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

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Wei Liu [mailto:wei.liu2@xxxxxxxxxx]
> Sent: 28 November 2013 13:13
> To: Paul Durrant
> Cc: xen-devel@xxxxxxxxxxxxx; Wei Liu; Ian Campbell; David Vrabel
> Subject: Re: [PATCH net] xen-netback: fix fragment detection in checksum
> setup
> 
> You seem to have forgotten to Cc netdev@.
> 

No, that was deliberate: I thought first review was supposed to be limited to 
xen-devel. Is that not the case any more?

  Paul

> On Thu, Nov 28, 2013 at 12:40:20PM +0000, Paul Durrant wrote:
> > The code to detect fragments in checksum_setup() was missing for IPv4
> and
> > too eager for IPv6. (It transpires that Windows seems to send IPv6 packets
> > with a fragment header even if they are not a fragment - i.e. offset is 
> > zero,
> > and M bit is not set).
> >
> > Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
> > Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
> > Cc: Ian Campbell <ian.campbell@xxxxxxxxxx>
> > Cc: David Vrabel <david.vrabel@xxxxxxxxxx>
> > ---
> >  drivers/net/xen-netback/netback.c |   31
> ++++++++++++++++++++++++++++---
> >  1 file changed, 28 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-
> netback/netback.c
> > index 919b650..eea7ff2 100644
> > --- a/drivers/net/xen-netback/netback.c
> > +++ b/drivers/net/xen-netback/netback.c
> > @@ -1165,15 +1165,27 @@ static int checksum_setup_ip(struct xenvif *vif,
> struct sk_buff *skb,
> >     struct iphdr *iph = (void *)skb->data;
> >     unsigned int header_size;
> >     unsigned int off;
> > +   bool fragment;
> >     int err = -EPROTO;
> >
> > +   fragment = false;
> > +
> >     off = sizeof(struct iphdr);
> >
> >     header_size = skb->network_header + off + MAX_IPOPTLEN;
> >     maybe_pull_tail(skb, header_size);
> >
> > +   if (ntohs(iph->frag_off) & 0x3fff)
> 
> I'm actually quite suprised that these magic numbers don't have a #define
> in network core code.
> 
> 0x3fff, if I'm not mistaken, is to check if a) M bit is set b) frag
> offset is not zero. I think this warrants a line of comment.
> 
> > +           fragment = true;
> > +
> >     off = iph->ihl * 4;
> >
> > +   if (fragment) {
> > +           if (net_ratelimit())
> > +                   netdev_err(vif->dev, "Packet is a fragment!\n");
> > +           goto out;
> > +   }
> > +
> >     switch (iph->protocol) {
> >     case IPPROTO_TCP:
> >             if (!skb_partial_csum_set(skb, off,
> > @@ -1237,6 +1249,7 @@ static int checksum_setup_ipv6(struct xenvif *vif,
> struct sk_buff *skb,
> >     bool fragment;
> >     bool done;
> >
> > +   fragment = false;
> >     done = false;
> >
> >     off = sizeof(struct ipv6hdr);
> > @@ -1275,9 +1288,21 @@ static int checksum_setup_ipv6(struct xenvif
> *vif, struct sk_buff *skb,
> >                     off += (hp->hdrlen+2)<<2;
> >                     break;
> >             }
> > -           case IPPROTO_FRAGMENT:
> > -                   fragment = true;
> > -                   /* fall through */
> > +           case IPPROTO_FRAGMENT: {
> > +                   struct frag_hdr *hp = (void *)(skb->data + off);
> > +
> > +                   header_size = skb->network_header +
> > +                           off +
> > +                           sizeof(struct frag_hdr);
> > +                   maybe_pull_tail(skb, header_size);
> > +
> > +                   if (ntohs(hp->frag_off) & 0xFFF9)
> > +                           fragment = true;
> > +
> 
> Same here, would be helpful if you add comment to clarify what 0xFFF9
> is.
> 
> Wei.
> 
> > +                   nexthdr = hp->nexthdr;
> > +                   off += sizeof(struct frag_hdr);
> > +                   break;
> > +           }
> >             default:
> >                     done = true;
> >                     break;
> > --
> > 1.7.10.4

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