netdev - [PATCH net] net: selftests: fix TCP packet checksum

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Message-ID: <20250624183258.3377740-1-kuba@kernel.org>
Date: Tue, 24 Jun 2025 11:32:58 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: davem@...emloft.net
Cc: netdev@...r.kernel.org,
	edumazet@...gle.com,
	pabeni@...hat.com,
	andrew+netdev@...n.ch,
	horms@...nel.org,
	Jakub Kicinski <kuba@...nel.org>,
	o.rempel@...gutronix.de,
	gerhard@...leder-embedded.com
Subject: [PATCH net] net: selftests: fix TCP packet checksum

The length in the pseudo header should be the length of the L3 payload
AKA the L4 header+payload. The selftest code builds the packet from
the lower layers up, so all the headers are pushed already when it
constructs L4. We need to subtract the lower layer headers from skb->len.

Fixes: 3e1e58d64c3d ("net: add generic selftest support")
Signed-off-by: Jakub Kicinski <kuba@...nel.org>
---
CC: o.rempel@...gutronix.de
CC: gerhard@...leder-embedded.com

I changed the math from the pointers to the offsets (which is what
udp4_hwcsum() does). Oleksij, would you be willing to retest and send
your Reported-and-tested-by: tag?
---
 net/core/selftests.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/net/core/selftests.c b/net/core/selftests.c
index 35f807ea9952..406faf8e5f3f 100644
--- a/net/core/selftests.c
+++ b/net/core/selftests.c
@@ -160,8 +160,9 @@ static struct sk_buff *net_test_get_skb(struct net_device *ndev,
 	skb->csum = 0;
 	skb->ip_summed = CHECKSUM_PARTIAL;
 	if (attr->tcp) {
-		thdr->check = ~tcp_v4_check(skb->len, ihdr->saddr,
-					    ihdr->daddr, 0);
+		int l4len = skb->len - skb_transport_offset(skb);
+
+		thdr->check = ~tcp_v4_check(l4len, ihdr->saddr, ihdr->daddr, 0);
 		skb->csum_start = skb_transport_header(skb) - skb->head;
 		skb->csum_offset = offsetof(struct tcphdr, check);
 	} else {
-- 
2.49.0