Subject: [PATCH net-next 2/6] selftests: drv-net: gro: add 1 byte payload test

From: Jakub Kicinski <[email-masked]>
Date: Wed, 1 Apr 2026 11:26:21 -0700

Small IPv4 packets get padded to 60B, this may break / confuse
some buggy implementations. Add a test to coalesce a 1B payload.
Keep this separate from the lrg_sml test because I suspect some
implementations may not handle this case (treat padded frames
as ineligible for coalescing).

Signed-off-by: Jakub Kicinski <[email-masked]>
---
 tools/testing/selftests/net/lib/gro.c      | 12 ++++++++++--
 tools/testing/selftests/drivers/net/gro.py |  4 +++-
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/net/lib/gro.c b/tools/testing/selftests/net/lib/gro.c
index 4d002af4a7aa..27ccd742615a 100644
--- a/tools/testing/selftests/net/lib/gro.c
+++ b/tools/testing/selftests/net/lib/gro.c
@@ -10,8 +10,9 @@
  *  packet coalesced: it can be smaller than the rest and coalesced
  *  as long as it is in the same flow.
  *   - data_same:    same size packets coalesce
- *   - data_lrg_sml: large then small coalesces
- *   - data_sml_lrg: small then large doesn't coalesce
+ *   - data_lrg_sml:   large then small coalesces
+ *   - data_lrg_1byte: large then 1 byte coalesces (Ethernet padding)
+ *   - data_sml_lrg:   small then large doesn't coalesce
  *   - data_burst:   two bursts of two, separated by 100ms
  *
  * ack:
@@ -1296,6 +1297,9 @@ static void gro_sender(void)
 	} else if (strcmp(testname, "data_lrg_sml") == 0) {
 		send_data_pkts(txfd, &daddr, PAYLOAD_LEN, PAYLOAD_LEN / 2);
 		write_packet(txfd, fin_pkt, total_hdr_len, &daddr);
+	} else if (strcmp(testname, "data_lrg_1byte") == 0) {
+		send_data_pkts(txfd, &daddr, PAYLOAD_LEN, 1);
+		write_packet(txfd, fin_pkt, total_hdr_len, &daddr);
 	} else if (strcmp(testname, "data_sml_lrg") == 0) {
 		send_data_pkts(txfd, &daddr, PAYLOAD_LEN / 2, PAYLOAD_LEN);
 		write_packet(txfd, fin_pkt, total_hdr_len, &daddr);
@@ -1474,6 +1478,10 @@ static void gro_receiver(void)
 		printf("large data packets followed by a smaller one: ");
 		correct_payload[0] = PAYLOAD_LEN * 1.5;
 		check_recv_pkts(rxfd, correct_payload, 1);
+	} else if (strcmp(testname, "data_lrg_1byte") == 0) {
+		printf("large data packet followed by a 1 byte one: ");
+		correct_payload[0] = PAYLOAD_LEN + 1;
+		check_recv_pkts(rxfd, correct_payload, 1);
 	} else if (strcmp(testname, "data_sml_lrg") == 0) {
 		printf("small data packets followed by a larger one: ");
 		correct_payload[0] = PAYLOAD_LEN / 2;
diff --git a/tools/testing/selftests/drivers/net/gro.py b/tools/testing/selftests/drivers/net/gro.py
index 10da5d4bee9b..73436d16b40d 100755
--- a/tools/testing/selftests/drivers/net/gro.py
+++ b/tools/testing/selftests/drivers/net/gro.py
@@ -11,6 +11,7 @@ coalescing behavior.
 Test cases:
   - data_same: Same size data packets coalesce
   - data_lrg_sml: Large packet followed by smaller one coalesces
+  - data_lrg_1byte: Large packet followed by 1B one coalesces (Ethernet padding)
   - data_sml_lrg: Small packet followed by larger one doesn't coalesce
   - ack: Pure ACK packets do not coalesce
   - flags_psh: Packets with PSH flag don't coalesce
@@ -289,7 +290,8 @@ def _run_gro_bin(cfg, test_name, protocol=None, num_flows=None,
 
     # Tests that work for all protocols
     common_tests = [
-        "data_same", "data_lrg_sml", "data_sml_lrg", "data_burst",
+        "data_same", "data_lrg_sml", "data_sml_lrg", "data_lrg_1byte",
+        "data_burst",
         "ack",
         "flags_psh", "flags_syn", "flags_rst", "flags_urg", "flags_cwr",
         "tcp_csum", "tcp_seq", "tcp_ts", "tcp_opt",
-- 
2.53.0