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

[Haiyang Zhang <haiyangz@microsoft.com>]

> -----Original Message-----
> From: Dexuan Cui <decui@microsoft.com>
> Sent: Saturday, February 15, 2020 12:24 AM
> To: Haiyang Zhang <haiyangz@microsoft.com>; Stephen Hemminger
> <sthemmin@microsoft.com>; David S. Miller <davem@davemloft.net>;
> netdev@vger.kernel.org; KY Srinivasan <kys@microsoft.com>; linux-
> kernel@vger.kernel.org
> Cc: linux-hyperv@vger.kernel.org
> Subject: Is it safe for a NIC driver to use all the 48 bytes of skb->cb?
> 
> Hi,
> It looks all the layers of drivers among the network stack can use the 48-byte
> skb->cb array. Is there any rule how they should coordinate with each other?
> 
> I noticed the last 16 bytes are used by struct skb_gso_cb:
> 
> include/linux/skbuff.h:
> struct skb_gso_cb {
>         union {
>                 int     mac_offset;
>                 int     data_offset;
>         };
>         int     encap_level;
>         __wsum  csum;
>         __u16   csum_start;
> };
> #define SKB_SGO_CB_OFFSET       32
> #define SKB_GSO_CB(skb) ((struct skb_gso_cb *)((skb)->cb +
> SKB_SGO_CB_OFFSET))
> 
> Does this mean a low level NIC driver (e.g. hv_netvsc) should only use the first
> 32 bytes? What if the upper layer network stack starts to take up more space in
> the future?

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);

> 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
> 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