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

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Message-ID: <c182ba60-81fc-4776-b3bb-6997e87f7749@engleder-embedded.com>
Date: Tue, 24 Jun 2025 22:53:41 +0200
From: Gerhard Engleder <gerhard@...leder-embedded.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: netdev@...r.kernel.org, edumazet@...gle.com, pabeni@...hat.com,
 andrew+netdev@...n.ch, horms@...nel.org, o.rempel@...gutronix.de,
 davem@...emloft.net
Subject: Re: [PATCH net] net: selftests: fix TCP packet checksum

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@...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 {

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

gerhard