netdev - Re: [net v1 PATCH 4/4] net: fix bpf_xdp_adjust

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Date:   Fri, 2 Aug 2019 09:53:54 +0200
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        xdp-newbies@...r.kernel.org,
        Daniel Borkmann <borkmann@...earbox.net>,
        brandon.cazander@...tapplied.net,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        brouer@...hat.com
Subject: Re: [net v1 PATCH 4/4] net: fix bpf_xdp_adjust_head regression for
 generic-XDP

On Thu, 1 Aug 2019 17:44:06 -0700
Jakub Kicinski <jakub.kicinski@...ronome.com> wrote:

> On Thu, 01 Aug 2019 20:00:31 +0200, Jesper Dangaard Brouer wrote:
> > When generic-XDP was moved to a later processing step by commit
> > 458bf2f224f0 ("net: core: support XDP generic on stacked devices.")
> > a regression was introduced when using bpf_xdp_adjust_head.
> > 
> > The issue is that after this commit the skb->network_header is now
> > changed prior to calling generic XDP and not after. Thus, if the header
> > is changed by XDP (via bpf_xdp_adjust_head), then skb->network_header
> > also need to be updated again.  Fix by calling skb_reset_network_header().
> > 
> > Fixes: 458bf2f224f0 ("net: core: support XDP generic on stacked devices.")
> > Reported-by: Brandon Cazander <brandon.cazander@...tapplied.net>
> > Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>  
> 
> Out of curiosity what was your conclusion regarding resetting the
> transport header as well?

Well, I don't know... I need some review, from e.g. Stephen that
changed this... I've added code snippets below signature to helper
reviewers (also helps understand below paragraph).

I think, we perhaps should call skb_reset_transport_header(), as we
change skb->data (via either __skb_pull() or __skb_push()), *BUT* I'm
not sure it is needed/required, as someone/something afterwards still
need to call skb_set_transport_header(), which also calls
skb_reset_transport_header() anyway.

I also want to ask reviewers, if we should move the call to
skb_reset_mac_len() (which Stephen added) in __netif_receive_skb_core()
into netif_receive_generic_xdp() (after the call to
skb_reset_network_header()), (it would be more natural to keep them
together)?

- - 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer

Code snippet
	/* check if bpf_xdp_adjust_head was used */
	off = xdp->data - orig_data;
	if (off) {
		if (off > 0)
			__skb_pull(skb, off);
		else if (off < 0)
			__skb_push(skb, -off);

		skb->mac_header += off;
		skb_reset_network_header(skb);
	}


static inline bool skb_transport_header_was_set(const struct sk_buff *skb)
{
	return skb->transport_header != (typeof(skb->transport_header))~0U;
}

static inline unsigned char *skb_transport_header(const struct sk_buff *skb)
{
	return skb->head + skb->transport_header;
}

static inline void skb_reset_transport_header(struct sk_buff *skb)
{
	skb->transport_header = skb->data - skb->head;
}

static inline void skb_set_transport_header(struct sk_buff *skb,
					    const int offset)
{
	skb_reset_transport_header(skb);
	skb->transport_header += offset;
}