RE: Is it safe for a NIC driver to use all the 48 bytes of skb->cb?

From: Haiyang Zhang <haiyangz@microsoft.com>
Sent: Saturday, February 15, 2020 7:20 AM
To: Dexuan Cui <decui@microsoft.com>; Stephen Hemminger

According to the comments in skbuff.h below, it is the responsibility of the
owning layer to make a SKB clone, if it wants to keep the data across layers. 
So, every layer can still use all of the 48 bytes.

        /*
         * This is the control buffer. It is free to use for every
         * layer. Please put your private variables there. If you
         * want to keep them across layers you have to do a skb_clone()
         * first. This is owned by whoever has the skb queued ATM.
         */
        char                    cb[48] __aligned(8);

quoted

Now hv_netvsc assumes it can use all of the 48-bytes, though it uses only
20 bytes, but just in case the struct hv_netvsc_packet grows to >32 bytes in

the

quoted

future, should we change the BUILD_BUG_ON() in netvsc_start_xmit() to
BUILD_BUG_ON(sizeof(struct hv_netvsc_packet) > SKB_SGO_CB_OFFSET); ?

Based on the explanation above, the existing hv_netvsc code is correct.

Thanks,
- Haiyang

Got it. So if the upper layer saves something in the cb, it must do a skb_clone()
and pass the new skb to hv_netvsc. hv_netvsc is the lowest layer in the network 
stack, so it can use all the 48 bytes without calling skb_clone().

BTW, now I happen to have a different question: in netvsc_probe() we have 
net->needed_headroom = RNDIS_AND_PPI_SIZE;
I think this means when the network stack (ARP, IP, ICMP, TCP, UDP,etc) passes a 
skb to hv_netvsc, the skb's headroom is increased by an extra size of 
net->needed_headroom, right? Then in netvsc_xmit(), why do we still need to
call skb_cow_head(skb, RNDIS_AND_PPI_SIZE)? -- this looks unnecessary to me?

PS, what does the "cow" here mean? Copy On Write? It looks skb_cow_head()
just copies the data (if necessary) and it has nothing to do with the 
write-protection in the MMU code.

Thanks,
Dexuan