Re: [PATCH bpf v3 2/2] selftests/bpf: Add protocol check test for bpf_sk_assign_tcp_reqsk()

[Martin KaFai Lau <martin.lau@linux.dev>]

On 3/27/26 6:38 AM, Jiayuan Chen wrote:
> From: Jiayuan Chen <jiayuan.chen@shopee.com>
> 
> Add test_tcp_custom_syncookie_protocol_check to verify that
> bpf_sk_assign_tcp_reqsk() rejects non-TCP skbs. The test sends a UDP
> packet through TC ingress where a BPF program calls
> bpf_sk_assign_tcp_reqsk() on it and checks that the kfunc returns an
> error. A UDP server recv() is used as synchronization to ensure the
> BPF program has finished processing before checking the result.
> 
> Without the fix in bpf_sk_assign_tcp_reqsk(), the kfunc succeeds and
> attaches a TCP reqsk to the UDP skb, which causes a null pointer
> dereference panic when the kernel processes it through the UDP receive
> path.
> 
> Test result:
> 
>    ./test_progs -a tcp_custom_syncookie_protocol_check -v
>    setup_netns:PASS:create netns 0 nsec
>    setup_netns:PASS:ip 0 nsec
>    write_sysctl:PASS:open sysctl 0 nsec
>    write_sysctl:PASS:write sysctl 0 nsec
>    setup_netns:PASS:write_sysctl 0 nsec
>    test_tcp_custom_syncookie_protocol_check:PASS:open_and_load 0 nsec
>    setup_tc:PASS:qdisc add dev lo clsact 0 nsec
>    setup_tc:PASS:filter add dev lo ingress 0 nsec
>    run_protocol_check:PASS:start tcp_server 0 nsec
>    run_protocol_check:PASS:start udp_server 0 nsec
>    run_protocol_check:PASS:connect udp_client 0 nsec
>    run_protocol_check:PASS:send udp 0 nsec
>    run_protocol_check:PASS:recv udp 0 nsec
>    run_protocol_check:PASS:udp_intercepted 0 nsec
>    run_protocol_check:PASS:assign_ret 0 nsec
>    #471/1   tcp_custom_syncookie_protocol_check/IPv4 TCP:OK
>    run_protocol_check:PASS:start tcp_server 0 nsec
>    run_protocol_check:PASS:start udp_server 0 nsec
>    run_protocol_check:PASS:connect udp_client 0 nsec
>    run_protocol_check:PASS:send udp 0 nsec
>    run_protocol_check:PASS:recv udp 0 nsec
>    run_protocol_check:PASS:udp_intercepted 0 nsec
>    run_protocol_check:PASS:assign_ret 0 nsec
>    #471/2   tcp_custom_syncookie_protocol_check/IPv6 TCP:OK
>    #471     tcp_custom_syncookie_protocol_check:OK
>    Summary: 1/2 PASSED, 0 SKIPPED, 0 FAILED
> 
> Cc: Jiayuan Chen <jiayuan.chen@linux.dev>
> Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
> ---
>   .../bpf/prog_tests/tcp_custom_syncookie.c     |  94 ++++++++++++++-
>   .../bpf/progs/test_tcp_custom_syncookie.c     | 109 ++++++++++++++++++
>   2 files changed, 199 insertions(+), 4 deletions(-)
> 
> diff --git a/tools/testing/selftests/bpf/prog_tests/tcp_custom_syncookie.c b/tools/testing/selftests/bpf/prog_tests/tcp_custom_syncookie.c
> index eaf441dc7e79..e46031d0786b 100644
> --- a/tools/testing/selftests/bpf/prog_tests/tcp_custom_syncookie.c
> +++ b/tools/testing/selftests/bpf/prog_tests/tcp_custom_syncookie.c
> @@ -5,6 +5,7 @@
>   #include <sched.h>
>   #include <stdlib.h>
>   #include <net/if.h>
> +#include <netinet/in.h>
>   
>   #include "test_progs.h"
>   #include "cgroup_helpers.h"
