Subject: Re: [net-next 2/3] selftests/net: Test PACKET_STATISTICS drops

From: Willem de Bruijn <[email-masked]>
Date: Fri, 03 Apr 2026 11:26:01 -0400

Joe Damato wrote:
> Extend psock_snd to test drops by setting a tiny receive buffer and
> sending a large burst of packets.
> 
> Signed-off-by: Joe Damato <[email-masked]>
> ---
>  tools/testing/selftests/net/psock_snd.c  | 52 ++++++++++++++++++++----
>  tools/testing/selftests/net/psock_snd.sh |  5 +++
>  2 files changed, 48 insertions(+), 9 deletions(-)
> 
> diff --git a/tools/testing/selftests/net/psock_snd.c b/tools/testing/selftests/net/psock_snd.c
> index 5be481a3d2bd..7bbce274b9f0 100644
> --- a/tools/testing/selftests/net/psock_snd.c
> +++ b/tools/testing/selftests/net/psock_snd.c
> @@ -39,6 +39,7 @@ static bool	cfg_use_gso;
>  static bool	cfg_use_qdisc_bypass;
>  static bool	cfg_use_vlan;
>  static bool	cfg_use_vnet;
> +static bool	cfg_drop;
>  
>  static char	*cfg_ifname = "lo";
>  static int	cfg_mtu	= 1500;
> @@ -49,6 +50,8 @@ static uint16_t	cfg_port = 8000;
>  /* test sending up to max mtu + 1 */
>  #define TEST_SZ	(sizeof(struct virtio_net_hdr) + ETH_HLEN + ETH_MAX_MTU + 1)
>  
> +#define BURST_CNT (1000)
> +
>  static char tbuf[TEST_SZ], rbuf[TEST_SZ];
>  
>  static unsigned long add_csum_hword(const uint16_t *start, int num_u16)
> @@ -212,13 +215,14 @@ static void do_send(int fd, char *buf, int len)
>  	if (ret != len)
>  		error(1, 0, "write: %u %u", ret, len);
>  
> -	fprintf(stderr, "tx: %u\n", ret);
> +	if (!cfg_drop)
> +		fprintf(stderr, "tx: %u\n", ret);
>  }
>  
> -static int do_tx(void)
> +static int do_tx(bool burst)
>  {
>  	const int one = 1;
> -	int fd, len;
> +	int i, fd, len;
>  
>  	fd = socket(PF_PACKET, cfg_use_dgram ? SOCK_DGRAM : SOCK_RAW, 0);
>  	if (fd == -1)
> @@ -242,6 +246,10 @@ static int do_tx(void)
>  
>  	do_send(fd, tbuf, len);
>  
> +	if (burst)
> +		for (i = 0; i < BURST_CNT; i++)
> +			do_send(fd, tbuf, len);
> +
>  	if (close(fd))
>  		error(1, errno, "close t");
>  
> @@ -290,6 +298,7 @@ static void do_rx(int fd, int expected_len, char *expected)
>  static int setup_sniffer(void)
>  {
>  	struct timeval tv = { .tv_usec = 100 * 1000 };
> +	const int one = 1;
>  	int fd;
>  
>  	fd = socket(PF_PACKET, SOCK_RAW, 0);
> @@ -299,6 +308,10 @@ static int setup_sniffer(void)
>  	if (setsockopt(fd, SOL_SOCKET, SO_RCVTIMEO, &tv, sizeof(tv)))
>  		error(1, errno, "setsockopt rcv timeout");
>  
> +	if (cfg_drop)
> +		if (setsockopt(fd, SOL_SOCKET, SO_RCVBUF, &one, sizeof(one)))
> +			error(1, errno, "setsockopt SO_RCVBUF");
> +
>  	pair_udp_setfilter(fd);
>  	do_bind(fd);
>  
> @@ -309,7 +322,7 @@ static void parse_opts(int argc, char **argv)
>  {
>  	int c;
>  
> -	while ((c = getopt(argc, argv, "bcCdgl:qt:vV")) != -1) {
> +	while ((c = getopt(argc, argv, "bcCdDgl:qt:vV")) != -1) {
>  		switch (c) {
>  		case 'b':
>  			cfg_use_bind = true;
> @@ -323,6 +336,9 @@ static void parse_opts(int argc, char **argv)
>  		case 'd':
>  			cfg_use_dgram = true;
>  			break;
> +		case 'D':
> +			cfg_drop = true;
> +			break;
>  		case 'g':
>  			cfg_use_gso = true;
>  			break;
> @@ -367,11 +383,23 @@ static void check_packet_stats(int fd)
>  	if (getsockopt(fd, SOL_PACKET, PACKET_STATISTICS, &st, &len))
>  		error(1, errno, "getsockopt packet statistics");
>  
> -	if (st.tp_packets != 1)
> -		error(1, 0, "stats: tp_packets %u != 1", st.tp_packets);
> +	if (cfg_drop) {
> +		/* PACKET_STATISTICS reports all packets seen (including
> +		 * drops) in tp_packets
> +		 */
> +		if (st.tp_packets < st.tp_drops)
> +			error(1, 0, "stats: tp_packets %u < tp_drops %u",
> +			      st.tp_packets, st.tp_drops);
>  
> -	if (st.tp_drops != 0)
> -		error(1, 0, "stats: tp_drops %u != 0", st.tp_drops);
> +		if (st.tp_drops == 0)
> +			error(1, 0, "stats: expected drops but tp_drops == 0");
> +	} else {
> +		if (st.tp_packets != 1)
> +			error(1, 0, "stats: tp_packets %u != 1", st.tp_packets);
> +
> +		if (st.tp_drops != 0)
> +			error(1, 0, "stats: tp_drops %u != 0", st.tp_drops);
> +	}
>  
>  	/* verify clear on read */
>  	memset(&st, 0xff, sizeof(st));
> @@ -394,7 +422,12 @@ static void run_test(void)
>  	fdr = setup_rx();
>  	fds = setup_sniffer();
>  
> -	total_len = do_tx();
> +	total_len = do_tx(cfg_drop);

only if respinning: no need to pass global cfg_.. variables through
function arguments.

> +
> +	if (cfg_drop) {
> +		check_packet_stats(fds);
> +		goto out;
> +	}

can this not use the check_packet_stats added in the previous patch
right below?

>  
>  	/* BPF filter accepts only this length, vlan changes MAC */
>  	if (cfg_payload_len == DATA_LEN && !cfg_use_vlan) {
> @@ -405,6 +438,7 @@ static void run_test(void)
>  
>  	do_rx(fdr, cfg_payload_len, tbuf + total_len - cfg_payload_len);
>  
> +out:
>  	if (close(fds))
>  		error(1, errno, "close s");
>  	if (close(fdr))
> diff --git a/tools/testing/selftests/net/psock_snd.sh b/tools/testing/selftests/net/psock_snd.sh
> index 1cbfeb5052ec..b6ef12fad5d5 100755
> --- a/tools/testing/selftests/net/psock_snd.sh
> +++ b/tools/testing/selftests/net/psock_snd.sh
> @@ -92,4 +92,9 @@ echo "raw gso max size"
>  echo "raw gso max size + 1 (expected to fail)"
>  (! ./in_netns.sh ./psock_snd -v -c -g -l "${max_mss_exceeds}")
>  
> +# test drops statistics
> +
> +echo "test drops statistics"
> +./in_netns.sh ./psock_snd -D
> +
>  echo "OK. All tests passed"
> -- 
> 2.52.0
>