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

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Date:   Sat, 15 Feb 2020 18:04:13 +0000
From:   Dexuan Cui <decui@...rosoft.com>
To:     Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "David S. Miller" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        KY Srinivasan <kys@...rosoft.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>
Subject: RE: Is it safe for a NIC driver to use all the 48 bytes of skb->cb?

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

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