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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Tue, Dec 03, 2013 at 02:34:56PM +0000, Paul Durrant wrote:
> > -----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.

How? __pskb_pull_tail doesn't seem to care about upper layer protocols.
And maybe_pull_tail has already done some lenght comparisions.

Wei.

> 
>   Paul

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