storage->token = 0;
+		__builtin_memset(storage->ca_name, 0, TCP_CA_NAME_MAX);
 	} else {
 		msk = bpf_skc_to_mptcp_sock(sk);
 		if (!msk)
···
 			return 1;
 
 		storage->token = msk->token;
+		__builtin_memcpy(storage->ca_name, msk->ca_name, TCP_CA_NAME_MAX);
 	}
 	storage->invoked++;
 	storage->is_mptcp = is_mptcp;