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

[Haiyang Zhang]

> -----Original Message-----
> From: Dexuan Cui <decui@xxxxxxxxxxxxx>
> Sent: Saturday, February 15, 2020 1:04 PM
> To: Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>; Stephen Hemminger
> <sthemmin@xxxxxxxxxxxxx>; David S. Miller <davem@xxxxxxxxxxxxx>;
> netdev@xxxxxxxxxxxxxxx; KY Srinivasan <kys@xxxxxxxxxxxxx>; linux-
> kernel@xxxxxxxxxxxxxxx
> Cc: linux-hyperv@xxxxxxxxxxxxxxx
> Subject: RE: Is it safe for a NIC driver to use all the 48 bytes of skb->cb?
> 
> > From: Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>
> > Sent: Saturday, February 15, 2020 7:20 AM
> > To: Dexuan Cui <decui@xxxxxxxxxxxxx>; 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?

skb_cow_head() only expands the headroom if it is not enough, in case some 
upper layer path didn't reserve enough.

> 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.

Unrelated to MMU. It just copies some data to make room for writing.

Thanks,
- Haiyang