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

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Message-ID: <20131203145749.GA728@zion.uk.xensource.com>
Date:	Tue, 3 Dec 2013 14:57:49 +0000
From:	Wei Liu <wei.liu2@...rix.com>
To:	Paul Durrant <Paul.Durrant@...rix.com>
CC:	Wei Liu <wei.liu2@...rix.com>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Zoltan Kiss <zoltan.kiss@...rix.com>,
	Ian Campbell <Ian.Campbell@...rix.com>,
	David Vrabel <david.vrabel@...rix.com>,
	David Miller <davem@...emloft.net>
Subject: Re: [PATCH net v4] xen-netback: fix fragment detection in checksum
 setup

On Tue, Dec 03, 2013 at 02:34:56PM +0000, Paul Durrant wrote:
> > -----Original Message-----
> > From: Wei Liu [mailto:wei.liu2@...rix.com]
> > Sent: 03 December 2013 14:29
> > To: Paul Durrant
> > Cc: Wei Liu; xen-devel@...ts.xen.org; netdev@...r.kernel.org; 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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