Re: [PATCH] selftests/bpf: tc_tunnel - validate decap tunnel state without false-pass gaps

[Hudson, Nick]

> On Aug 19, 2026, at 10:34 PM, Emil Tsalapatis <emil@xxxxxxxxxxxxxxx> wrote:
> 
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> On Tue Aug 18, 2026 at 8:03 AM EDT, Nick Hudson wrote:
>> This follow-up tightens the selftest to guard against false passes in
>> GSO decap validation.
>> 
>> It validates the expected post-decap state for both GSO and non-GSO
>> packets: expected tunnel gso_type bits must be cleared and
>> skb->encapsulation must match the remaining tunnel state.
>> 
>> It also adds explicit assertions that the decap validation path was
>> executed, including the large-send phase for GSO-marked subtests, and
>> asserts that the large-send marker is reset after the send.
>> 
>> This keeps the tc_tunnel decap checks meaningful and avoids false-pass
>> regressions without over-constraining behavior across tunnel modes.
>> 
>> Signed-off-by: Nick Hudson <nhudson@xxxxxxxxxx>
>> ---
>> .../selftests/bpf/prog_tests/test_tc_tunnel.c | 33 +++++++++++++++----
>> .../selftests/bpf/progs/test_tc_tunnel.c      | 19 +++++++++++
>> 2 files changed, 46 insertions(+), 6 deletions(-)
>> 
>> diff --git a/tools/testing/selftests/bpf/prog_tests/test_tc_tunnel.c b/tools/testing/selftests/bpf/prog_tests/test_tc_tunnel.c
>> index 67ba27d69347..08d7d90f7772 100644
>> --- a/tools/testing/selftests/bpf/prog_tests/test_tc_tunnel.c
>> +++ b/tools/testing/selftests/bpf/prog_tests/test_tc_tunnel.c
>> @@ -206,7 +206,7 @@ static void disconnect_client_from_server(struct subtest_cfg *cfg,
>> 	free(conn);
>> }
>> 
>> -static int send_and_test_data(struct subtest_cfg *cfg)
>> +static int send_and_test_data(struct subtest_cfg *cfg, struct test_tc_tunnel *skel)
>> {
>> 	struct connection *conn;
>> 	int err, res = -1;
>> @@ -215,6 +215,7 @@ static int send_and_test_data(struct subtest_cfg *cfg)
>> 	if (!ASSERT_OK_PTR(conn, "connect to server"))
>> 		return -1;
>> 
>> +	skel->bss->decap_expect_large_send = 0;
>> 	err = send(conn->client_fd, tx_buffer, DEFAULT_TEST_DATA_SIZE, 0);
>> 	if (!ASSERT_EQ(err, DEFAULT_TEST_DATA_SIZE, "send data from client"))
>> 		goto end;
>> @@ -226,14 +227,17 @@ static int send_and_test_data(struct subtest_cfg *cfg)
>> 		goto end;
>> 	}
>> 
>> +	skel->bss->decap_expect_large_send = 1;
>> 	err = send(conn->client_fd, tx_buffer, GSO_TEST_DATA_SIZE, 0);
>> 	if (!ASSERT_EQ(err, GSO_TEST_DATA_SIZE, "send (large) data from client"))
>> 		goto end;
>> 	if (check_server_rx_data(cfg, conn, DEFAULT_TEST_DATA_SIZE))
>> 		goto end;
>> +	skel->bss->decap_expect_large_send = 0;
>> 
>> 	res = 0;
>> end:
>> +	skel->bss->decap_expect_large_send = 0;
>> 	disconnect_client_from_server(cfg, conn);
>> 	return res;
>> }
>> @@ -374,10 +378,15 @@ static int configure_ebpf_decapsulation(struct subtest_cfg *cfg)
>> 	return ret;
>> }
>> 
>> -static void run_test(struct subtest_cfg *cfg)
>> +static void run_test(struct subtest_cfg *cfg, struct test_tc_tunnel *skel)
>> {
>> 	struct nstoken *nstoken;
>> 
>> +	skel->bss->decap_validation_seen = 0;
>> +	skel->bss->decap_gso_validation_seen = 0;
> 
> Sashiko points out correctly that gso_validation_seen is never checked.
> Can we assert it with the others?
> 

Yeah, I’m fixing this and removing

    decap_{large_send_validation_seen,expect_large_send}


> The CI bot's point on the other hand doesn't matter that much imo, the
> assert for the config variable is good in case we ever add any other
> subtests.

Which point, sorry? 

