Subject: Re: [net-next 3/3] selftests/net: Test PACKET_AUXDATA

From: Willem de Bruijn <[email-masked]>
Date: Fri, 03 Apr 2026 11:37:10 -0400

Joe Damato wrote:
> Extend the packet socket selftest, adding a recvmsg path, to test
> PACKET_AUXDATA. Check basic attributes of tpacket_auxdata.
> 
> Signed-off-by: Joe Damato <[email-masked]>
> ---
>  tools/testing/selftests/net/psock_snd.c  | 57 ++++++++++++++++++++++--
>  tools/testing/selftests/net/psock_snd.sh |  5 +++
>  2 files changed, 59 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/testing/selftests/net/psock_snd.c b/tools/testing/selftests/net/psock_snd.c
> index 7bbce274b9f0..682e0afe364d 100644
> --- a/tools/testing/selftests/net/psock_snd.c
> +++ b/tools/testing/selftests/net/psock_snd.c
> @@ -40,6 +40,7 @@ static bool	cfg_use_qdisc_bypass;
>  static bool	cfg_use_vlan;
>  static bool	cfg_use_vnet;
>  static bool	cfg_drop;
> +static bool	cfg_aux_data;
>  
>  static char	*cfg_ifname = "lo";
>  static int	cfg_mtu	= 1500;
> @@ -281,9 +282,26 @@ static int setup_rx(void)
>  
>  static void do_rx(int fd, int expected_len, char *expected)
>  {
> +	char cmsg_buf[1024] = {};
> +	struct msghdr msg = {};
> +	struct iovec iov[1];
>  	int ret;
>  
> -	ret = recv(fd, rbuf, sizeof(rbuf), 0);
> +	if (cfg_aux_data) {
> +		iov[0].iov_base = rbuf;
> +		iov[0].iov_len = sizeof(rbuf);
> +
> +		msg.msg_iov = iov;
> +		msg.msg_iovlen = 1;
> +
> +		msg.msg_control = cmsg_buf;
> +		msg.msg_controllen = sizeof(cmsg_buf);
> +
> +		ret = recvmsg(fd, &msg, 0);
> +	} else {
> +		ret = recv(fd, rbuf, sizeof(rbuf), 0);
> +	}
> +
>  	if (ret == -1)
>  		error(1, errno, "recv");
>  	if (ret != expected_len)
> @@ -292,6 +310,31 @@ static void do_rx(int fd, int expected_len, char *expected)
>  	if (memcmp(rbuf, expected, ret))
>  		error(1, 0, "recv: data mismatch");
>  
> +	if (cfg_aux_data) {
> +		struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
> +		struct tpacket_auxdata *adata;
> +
> +		if (!cmsg)
> +			error(1, 0, "auxdata null");
> +
> +		if (cmsg->cmsg_level != SOL_PACKET)
> +			error(1, 0, "cmsg_level != SOL_PACKET");
> +
> +		if (cmsg->cmsg_type != PACKET_AUXDATA)
> +			error(1, 0, "cmsg_type != PACKET_AUXDATA");
> +
> +		adata = (struct tpacket_auxdata *)CMSG_DATA(cmsg);
> +
> +		if (adata->tp_net != ETH_HLEN)
> +			error(1, 0, "cmsg tp_net != ETH_HLEN");
> +
> +		if (adata->tp_len != expected_len)
> +			error(1, 0, "cmsg tp_len != %u", expected_len);
> +
> +		if (adata->tp_snaplen != expected_len)
> +			error(1, 0, "cmsg tp_snaplen != %u", expected_len);
> +	}
> +
>  	fprintf(stderr, "rx: %u\n", ret);
>  }
>  
> @@ -312,6 +355,10 @@ static int setup_sniffer(void)
>  		if (setsockopt(fd, SOL_SOCKET, SO_RCVBUF, &one, sizeof(one)))
>  			error(1, errno, "setsockopt SO_RCVBUF");
>  
> +	if (cfg_aux_data)
> +		if (setsockopt(fd, SOL_PACKET, PACKET_AUXDATA, &one, sizeof(one)))
> +			error(1, errno, "setsockopt PACKET_AUXDATA");
> +
>  	pair_udp_setfilter(fd);
>  	do_bind(fd);
>  
> @@ -322,8 +369,11 @@ static void parse_opts(int argc, char **argv)
>  {
>  	int c;
>  
> -	while ((c = getopt(argc, argv, "bcCdDgl:qt:vV")) != -1) {
> +	while ((c = getopt(argc, argv, "abcCdDgl:qt:vV")) != -1) {
>  		switch (c) {
> +		case 'a':
> +			cfg_aux_data = true;
> +			break;
>  		case 'b':
>  			cfg_use_bind = true;
>  			break;
> @@ -436,7 +486,8 @@ static void run_test(void)
>  		check_packet_stats(fds);
>  	}
>  
> -	do_rx(fdr, cfg_payload_len, tbuf + total_len - cfg_payload_len);
> +	if (!cfg_aux_data)
> +		do_rx(fdr, cfg_payload_len, tbuf + total_len - cfg_payload_len);

Slight preference to instead detect that this is a read not from a packet
socket and skip the cmsg processing inside do_rx as a result.

A roundabout way would be using getsockname. Simpler may be just to pass a new
boolean to do_rx is_psock or os.