Subject: Re: [PATCH net] {xfrm, sctp} Stick to software crc32 even if hardware is capable of that

From: Vlad Yasevich <[email-masked]>
Date: Thu, 10 Oct 2013 10:11:04 -0400

On 10/10/2013 01:51 AM, Fan Du wrote:
> igb/ixgbe have hardware sctp checksum support, when this feature is enabled
> and also IPsec is armed to protect sctp traffic, ugly things happened as
> xfrm_output checks CHECKSUM_PARTIAL to do check sum operation(sum every thing
> up and pack the 16bits result in the checksum field). The result is fail
> establishment of sctp communication.
>
> And I saw another point in this part of code, when IPsec is not armed,
> sctp communication is good, however setting setting CHECKSUM_PARTIAL will
> make xfrm_output compute dummy checksum values which will be overwritten by
> hardware lately.
>
> So this patch try to solve above two issues together.
>
> Signed-off-by: Fan Du <[email-masked]>
> ---
> note:
>    igb/ixgbe hardware is not handy on my side, so just build test only.
>
> ---
>   net/sctp/output.c |   27 +++++++++++++++++++--------
>   1 file changed, 19 insertions(+), 8 deletions(-)
>
> diff --git a/net/sctp/output.c b/net/sctp/output.c
> index 0ac3a65..f0b9cc5 100644
> --- a/net/sctp/output.c
> +++ b/net/sctp/output.c
> @@ -372,6 +372,16 @@ static void sctp_packet_set_owner_w(struct sk_buff *skb, struct sock *sk)
>   	atomic_inc(&sk->sk_wmem_alloc);
>   }
>
> +static int is_xfrm_armed(struct dst_entry *dst)
> +{
> +#ifdef CONFIG_XFRM
> +	/* If dst->xfrm is valid, this skb needs to be transformed */
> +	return dst->xfrm != NULL;
> +#else
> +	return 0;
> +#endif
> +}
> +
>   /* All packets are sent to the network through this function from
>    * sctp_outq_tail().
>    *
> @@ -536,20 +546,21 @@ int sctp_packet_transmit(struct sctp_packet *packet)
>   	 * by CRC32-C as described in <draft-ietf-tsvwg-sctpcsum-02.txt>.
>   	 */
>   	if (!sctp_checksum_disable) {
> -		if (!(dst->dev->features & NETIF_F_SCTP_CSUM)) {
> +		if ((!(dst->dev->features & NETIF_F_SCTP_CSUM)) ||
> +			is_xfrm_armed(dst)) {
> +
>   			__u32 crc32 = sctp_start_cksum((__u8 *)sh, cksum_buf_len);
>
>   			/* 3) Put the resultant value into the checksum field in the
>   			 *    common header, and leave the rest of the bits unchanged.
>   			 */
>   			sh->checksum = sctp_end_cksum(crc32);
> -		} else {
> -			/* no need to seed pseudo checksum for SCTP */
> -			nskb->ip_summed = CHECKSUM_PARTIAL;
> -			nskb->csum_start = (skb_transport_header(nskb) -
> -			                    nskb->head);
> -			nskb->csum_offset = offsetof(struct sctphdr, checksum);
> -		}
> +		} else
> +			/* Mark skb as CHECKSUM_UNNECESSARY to let hardware compute
> +			 * the checksum, and also avoid xfrm_output to do unceccessary
> +			 * checksum.
> +			 */
> +			nskb->ip_summed = CHECKSUM_UNNECESSARY;
>   	}

In addition to what Niel said, the use of CHECKSUM_UNNECESSARY is
incorrect as it will cause the nic to not compute the checksum.
The checksum offload depends on the use of CHECKSUM_PARTIAL.

-vlad

>   	/* IP layer ECN support