> @@ -47,11 +48,10 @@ static int setup_netns(void)
>   	return -1;
>   }
>   
> -static int setup_tc(struct test_tcp_custom_syncookie *skel)
> +static int setup_tc(int prog_fd)
>   {
>   	LIBBPF_OPTS(bpf_tc_hook, qdisc_lo, .attach_point = BPF_TC_INGRESS);
> -	LIBBPF_OPTS(bpf_tc_opts, tc_attach,
> -		    .prog_fd = bpf_program__fd(skel->progs.tcp_custom_syncookie));
> +	LIBBPF_OPTS(bpf_tc_opts, tc_attach, .prog_fd = prog_fd);
>   
>   	qdisc_lo.ifindex = if_nametoindex("lo");
>   	if (!ASSERT_OK(bpf_tc_hook_create(&qdisc_lo), "qdisc add dev lo clsact"))
> @@ -127,7 +127,7 @@ void test_tcp_custom_syncookie(void)
>   	if (!ASSERT_OK_PTR(skel, "open_and_load"))
>   		return;
>   
> -	if (setup_tc(skel))
> +	if (setup_tc(bpf_program__fd(skel->progs.tcp_custom_syncookie)))
>   		goto destroy_skel;
>   
>   	for (i = 0; i < ARRAY_SIZE(test_cases); i++) {
> @@ -145,6 +145,92 @@ void test_tcp_custom_syncookie(void)
>   
>   destroy_skel:
>   	system("tc qdisc del dev lo clsact");
> +	test_tcp_custom_syncookie__destroy(skel);
> +}
> +
> +/* Test: bpf_sk_assign_tcp_reqsk() should reject non-TCP skb.
> + *
> + * Send a UDP packet through TC ingress where a BPF program calls
> + * bpf_sk_assign_tcp_reqsk() on it. The kfunc should return an error
> + * because the skb carries UDP, not TCP.
> + *
> + * TCP and UDP servers share the same port. The BPF program intercepts
> + * the UDP packet, looks up the TCP listener via the dest port, and
> + * attempts to assign a TCP reqsk to the UDP skb.
> + */
> +static void run_protocol_check(struct test_tcp_custom_syncookie *skel,
> +			       int family, const char *addr)
> +{
> +	int tcp_server = -1, udp_server = -1, udp_client = -1;
> +	char buf[32] = "test";
> +	int port, ret;
> +
> +	tcp_server = start_server(family, SOCK_STREAM, addr, 0, 0);
> +	if (!ASSERT_NEQ(tcp_server, -1, "start tcp_server"))
> +		return;
> +
> +	port = ntohs(get_socket_local_port(tcp_server));
> +
> +	/* UDP server on same port for synchronization and port sharing */
> +	udp_server = start_server(family, SOCK_DGRAM, addr, port, 0);
> +	if (!ASSERT_NEQ(udp_server, -1, "start udp_server"))
> +		goto close_tcp;
> +
> +	skel->bss->udp_intercepted = false;
> +	skel->data->assign_ret = -1;

assign_ret is init to -1

> +
> +	udp_client = connect_to_fd(udp_server, 0);
> +	if (!ASSERT_NEQ(udp_client, -1, "connect udp_client"))
> +		goto close_udp_server;
>   
> +	ret = send(udp_client, buf, sizeof(buf), 0);
> +	if (!ASSERT_EQ(ret, sizeof(buf), "send udp"))
> +		goto close_udp_client;
> +
> +	/* recv() ensures TC ingress BPF has processed the skb */
> +	ret = recv(udp_server, buf, sizeof(buf), 0);
> +	if (!ASSERT_EQ(ret, sizeof(buf), "recv udp"))
> +		goto close_udp_client;
> +
> +	ASSERT_EQ(skel->bss->udp_intercepted, true, "udp_intercepted");
> +
> +	/* assign_ret == 0 means kfunc accepted UDP skb (bug).
> +	 * assign_ret < 0 means kfunc correctly rejected it (fixed).
> +	 */
> +	ASSERT_NEQ(skel->data->assign_ret, 0, "assign_ret");

and here it checks NEQ 0....

Maybe init assign_ret to 0?

Also, why not specifically check for -EINVAL?