> 
> pw-bot: cr
> 
>> +	skel->bss->decap_large_send_validation_seen = 0;
>> +	skel->bss->decap_expect_large_send = 0;
>> +
>> 	if (!ASSERT_OK(run_server(cfg), "run server"))
>> 		return;
>> 
>> @@ -386,7 +395,7 @@ static void run_test(struct subtest_cfg *cfg)
>> 		goto fail;
>> 
>> 	/* Basic communication must work */
>> -	if (!ASSERT_OK(send_and_test_data(cfg), "connect without any encap"))
>> +	if (!ASSERT_OK(send_and_test_data(cfg, skel), "connect without any encap"))
>> 		goto fail;
>> 
>> 	/* Attach encapsulation program to client */
>> @@ -398,7 +407,7 @@ static void run_test(struct subtest_cfg *cfg)
>> 		if (!ASSERT_OK(configure_kernel_decapsulation(cfg),
>> 					"configure kernel decapsulation"))
>> 			goto fail;
>> -		if (!ASSERT_OK(send_and_test_data(cfg),
>> +		if (!ASSERT_OK(send_and_test_data(cfg, skel),
>> 			       "connect with encap prog and kern decap"))
>> 			goto fail;
>> 	}
>> @@ -406,7 +415,18 @@ static void run_test(struct subtest_cfg *cfg)
>> 	/* Replace kernel decapsulation with BPF decapsulation, test must pass */
>> 	if (!ASSERT_OK(configure_ebpf_decapsulation(cfg), "configure ebpf decapsulation"))
>> 		goto fail;
>> -	ASSERT_OK(send_and_test_data(cfg), "connect with encap and decap progs");
>> +	if (!ASSERT_OK(send_and_test_data(cfg, skel), "connect with encap and decap progs"))
>> +		goto fail;
>> +	if (!ASSERT_NEQ(skel->bss->decap_validation_seen, 0,
>> +			"decap validation executed"))
>> +		goto fail;
>> +	if (!ASSERT_EQ(skel->bss->decap_expect_large_send, 0,
>> +		       "decap large-send marker reset"))
>> +		goto fail;
>> +	if (cfg->test_gso) {
>> +		ASSERT_NEQ(skel->bss->decap_large_send_validation_seen, 0,
>> +			   "decap validation executed for large send");
>> +	}
>> 
>> fail:
>> 	close_netns(nstoken);
>> @@ -438,6 +458,7 @@ static int setup(void)
>> 	SYS(fail_close_ns_client, "ip link add %s type veth peer name %s",
>> 	    "veth1 mtu 1500 netns " CLIENT_NS " address " MAC_ADDR_VETH1,
>> 	    "veth2 mtu 1500 netns " SERVER_NS " address " MAC_ADDR_VETH2);
>> +	SYS(fail_close_ns_client, "ethtool -K veth1 tso off");
>> 	SYS(fail_close_ns_client, "ip link set veth1 up");
>> 	nstoken_server = open_netns(SERVER_NS);
>> 	if (!ASSERT_OK_PTR(nstoken_server, "open server ns"))
>> @@ -701,7 +722,7 @@ void test_tc_tunnel(void)
>> 		if (ret < 0 || !test__start_subtest(cfg->name))
>> 			continue;
>> 		if (subtest_setup(skel, cfg) == 0)
>> -			run_test(cfg);
>> +			run_test(cfg, skel);
>> 		subtest_cleanup(cfg);
>> 	}
>> 	cleanup();
>> diff --git a/tools/testing/selftests/bpf/progs/test_tc_tunnel.c b/tools/testing/selftests/bpf/progs/test_tc_tunnel.c
>> index 853bca962910..e9bd1c9781f7 100644
>> --- a/tools/testing/selftests/bpf/progs/test_tc_tunnel.c
>> +++ b/tools/testing/selftests/bpf/progs/test_tc_tunnel.c
>> @@ -16,6 +16,11 @@ static const int cfg_port = 8000;
>> 
>> static const int cfg_udp_src = 20000;
>> 
>> +__u64 decap_validation_seen;
>> +__u64 decap_gso_validation_seen;
>> +__u64 decap_large_send_validation_seen;
>> +__u32 decap_expect_large_send;
>> +
>> #define ETH_P_MPLS_UC	0x8847
>> #define ETH_P_TEB	0x6558
>> 
>> @@ -690,7 +695,21 @@ static int decap_internal(struct __sk_buff *skb, int off, int len, char proto,
>> 
>> 	kskb = bpf_cast_to_kern_ctx(skb);
>> 	shinfo = bpf_core_cast(kskb->head + kskb->end, struct skb_shared_info);
>> +
>> +	if (flags & (BPF_F_ADJ_ROOM_DECAP_L4_MASK |
>> +		     BPF_F_ADJ_ROOM_DECAP_IPXIP_MASK))
>> +		decap_validation_seen++;
>> +
>> +	if ((flags & (BPF_F_ADJ_ROOM_DECAP_L4_MASK |
>> +		      BPF_F_ADJ_ROOM_DECAP_IPXIP_MASK)) &&
>> +	    decap_expect_large_send)
>> +		decap_large_send_validation_seen++;
>> +
>> 	if (shinfo->gso_size) {
>> +		if (flags & (BPF_F_ADJ_ROOM_DECAP_L4_MASK |
>> +			     BPF_F_ADJ_ROOM_DECAP_IPXIP_MASK))
>> +			decap_gso_validation_seen++;
>> +
>> 		if ((flags & BPF_F_ADJ_ROOM_DECAP_L4_UDP) &&
>> 		    (shinfo->gso_type & SKB_GSO_UDP_TUNNEL_MASK))
>> 			return TC_ACT_SHOT;
>

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