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

From: Jakub Kicinski <[email-masked]>
Date: Wed, 1 Apr 2026 11:26:25 -0700

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);
 	} else if (strcmp(testname, "ip_ttl") == 0) {
 		correct_payload[0] = PAYLOAD_LEN;
 		correct_payload[1] = PAYLOAD_LEN;
diff --git a/tools/testing/selftests/drivers/net/gro.py b/tools/testing/selftests/drivers/net/gro.py
index 68b7b29ea2ee..ba9f4146034c 100755
--- a/tools/testing/selftests/drivers/net/gro.py
+++ b/tools/testing/selftests/drivers/net/gro.py
@@ -306,6 +306,7 @@ def _run_gro_bin(cfg, test_name, protocol=None, num_flows=None,
 
     # Tests specific to IPv4
     ipv4_tests = [
+        "ip_csum",
         "ip_ttl", "ip_opt", "ip_frag4",
         "ip_id_df1_inc", "ip_id_df1_fixed",
         "ip_id_df0_inc", "ip_id_df0_fixed",
-- 
2.53.0