tcp: Set CHECKSUM_UNNECESSARY in tcp_init_nondata_skb

Back in commit 04a0551c87363f100b04d28d7a15a632b70e18e7
("loopback: Drop obsolete ip_summed setting") we stopped
setting CHECKSUM_UNNECESSARY in the loopback xmit.

This is because such a setting was a lie since it implies that the
checksum field of the packet is properly filled in.

Instead what happens normally is that CHECKSUM_PARTIAL is set and
skb->csum is calculated as needed.

But this was only happening for TCP data packets (via the
skb->ip_summed assignment done in tcp_sendmsg()).  It doesn't
happen for non-data packets like ACKs etc.

Fix this by setting skb->ip_summed in the common non-data packet
constructor.  It already is setting skb->csum to zero.

But this reminds us that we still have things like ip_output.c's
ip_dev_loopback_xmit() which sets skb->ip_summed to the value
CHECKSUM_UNNECESSARY, which Herbert's patch teaches us is not
valid.  So we'll have to address that at some point too.

Signed-off-by: David S. Miller <davem@davemloft.net>

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On Thu, 2010-04-08 at 01:26 -0700, David Miller wrote:
> Fix this by setting skb->ip_summed in the common non-data packet
> constructor.  It already is setting skb->csum to zero.
> Signed-off-by: David S. Miller <davem@davemloft.net>
> 
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index f181b78..00afbb0 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -349,6 +349,7 @@ static inline void TCP_ECN_send(struct sock *sk, struct sk_buff *skb,
>   */
>  static void tcp_init_nondata_skb(struct sk_buff *skb, u32 seq, u8 flags)
>  {
> +	skb->ip_summed = CHECKSUM_PARTIAL;
>  	skb->csum = 0;
>  
>  	TCP_SKB_CB(skb)->flags = flags;

There might be trivial value in using the
struct layout order for the sets avoiding
crossing cachelines.

from:

static void tcp_init_nondata_skb(struct sk_buff *skb, u32 seq, u8 flags)
{
	skb->ip_summed = CHECKSUM_PARTIAL;
	skb->csum = 0;

	TCP_SKB_CB(skb)->flags = flags;
	TCP_SKB_CB(skb)->sacked = 0;

	skb_shinfo(skb)->gso_segs = 1;
	skb_shinfo(skb)->gso_size = 0;
	skb_shinfo(skb)->gso_type = 0;

	TCP_SKB_CB(skb)->seq = seq;
	if (flags & (TCPCB_FLAG_SYN | TCPCB_FLAG_FIN))
		seq++;
	TCP_SKB_CB(skb)->end_seq = seq;
}

to:

static void tcp_init_nondata_skb(struct sk_buff *skb, u32 seq, u8 flags)
{
	skb->ip_summed = CHECKSUM_PARTIAL; 
	skb->csum = 0;

	TCP_SKB_CB(skb)->seq = seq;
	if (flags & (TCPCB_FLAG_SYN | TCPCB_FLAG_FIN))
		seq++;
	TCP_SKB_CB(skb)->end_seq = seq;
	TCP_SKB_CB(skb)->sacked = 0;
	TCP_SKB_CB(skb)->flags = flags;

	skb_shinfo(skb)->gso_size = 0;
	skb_shinfo(skb)->gso_segs = 1;
	skb_shinfo(skb)->gso_type = 0;
}


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On Thu, Apr 08, 2010 at 01:26:17AM -0700, David Miller wrote:
> 
> Back in commit 04a0551c87363f100b04d28d7a15a632b70e18e7
> ("loopback: Drop obsolete ip_summed setting") we stopped
> setting CHECKSUM_UNNECESSARY in the loopback xmit.
> 
> This is because such a setting was a lie since it implies that the
> checksum field of the packet is properly filled in.
> 
> Instead what happens normally is that CHECKSUM_PARTIAL is set and
> skb->csum is calculated as needed.
> 
> But this was only happening for TCP data packets (via the
> skb->ip_summed assignment done in tcp_sendmsg()).  It doesn't
> happen for non-data packets like ACKs etc.

I don't understand.  If said packet was going across the network,
how could it possibly have worked?

Cheers,