Subject: [PATCH net-next v2 6/8] selftests: drv-net: gro: make large packet math more precise

From: Jakub Kicinski <[email-masked]>
Date: Thu, 2 Apr 2026 13:59:58 -0700

When constructing the packets for large_* test cases we use
a static value for packet count and MSS. It works okay for
ipv4 vs ipv6 but the gap between ipv4 and ip6ip6 is going to
be quite significant.

Make the defines calculate the worst case values, those
are only used for sizing stack arrays. Create helpers for
calculating precise values based on the exact test case.

Signed-off-by: Jakub Kicinski <[email-masked]>
---
 tools/testing/selftests/net/lib/gro.c | 45 +++++++++++++++++----------
 1 file changed, 29 insertions(+), 16 deletions(-)

diff --git a/tools/testing/selftests/net/lib/gro.c b/tools/testing/selftests/net/lib/gro.c
index 3616ecc42a01..dc8638d5d74d 100644
--- a/tools/testing/selftests/net/lib/gro.c
+++ b/tools/testing/selftests/net/lib/gro.c
@@ -94,10 +94,11 @@
 #define START_SEQ 100
 #define START_ACK 100
 #define ETH_P_NONE 0
-#define MSS (4096 - sizeof(struct tcphdr) - sizeof(struct ipv6hdr))
-#define MAX_PAYLOAD (IP_MAXPACKET - sizeof(struct tcphdr) - sizeof(struct ipv6hdr))
-#define NUM_LARGE_PKT (MAX_PAYLOAD / MSS)
+#define ASSUMED_MTU 4096
+#define MAX_MSS (ASSUMED_MTU - sizeof(struct iphdr) - sizeof(struct tcphdr))
 #define MAX_HDR_LEN (ETH_HLEN + sizeof(struct ipv6hdr) + sizeof(struct tcphdr))
+#define MAX_LARGE_PKT_CNT ((IP_MAXPACKET - (MAX_HDR_LEN - ETH_HLEN)) /	\
+			   (ASSUMED_MTU - (MAX_HDR_LEN - ETH_HLEN)))
 #define MIN_EXTHDR_SIZE 8
 #define EXT_PAYLOAD_1 "\x00\x00\x00\x00\x00\x00"
 #define EXT_PAYLOAD_2 "\x11\x11\x11\x11\x11\x11"
@@ -146,6 +147,16 @@ static int max_payload(void)
 	return IP_MAXPACKET - (total_hdr_len - ETH_HLEN);
 }
 
+static int calc_mss(void)
+{
+	return ASSUMED_MTU - (total_hdr_len - ETH_HLEN);
+}
+
+static int num_large_pkt(void)
+{
+	return max_payload() / calc_mss();
+}
+
 static void vlog(const char *fmt, ...)
 {
 	va_list args;
@@ -525,18 +536,20 @@ static void send_data_pkts(int fd, struct sockaddr_ll *daddr,
  */
 static void send_large(int fd, struct sockaddr_ll *daddr, int remainder)
 {
-	static char pkts[NUM_LARGE_PKT][MAX_HDR_LEN + MSS];
-	static char last[MAX_HDR_LEN + MSS];
-	static char new_seg[MAX_HDR_LEN + MSS];
+	static char pkts[MAX_LARGE_PKT_CNT][MAX_HDR_LEN + MAX_MSS];
+	static char new_seg[MAX_HDR_LEN + MAX_MSS];
+	static char last[MAX_HDR_LEN + MAX_MSS];
+	const int num_pkt = num_large_pkt();
+	const int mss = calc_mss();
 	int i;
 
-	for (i = 0; i < NUM_LARGE_PKT; i++)
-		create_packet(pkts[i], i * MSS, 0, MSS, 0);
-	create_packet(last, NUM_LARGE_PKT * MSS, 0, remainder, 0);
-	create_packet(new_seg, (NUM_LARGE_PKT + 1) * MSS, 0, remainder, 0);
+	for (i = 0; i < num_pkt; i++)
+		create_packet(pkts[i], i * mss, 0, mss, 0);
+	create_packet(last, num_pkt * mss, 0, remainder, 0);
+	create_packet(new_seg, (num_pkt + 1) * mss, 0, remainder, 0);
 
-	for (i = 0; i < NUM_LARGE_PKT; i++)
-		write_packet(fd, pkts[i], total_hdr_len + MSS, daddr);
+	for (i = 0; i < num_pkt; i++)
+		write_packet(fd, pkts[i], total_hdr_len + mss, daddr);
 	write_packet(fd, last, total_hdr_len + remainder, daddr);
 	write_packet(fd, new_seg, total_hdr_len + remainder, daddr);
 }
@@ -1437,12 +1450,12 @@ static void gro_sender(void)
 
 	/* large sub-tests */
 	} else if (strcmp(testname, "large_max") == 0) {
-		int remainder = max_payload() % MSS;
+		int remainder = max_payload() % calc_mss();
 
 		send_large(txfd, &daddr, remainder);
 		write_packet(txfd, fin_pkt, total_hdr_len, &daddr);
 	} else if (strcmp(testname, "large_rem") == 0) {
-		int remainder = max_payload() % MSS;
+		int remainder = max_payload() % calc_mss();
 
 		send_large(txfd, &daddr, remainder + 1);
 		write_packet(txfd, fin_pkt, total_hdr_len, &daddr);
@@ -1646,14 +1659,14 @@ static void gro_receiver(void)
 
 	/* large sub-tests */
 	} else if (strcmp(testname, "large_max") == 0) {
-		int remainder = max_payload() % MSS;
+		int remainder = max_payload() % calc_mss();
 
 		correct_payload[0] = max_payload();
 		correct_payload[1] = remainder;
 		printf("Shouldn't coalesce if exceed IP max pkt size: ");
 		check_recv_pkts(rxfd, correct_payload, 2);
 	} else if (strcmp(testname, "large_rem") == 0) {
-		int remainder = max_payload() % MSS;
+		int remainder = max_payload() % calc_mss();
 
 		/* last segment sent individually, doesn't start new segment */
 		correct_payload[0] = max_payload() - remainder;
-- 
2.53.0