selftests/bpf: extend sockopt_sk selftest with TCP_CONGESTION use case

Ignore SOL_TCP:TCP_CONGESTION in getsockopt and always override
SOL_TCP:TCP_CONGESTION with "cubic" in setsockopt hook.

Call setsockopt(SOL_TCP, TCP_CONGESTION) with short optval ("nv")
to make sure BPF program has enough buffer space to replace it
with "cubic".
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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

 // SPDX-License-Identifier: GPL-2.0
+#include <string.h>
 #include <netinet/in.h>
+#include <netinet/tcp.h>
 #include <linux/bpf.h>
 #include "bpf_helpers.h"
 
@@ -42,6 +44,14 @@ int _getsockopt(struct bpf_sockopt *ctx)
 		return 1;
 	}
 
+	if (ctx->level == SOL_TCP && ctx->optname == TCP_CONGESTION) {
+		/* Not interested in SOL_TCP:TCP_CONGESTION;
+		 * let next BPF program in the cgroup chain or kernel
+		 * handle it.
+		 */
+		return 1;
+	}
+
 	if (ctx->level != SOL_CUSTOM)
 		return 0; /* EPERM, deny everything except custom level */
 
@@ -91,6 +101,18 @@ int _setsockopt(struct bpf_sockopt *ctx)
 		return 1;
 	}
 
+	if (ctx->level == SOL_TCP && ctx->optname == TCP_CONGESTION) {
+		/* Always use cubic */
+
+		if (optval + 5 > optval_end)
+			return 0; /* EPERM, bounds check */
+
+		memcpy(optval, "cubic", 5);
+		ctx->optlen = 5;
+
+		return 1;
+	}
+
 	if (ctx->level != SOL_CUSTOM)
 		return 0; /* EPERM, deny everything except custom level */
 

tools/testing/selftests/bpf/test_sockopt_sk.c

@@ -6,6 +6,7 @@
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <netinet/in.h>
+#include <netinet/tcp.h>
 
 #include <linux/filter.h>
 #include <bpf/bpf.h>
@@ -25,6 +26,7 @@ static int getsetsockopt(void)
 	union {
 		char u8[4];
 		__u32 u32;
+		char cc[16]; /* TCP_CA_NAME_MAX */
 	} buf = {};
 	socklen_t optlen;
 
@@ -115,6 +117,29 @@ static int getsetsockopt(void)
 		goto err;
 	}
 
+	/* TCP_CONGESTION can extend the string */
+
+	strcpy(buf.cc, "nv");
+	err = setsockopt(fd, SOL_TCP, TCP_CONGESTION, &buf, strlen("nv"));
+	if (err) {
+		log_err("Failed to call setsockopt(TCP_CONGESTION)");
+		goto err;
+	}
+
+
+	optlen = sizeof(buf.cc);
+	err = getsockopt(fd, SOL_TCP, TCP_CONGESTION, &buf, &optlen);
+	if (err) {
+		log_err("Failed to call getsockopt(TCP_CONGESTION)");
+		goto err;
+	}
+
+	if (strcmp(buf.cc, "cubic") != 0) {
+		log_err("Unexpected getsockopt(TCP_CONGESTION) %s != %s",
+			buf.cc, "cubic");
+		goto err;
+	}
+
 	close(fd);
 	return 0;
 err: