[PATCH mptcp-next 2/2] selftests: mptcp: bpf: exercise the subflow avoid flag

Add a BPF struct_ops scheduler selftest for the new
mptcp_subflow_set_avoid() kfunc. The scheduler marks every subflow except
the first as "avoid", then schedules only a subflow whose avoid flag is
clear. The existing send_data_and_verify() harness confirms the avoided
path carried no data, exercising the kfunc round-trip from eBPF: set the
flag, read it back, and honour it.

This also adds the BPF-side extern declaration for
mptcp_subflow_set_avoid(); without it a BPF program cannot compile a call
to the kfunc, as the kernel-side BTF registration alone is not enough.

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/349
Signed-off-by: Kalpan Jani <[email protected]>
---
 .../testing/selftests/bpf/prog_tests/mptcp.c  | 15 ++++++
 tools/testing/selftests/bpf/progs/mptcp_bpf.h |  2 +
 .../selftests/bpf/progs/mptcp_bpf_avoid.c     | 49 +++++++++++++++++++
 3 files changed, 66 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/progs/mptcp_bpf_avoid.c

diff --git a/tools/testing/selftests/bpf/prog_tests/mptcp.c b/tools/testing/selftests/bpf/prog_tests/mptcp.c
index 7f48fd9e94e1..1b88806a7a86 100644
--- a/tools/testing/selftests/bpf/prog_tests/mptcp.c
+++ b/tools/testing/selftests/bpf/prog_tests/mptcp.c
@@ -17,6 +17,7 @@
 #include "mptcp_bpf_bkup.skel.h"
 #include "mptcp_bpf_rr.skel.h"
 #include "mptcp_bpf_red.skel.h"
+#include "mptcp_bpf_avoid.skel.h"
 #include "mptcp_bpf_burst.skel.h"
 
 #define NS_TEST "mptcp_ns"
@@ -813,6 +814,18 @@ static void test_burst(void)
 	mptcp_bpf_burst__destroy(skel);
 }
 
+static void test_avoid(void)
+{
+	struct mptcp_bpf_avoid *skel;
+
+	skel = mptcp_bpf_avoid__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "open_and_load: avoid"))
+		return;
+
+	test_bpf_sched(skel->maps.avoid, "avoid", WITH_DATA, WITHOUT_DATA);
+	mptcp_bpf_avoid__destroy(skel);
+}
+
 void test_mptcp(void)
 {
 	if (test__start_subtest("base"))
@@ -835,4 +848,6 @@ void test_mptcp(void)
 		test_red();
 	if (test__start_subtest("burst"))
 		test_burst();
+	if (test__start_subtest("avoid"))
+		test_avoid();
 }
diff --git a/tools/testing/selftests/bpf/progs/mptcp_bpf.h b/tools/testing/selftests/bpf/progs/mptcp_bpf.h
index de450b2eaf0f..a60b66762274 100644
--- a/tools/testing/selftests/bpf/progs/mptcp_bpf.h
+++ b/tools/testing/selftests/bpf/progs/mptcp_bpf.h
@@ -48,5 +48,7 @@ bpf_mptcp_subflow_tcp_sock(const struct mptcp_subflow_context *subflow) __ksym;
 
 extern void mptcp_subflow_set_scheduled(struct mptcp_subflow_context *subflow,
 					bool scheduled) __ksym;
+extern void mptcp_subflow_set_avoid(struct mptcp_subflow_context *subflow,
+				    bool avoid) __ksym;
 
 #endif
diff --git a/tools/testing/selftests/bpf/progs/mptcp_bpf_avoid.c b/tools/testing/selftests/bpf/progs/mptcp_bpf_avoid.c
new file mode 100644
index 000000000000..3678781f346a
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/mptcp_bpf_avoid.c
@@ -0,0 +1,49 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026, MPTCP. */
+
+#include "mptcp_bpf.h"
+#include <bpf/bpf_tracing.h>
+
+char _license[] SEC("license") = "GPL";
+
+SEC("struct_ops")
+void BPF_PROG(mptcp_sched_avoid_init, struct mptcp_sock *msk)
+{
+}
+
+SEC("struct_ops")
+void BPF_PROG(mptcp_sched_avoid_release, struct mptcp_sock *msk)
+{
+}
+
+SEC("struct_ops")
+int BPF_PROG(bpf_avoid_get_send, struct mptcp_sock *msk)
+{
+	struct mptcp_subflow_context *subflow;
+	int i = 0;
+
+	/* bench every subflow except the first one */
+	bpf_for_each(mptcp_subflow, subflow, (struct sock *)msk) {
+		if (i++ > 0)
+			mptcp_subflow_set_avoid(subflow, true);
+	}
+
+	/* only schedule a subflow that is NOT avoided */
+	bpf_for_each(mptcp_subflow, subflow, (struct sock *)msk) {
+		if (subflow->avoid)
+			continue;
+		if (mptcp_subflow_active(subflow)) {
+			mptcp_subflow_set_scheduled(subflow, true);
+			break;
+		}
+	}
+	return 0;
+}
+
+SEC(".struct_ops.link")
+struct mptcp_sched_ops avoid = {
+	.init		= (void *)mptcp_sched_avoid_init,
+	.release	= (void *)mptcp_sched_avoid_release,
+	.get_send	= (void *)bpf_avoid_get_send,
+	.name		= "bpf_avoid",
+};
-- 
2.43.0