Subject: Re: [PATCH net-next 6/6] selftests: drv-net: gro: add a test for bad IPv4 csum

From: Willem de Bruijn <[email-masked]>
Date: Wed, 01 Apr 2026 20:28:46 -0400

Jakub Kicinski wrote:
> We have a test for coalescing with bad TCP checksum, let's also
> test bad IPv4 header checksum.
> 
> Signed-off-by: Jakub Kicinski <[email-masked]>
> ---
>  tools/testing/selftests/net/lib/gro.c      | 25 ++++++++++++++++++++++
>  tools/testing/selftests/drivers/net/gro.py |  1 +
>  2 files changed, 26 insertions(+)
> 
> diff --git a/tools/testing/selftests/net/lib/gro.c b/tools/testing/selftests/net/lib/gro.c
> index 762e88932ed2..a458cfbd11eb 100644
> --- a/tools/testing/selftests/net/lib/gro.c
> +++ b/tools/testing/selftests/net/lib/gro.c
> @@ -36,6 +36,7 @@
>   *  Packets with different (ECN, TTL, TOS) header, IP options or
>   *  IP fragments shouldn't coalesce.
>   *   - ip_ecn, ip_tos:            shared between IPv4/IPv6
> + *   - ip_csum:                   IPv4 only, bad IP header checksum
>   *   - ip_ttl, ip_opt, ip_frag4:  IPv4 only
>   *   - ip_id_df*:                 IPv4 IP ID field coalescing tests
>   *   - ip_frag6, ip_v6ext_*:      IPv6 only
> @@ -675,6 +676,21 @@ static void send_changed_checksum(int fd, struct sockaddr_ll *daddr)
>  	write_packet(fd, buf, pkt_size, daddr);
>  }
>  
> +/* Packets with incorrect IPv4 header checksum don't coalesce. */
> +static void send_changed_ip_checksum(int fd, struct sockaddr_ll *daddr)
> +{
> +	static char buf[MAX_HDR_LEN + PAYLOAD_LEN];
> +	struct iphdr *iph = (struct iphdr *)(buf + ETH_HLEN);
> +	int pkt_size = total_hdr_len + PAYLOAD_LEN;
> +
> +	create_packet(buf, 0, 0, PAYLOAD_LEN, 0);
> +	write_packet(fd, buf, pkt_size, daddr);
> +
> +	create_packet(buf, PAYLOAD_LEN, 0, PAYLOAD_LEN, 0);
> +	iph->check = iph->check - 1;
> +	write_packet(fd, buf, pkt_size, daddr);
> +}
> +
>   /* Packets with non-consecutive sequence number don't coalesce.*/
>  static void send_changed_seq(int fd, struct sockaddr_ll *daddr)
>  {
> @@ -1392,6 +1408,10 @@ static void gro_sender(void)
>  		write_packet(txfd, fin_pkt, total_hdr_len, &daddr);
>  
>  	/* ip sub-tests - IPv4 only */
> +	} else if (strcmp(testname, "ip_csum") == 0) {
> +		send_changed_ip_checksum(txfd, &daddr);
> +		usleep(fin_delay_us);
> +		write_packet(txfd, fin_pkt, total_hdr_len, &daddr);
>  	} else if (strcmp(testname, "ip_ttl") == 0) {
>  		send_changed_ttl(txfd, &daddr);
>  		write_packet(txfd, fin_pkt, total_hdr_len, &daddr);
> @@ -1588,6 +1608,11 @@ static void gro_receiver(void)
>  		check_recv_pkts(rxfd, correct_payload, 2);
>  
>  	/* ip sub-tests - IPv4 only */
> +	} else if (strcmp(testname, "ip_csum") == 0) {
> +		correct_payload[0] = PAYLOAD_LEN;
> +		correct_payload[1] = PAYLOAD_LEN;
> +		printf("bad ip checksum doesn't coalesce: ");
> +		check_recv_pkts(rxfd, correct_payload, 2);

This verifies that a packet with bad csum does not coalesce to a valid
packet. Perhaps too paranoid but, do you also want to test the reverse
case?

Can be a single test by adding another write_packet and observing all
three separate packets.