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

From: Fan Du <[email-masked]>
Date: Fri, 11 Oct 2013 15:02:44 +0800

On 2013年10月10日 22:11, Vlad Yasevich wrote:
> 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.

My bad, my head is cramed with IPsec recently :( We should fix this in ipv4/v6 output path.

> -vlad
>> /* IP layer ECN support

-- 
浮沉随浪只记今朝笑

--fan