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 14:03
> To: Paul Durrant
> Cc: xen-devel@xxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; Zoltan Kiss; Wei Liu;
> Ian Campbell; David Vrabel; David Miller
> Subject: Re: [PATCH net v4] xen-netback: fix fragment detection in checksum
> setup
> 
> On Mon, Dec 02, 2013 at 05:35:15PM +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).
> > This patch also incorporates a fix to callers of maybe_pull_tail() where
> > skb->network_header was being erroneously added to the length
> argument.
> >
> > Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
> > Signed-off-by: Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>
> > Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
> > Cc: Ian Campbell <ian.campbell@xxxxxxxxxx>
> > Cc: David Vrabel <david.vrabel@xxxxxxxxxx>
> > cc: David Miller <davem@xxxxxxxxxxxxx>
> > ---
> > v4
> > - Re-work maybe_pull_tail() to have a failure path and update mac and
> >   network header pointers on success.
> > - Re-work callers of maybe_pull_tail() to handle failures and allow for
> >   movement of skb header pointers.
> > - Incorporate fixes from patch "Fix pull size in checksum_setup_ip" since
> >   it also modifies callers of maybe_pull_tail().
> > - Remove error messages in checksum routines that could be triggered by
> frontends
> >
> > v3
> > - Use defined values for 'fragment offset' and 'more fragments'
> >
> > v2
> > - Added comments noting what fragment/offset masks mean
> >
> >  drivers/net/xen-netback/netback.c |  215 ++++++++++++++++++++-------
> ----------
> >  include/linux/ipv6.h              |    1 +
> >  include/net/ipv6.h                |    3 +-
> >  3 files changed, 120 insertions(+), 99 deletions(-)
> >
> > diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-
> netback/netback.c
> > index 64f0e0d..e3d7216 100644
> > --- a/drivers/net/xen-netback/netback.c
> > +++ b/drivers/net/xen-netback/netback.c
> > @@ -1149,49 +1149,62 @@ static int xenvif_set_skb_gso(struct xenvif *vif,
> >     return 0;
> >  }
> >
> > -static inline void maybe_pull_tail(struct sk_buff *skb, unsigned int len)
> > +static inline bool maybe_pull_tail(struct sk_buff *skb, unsigned int len,
> > +                              unsigned int max)
> >  {
> > -   if (skb_is_nonlinear(skb) && skb_headlen(skb) < len) {
> > -           /* If we need to pullup then pullup to the max, so we
> > -            * won't need to do it again.
> > -            */
> > -           int target = min_t(int, skb->len, MAX_TCP_HEADER);
> > -           __pskb_pull_tail(skb, target - skb_headlen(skb));
> > -   }
> > +   if (skb_headlen(skb) >= len)
> > +           return true;
> > +
> > +   /* If we need to pullup then pullup to the max, so we
> > +    * won't need to do it again.
> > +    */
> > +   if (max > skb->len)
> > +           max = skb->len;
> > +
> > +   __pskb_pull_tail(skb, max - skb_headlen(skb));
> > +
> > +   return skb_headlen(skb) >= len;
> >  }
> >
> > +/* This value should be large enough to cover a tagged ethernet header
> plus
> > + * maximally sized IP and TCP or UDP headers.
> > + */
> > +#define MAX_IP_HDR_LEN 128
> > +
> >  static int checksum_setup_ip(struct xenvif *vif, struct sk_buff *skb,
> >                          int recalculate_partial_csum)
> >  {
> > -   struct iphdr *iph = (void *)skb->data;
> > -   unsigned int header_size;
> >     unsigned int off;
> > +   bool fragment;
> >     int err = -EPROTO;
> >
> > -   off = sizeof(struct iphdr);
> > +   fragment = false;
> >
> > -   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?

  Paul

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