Re: [Xen-devel] [PATCH net v4] xen-netback: fix fragment detection in checksum setup

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Wei Liu [mailto:wei.liu2@xxxxxxxxxx]
> Sent: 03 December 2013 14:29
> To: Paul Durrant
> Cc: Wei Liu; xen-devel@xxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; Zoltan Kiss;
> Ian Campbell; David Vrabel; David Miller
> Subject: Re: [PATCH net v4] xen-netback: fix fragment detection in checksum
> setup
> 
> On Tue, Dec 03, 2013 at 02:05:17PM +0000, Paul Durrant wrote:
> [...]
> > > >
> > > > -       header_size = skb->network_header + off + MAX_IPOPTLEN;
> > > > -       maybe_pull_tail(skb, header_size);
> > > > +       if (!maybe_pull_tail(skb, sizeof(struct iphdr), MAX_IP_HDR_LEN))
> > > > +               goto out;
> > > > +
> > >
> > > I think you need to correctly update err to reflect this failure.
> > > Using -EPROTO will wrongly blame frontend while it is backend that's
> > > failing to process the packet.
> > >
> >
> > But a failure should only occur if the packet is malformed, so that would be
> a frontend error wouldn't it?
> >
> 
> __pskb_pull_tail may fail due to malloc failure.
> 
> However the return value of __pskb_pull_tail cannot reflect the wether
> the failure is due to malformed packet or OOM. Not sure what's the best
> solution here. What's the malformed packet you were talking about?
> 

For example, the pull would fail if the packet had an either_type of IP but 
didn't contain an IP header, or perhaps an IPv6 packet that had an incomplete 
option header sequence. I would have thought such a packet was a more likely 
cause of failure than OOM, so -EPROTO seems a reasonable best guess.

  Paul

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