Re: [PATCH net-next v2] net: Loosen constraints for recalculating checksum in skb_segment()

[Eric Dumazet <eric.dumazet@gmail.com>]

On Fri, 2013-05-17 at 15:49 +0900, Simon Horman wrote:
> In the case where the ability to offload checksums changes
> then it may be necessary for software checksumming of an skb to occur.
> 
> An example of such a case is where a non-GRE packet is received but
> is to be encapsulated and transmitted as GRE.
> 
> Another example relates to my proposed support for for GSO packets
> that are non-MPLS when received but MPLS when transmitted.
> 
> The cost of this change is that csum may be checked when it previously
> was not. In the case where csum is true this is pure overhead. In the
> case where csum is false it leads to software checksumming, which
> I believe also leads to correct checksums in transmitted packets for
> the cases described above.
> 
> This patch relies on the return value of can_checksum_protocol()
> being correct and in turn the return value of skb_network_protocol(),
> used to provide the protocol parameter of can_checksum_protocol(),
> being correct. It also relies on the features passed to skb_segment()
> and in turn to can_checksum_protocol() being correct.
> 
> I believe that this problem does has not been observed for VLANs because it
> appears that all drivers except xgbe set vlan_features such that that the
> checksum offload support for VLAN packets is greater than or equal to that
> of non-VLAN packets.
> 
> I wonder if the code in xgbe may be an oversight and the hardware does
> support checksumming of VLAN packets.  If so it may be worth updating the
> vlan_features of the driver as this patch will force such checksums to be
> performed in software rather than hardware.
> 
> Signed-off-by: Simon Horman <horms@verge.net.au>
> 
> ---
> 
> Tested using the bnx2 driver.
> 
> v2
> Rewrite changlog after further analysis
> ---
>  net/core/skbuff.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index af9185d..3a43be4 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -2853,7 +2853,7 @@ struct sk_buff *skb_segment(struct sk_buff *skb, netdev_features_t features)
>  						 doffset + tnl_hlen);
>  
>  		if (fskb != skb_shinfo(skb)->frag_list)
> -			continue;
> +			goto csum;
>  
>  		if (!sg) {
>  			nskb->ip_summed = CHECKSUM_NONE;
> @@ -2917,6 +2917,7 @@ skip_fraglist:
>  		nskb->len += nskb->data_len;
>  		nskb->truesize += nskb->data_len;
>  
> +csum:
>  		if (!csum) {
>  			nskb->csum = skb_checksum(nskb, doffset,
>  						  nskb->len - doffset, 0);


I had hard time to understand this patch, but I think I now get it.

Changelog seems to be very specific, while the change looks like a more
generic problem about skbs having a frag_list ? Oh well...

skb_segment() is probably too complex for my understanding.

Could you please change the label name ?

perform_csum_check for example, and maybe it would be easier :)