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

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

[Jakub Kicinski]

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@kernel.org>
---
CC: o.rempel@pengutronix.de
CC: gerhard@engleder-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

Re: [PATCH net] net: selftests: fix TCP packet checksum

[Gerhard Engleder]

On 24.06.25 20:32, Jakub Kicinski wrote:
> 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@kernel.org>
> ---
> CC: o.rempel@pengutronix.de
> CC: gerhard@engleder-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 {

Reviewed-by: Gerhard Engleder <gerhard@engleder-embedded.com>

And also tested with my selftest patches for tsnep, which did not made
it into mainline. So I'm not sure if a Tested-by is allowed.

(Tested-by: Gerhard Engleder <gerhard@engleder-embedded.com>)

gerhard

Re: [PATCH net] net: selftests: fix TCP packet checksum

[Oleksij Rempel]

On Tue, Jun 24, 2025 at 11:32:58AM -0700, Jakub Kicinski wrote:
> 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@kernel.org>

Reported-by: Oleksij Rempel <o.rempel@pengutronix.de>
Tested-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Oleksij Rempel <o.rempel@pengutronix.de>

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Re: [PATCH net] net: selftests: fix TCP packet checksum

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (main)
by Paolo Abeni <pabeni@redhat.com>:

On Tue, 24 Jun 2025 11:32:58 -0700 you wrote:
> 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@kernel.org>
> 
> [...]

Here is the summary with links:
  - [net] net: selftests: fix TCP packet checksum
    https://git.kernel.org/netdev/net/c/8d89661a36dd

You are awesome, thank you!
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