netdev - [PATCH bpf-next 2/2] selftests/bpf: Test bpf_skb_check_mtu(BPF_MTU_CHK_SEGS) when transport

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Message-ID: <20251112232331.1566074-2-martin.lau@linux.dev>
Date: Wed, 12 Nov 2025 15:23:31 -0800
From: Martin KaFai Lau <martin.lau@...ux.dev>
To: bpf@...r.kernel.org
Cc: 'Alexei Starovoitov ' <ast@...nel.org>,
	'Andrii Nakryiko ' <andrii@...nel.org>,
	'Daniel Borkmann ' <daniel@...earbox.net>,
	netdev@...r.kernel.org
Subject: [PATCH bpf-next 2/2] selftests/bpf: Test bpf_skb_check_mtu(BPF_MTU_CHK_SEGS) when transport_header is not set

From: Martin KaFai Lau <martin.lau@...nel.org>

Add a test to check that bpf_skb_check_mtu(BPF_MTU_CHK_SEGS) is
rejected (-EINVAL) if skb->transport_header is not set. The test
needs to lower the MTU of the loopback device. Thus, take this
opportunity to run the test in a netns by adding "ns_" to the test
name. The "serial_" prefix can then be removed.

Signed-off-by: Martin KaFai Lau <martin.lau@...nel.org>
---
 .../selftests/bpf/prog_tests/check_mtu.c      | 23 ++++++++++++++++++-
 .../selftests/bpf/progs/test_check_mtu.c      | 12 ++++++++++
 2 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/bpf/prog_tests/check_mtu.c b/tools/testing/selftests/bpf/prog_tests/check_mtu.c
index 2a9a30650350..65b4512967e7 100644
--- a/tools/testing/selftests/bpf/prog_tests/check_mtu.c
+++ b/tools/testing/selftests/bpf/prog_tests/check_mtu.c
@@ -153,6 +153,26 @@ static void test_check_mtu_run_tc(struct test_check_mtu *skel,
 	ASSERT_EQ(mtu_result, mtu_expect, "MTU-compare-user");
 }
 
+static void test_chk_segs_flag(struct test_check_mtu *skel, __u32 mtu)
+{
+	int err, prog_fd = bpf_program__fd(skel->progs.tc_chk_segs_flag);
+	struct __sk_buff skb = {
+		.gso_size = 10,
+	};
+	LIBBPF_OPTS(bpf_test_run_opts, topts,
+		    .data_in = &pkt_v4,
+		    .data_size_in = sizeof(pkt_v4),
+		    .ctx_in = &skb,
+		    .ctx_size_in = sizeof(skb),
+	);
+
+	/* Lower the mtu to test the BPF_MTU_CHK_SEGS */
+	SYS_NOFAIL("ip link set dev lo mtu 10");
+	err = bpf_prog_test_run_opts(prog_fd, &topts);
+	SYS_NOFAIL("ip link set dev lo mtu %u", mtu);
+	ASSERT_OK(err, "test_run");
+	ASSERT_EQ(topts.retval, BPF_OK, "retval");
+}
 
 static void test_check_mtu_tc(__u32 mtu, __u32 ifindex)
 {
@@ -177,11 +197,12 @@ static void test_check_mtu_tc(__u32 mtu, __u32 ifindex)
 	test_check_mtu_run_tc(skel, skel->progs.tc_minus_delta, mtu);
 	test_check_mtu_run_tc(skel, skel->progs.tc_input_len, mtu);
 	test_check_mtu_run_tc(skel, skel->progs.tc_input_len_exceed, mtu);
+	test_chk_segs_flag(skel, mtu);
 cleanup:
 	test_check_mtu__destroy(skel);
 }
 
-void serial_test_check_mtu(void)
+void test_ns_check_mtu(void)
 {
 	int mtu_lo;
 
diff --git a/tools/testing/selftests/bpf/progs/test_check_mtu.c b/tools/testing/selftests/bpf/progs/test_check_mtu.c
index 2ec1de11a3ae..7b6b2b342c1d 100644
--- a/tools/testing/selftests/bpf/progs/test_check_mtu.c
+++ b/tools/testing/selftests/bpf/progs/test_check_mtu.c
@@ -7,6 +7,7 @@
 
 #include <stddef.h>
 #include <stdint.h>
+#include <errno.h>
 
 char _license[] SEC("license") = "GPL";
 
@@ -288,3 +289,14 @@ int tc_input_len_exceed(struct __sk_buff *ctx)
 	global_bpf_mtu_xdp = mtu_len;
 	return retval;
 }
+
+SEC("tc")
+int tc_chk_segs_flag(struct __sk_buff *ctx)
+{
+	__u32 mtu_len = 0;
+	int err;
+
+	err = bpf_check_mtu(ctx, GLOBAL_USER_IFINDEX, &mtu_len, 0, BPF_MTU_CHK_SEGS);
+
+	return err == -EINVAL ? BPF_OK : BPF_DROP;
+}
-- 
2.47.3