On Mon, Mar 05, 2012 at 09:33:54PM +0100, Hans Schillstrom wrote:
> Hello,
> On Monday, March 05, 2012 19:22:48 Pablo Neira Ayuso wrote:
> > Let me trim off parts that have been already discussed.
> >
> > On Mon, Mar 05, 2012 at 11:09:46AM +0100, Hans Schillstrom wrote:
> > [...]
> > > ...
> > > > > +
> > > > > +/*
> > > > > + * ICMP, get header offset if icmp error
> > > > > + */
> > > > > +static int get_inner_hdr(struct sk_buff *skb, int iphsz, int nhoff)
> > > > > +{
> > > > > + const struct icmphdr *icmph;
> > > > > + struct icmphdr _ih;
> > > > > +
> > > > > + /* Not enough header? */
> > > > > + icmph = skb_header_pointer(skb, nhoff + iphsz, sizeof(_ih), &_ih);
> > > > > + if (icmph == NULL)
> > > > > + return nhoff;
> > > >
> > > > I think this has to return -1 in this case.
> > >
> > > No, it must return the unchanged value of nhoffs.
> > > Unless I change the usge of it as described later.
> >
> > I see, you're defaulting on the original header if you cannot get the
> > ICMP header (eg. fragment case).
> >
> > > > > +
> > > > > + if (icmph->type > NR_ICMP_TYPES)
> > > > > + return nhoff;
> > > >
> > > > Same thing.
> > >
> > > Same comment.
> >
> > So if you get an unsupportted ICMP message, you rely on the original
> > IP header.
> >
>
> Yes, thats right.
>
> >
> [snip]
> >
> > I think you can do like in other parts of netfilter:
> >
> > union hmark_ports _uports = { ... };
> > union hmark_ports *uports;
> >
> > uports = skb_header_pointer(skb, nhoffs + poff, sizeof(_uports), &_uports);
> > if (uports == NULL)
> > return XT_CONTINUE;
> >
> > Then use uports->v32 in the rest of the code.
>
> If I do so, the original packet might be altered which is very bad.
> i.e. if skb_header_pointer() return skb->data + offset; then I will write
> right into the skb :-(
I see, then use skb_copy_bits(skb, offset, buffer, len) instead of
skb_header_pointer.
Thanks Hans.
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