2 files changed, 51 insertions, 0 deletions
diff --git a/tools/testing/selftests/bpf/progs/bpf_dctcp.c b/tools/testing/selftests/bpf/progs/bpf_dctcp.c
index fd42247da8b4..9573be6122be 100644
--- a/tools/testing/selftests/bpf/progs/bpf_dctcp.c
+++ b/tools/testing/selftests/bpf/progs/bpf_dctcp.c
@@ -17,6 +17,11 @@
 
 char _license[] SEC("license") = "GPL";
 
+volatile const char fallback[TCP_CA_NAME_MAX];
+const char bpf_dctcp[] = "bpf_dctcp";
+const char tcp_cdg[] = "cdg";
+char cc_res[TCP_CA_NAME_MAX];
+int tcp_cdg_res = 0;
 int stg_result = 0;
 
 struct {
@@ -57,6 +62,26 @@ void BPF_PROG(dctcp_init, struct sock *sk)
 	struct dctcp *ca = inet_csk_ca(sk);
 	int *stg;
 
+	if (!(tp->ecn_flags & TCP_ECN_OK) && fallback[0]) {
+		/* Switch to fallback */
+		bpf_setsockopt(sk, SOL_TCP, TCP_CONGESTION,
+			       (void *)fallback, sizeof(fallback));
+		/* Switch back to myself which the bpf trampoline
+		 * stopped calling dctcp_init recursively.
+		 */
+		bpf_setsockopt(sk, SOL_TCP, TCP_CONGESTION,
+			       (void *)bpf_dctcp, sizeof(bpf_dctcp));
+		/* Switch back to fallback */
+		bpf_setsockopt(sk, SOL_TCP, TCP_CONGESTION,
+			       (void *)fallback, sizeof(fallback));
+		/* Expecting -ENOTSUPP for tcp_cdg_res */
+		tcp_cdg_res = bpf_setsockopt(sk, SOL_TCP, TCP_CONGESTION,
+					     (void *)tcp_cdg, sizeof(tcp_cdg));
+		bpf_getsockopt(sk, SOL_TCP, TCP_CONGESTION,
+			       (void *)cc_res, sizeof(cc_res));
+		return;
+	}
+
 	ca->prior_rcv_nxt = tp->rcv_nxt;
 	ca->dctcp_alpha = min(dctcp_alpha_on_init, DCTCP_MAX_ALPHA);
 	ca->loss_cwnd = 0;
diff --git a/tools/testing/selftests/bpf/progs/bpf_dctcp_release.c b/tools/testing/selftests/bpf/progs/bpf_dctcp_release.c
new file mode 100644
index 000000000000..d836f7c372f0
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/bpf_dctcp_release.c
@@ -0,0 +1,26 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2021 Facebook */
+
+#include <stddef.h>
+#include <linux/bpf.h>
+#include <linux/types.h>
+#include <linux/stddef.h>
+#include <linux/tcp.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+#include "bpf_tcp_helpers.h"
+
+char _license[] SEC("license") = "GPL";
+const char cubic[] = "cubic";
+
+void BPF_STRUCT_OPS(dctcp_nouse_release, struct sock *sk)
+{
+	bpf_setsockopt(sk, SOL_TCP, TCP_CONGESTION,
+		       (void *)cubic, sizeof(cubic));
+}
+
+SEC(".struct_ops")
+struct tcp_congestion_ops dctcp_rel = {
+	.release	= (void *)dctcp_nouse_release,
+	.name		= "bpf_dctcp_rel",
+};