Re: [PATCH net-next 1/2] IPv6/GRO: generic helper to remove temporary HBH/jumbo header in driver

[Alexander Lobakin]

From: Alexander Lobakin <alexandr.lobakin@xxxxxxxxx>
Date: Wed, 23 Nov 2022 17:38:25 +0100

> From: Coco Li <lixiaoyan@xxxxxxxxxx>
> Date: Tue, 22 Nov 2022 15:27:39 -0800
> 
> > IPv6/TCP and GRO stacks can build big TCP packets with an added
> > temporary Hop By Hop header.
> > 
> > Is GSO is not involved, then the temporary header needs to be removed in
> > the driver. This patch provides a generic helper for drivers that need
> > to modify their headers in place.
> > 
> > Signed-off-by: Coco Li <lixiaoyan@xxxxxxxxxx>
> > ---
> >  include/net/ipv6.h | 33 +++++++++++++++++++++++++++++++++
> >  1 file changed, 33 insertions(+)
> > 
> > diff --git a/include/net/ipv6.h b/include/net/ipv6.h
> > index d383c895592a..a11d58c85c05 100644
> > --- a/include/net/ipv6.h
> > +++ b/include/net/ipv6.h
> > @@ -500,6 +500,39 @@ static inline int ipv6_has_hopopt_jumbo(const struct sk_buff *skb)
> >  	return jhdr->nexthdr;
> >  }
> >  
> > +/* Return 0 if HBH header is successfully removed
> > + * Or if HBH removal is unnecessary (packet is not big TCP)
> > + * Return error to indicate dropping the packet
> > + */
> > +static inline int ipv6_hopopt_jumbo_remove(struct sk_buff *skb)
> > +{
> > +	const int hophdr_len = sizeof(struct hop_jumbo_hdr);
> > +	int nexthdr = ipv6_has_hopopt_jumbo(skb);
> > +	struct ipv6hdr *h6;
> > +
> > +	if (!nexthdr)
> > +		return 0;
> > +
> > +	if (skb_cow_head(skb, 0))
> > +		return -1;
> 
> 	err = skb_cow_head(skb, 0);
> 	if (err)
> 		return err;
> 
> Alternatively, if you want to keep it simple, make the function bool
> and return false on `if (skb_cow_head(skb, 0)` and true otherwise.
> 
> > +
> > +	/* Remove the HBH header.
> > +	 * Layout: [Ethernet header][IPv6 header][HBH][L4 Header]
> > +	 */
> > +	memmove(skb->data + hophdr_len,
> > +		skb->data,
> 
> This can fit into the previous line.
> 
> > +		ETH_HLEN + sizeof(struct ipv6hdr));
> 
> Not correct at this point. I assume you took the implementation from
> ip6_offload.c[0], but ::gso_segment() and ::ndo_start_xmit() are two

Traditionally forgot to paste the links to the end of the mail. Pls
look at the end of this one for them (if I don't forget to paste
them again :clownface:).

> different entry points. Here you may have not only Eth header, but
> also VLAN, MPLS and whatnot.
> Correct way would be:
> 
> 	memmove(skb_mac_header(skb) + hophdr_len, skb_mac_header(skb),
> 		ipv6_hdr(skb) - skb_mac_header(skb) +
> 		sizeof(struct ipv6hdr));
> 
> > +
> > +	skb->data += hophdr_len;
> > +	skb->len -= hophdr_len;
> > +	skb->network_header += hophdr_len;
> 
> skb->mac_header also needs to be adjusted, the fact that it's equal
> to skb->data at the entry of ::ndo_start_xmit() doesn't mean
> anything.

Also, while I'm here: you should use skb_may_pull() +
{,__}skb_pull() here instead of manual maths. ::network_header and
::mac_header still need to be adjusted manually tho.

> 
> > +
> > +	h6 = ipv6_hdr(skb);
> > +	h6->nexthdr = nexthdr;
> > +
> > +	return 0;
> > +}
> 
> Please switch all the places where the same logics is used to your
> new helper.
> 
> > +
> >  static inline bool ipv6_accept_ra(struct inet6_dev *idev)
> >  {
> >  	/* If forwarding is enabled, RA are not accepted unless the special
> > -- 
> > 2.38.1.584.g0f3c55d4c2-goog
> 
> Thanks,
> Olek

[0] https://elixir.bootlin.com/linux/v6.1-rc6/source/net/ipv6/ip6_offload.c#L92

Thanks,
Olek