IPv4: ip_options_compile() how can we avoid blowing up on a NULL skb???

["Jesper Juhl" <jesper.juhl@gmail.com>]

Hi,

In net/ipv4/ip_options.c::ip_options_compile() we have the following
code at the start of the function :


int ip_options_compile(struct ip_options * opt, struct sk_buff * skb)
{
       int l;
       unsigned char * iph;
       unsigned char * optptr;
       int optlen;
       unsigned char * pp_ptr = NULL;
       struct rtable *rt = skb ? (struct rtable*)skb->dst : NULL;

       if (!opt) {
               opt = &(IPCB(skb)->opt);
               iph = skb->nh.raw;
               opt->optlen = ((struct iphdr *)iph)->ihl*4 -
sizeof(struct iphdr);
               optptr = iph + sizeof(struct iphdr);
               opt->is_data = 0;
       } else {
               optptr = opt->is_data ? opt->__data : (unsigned
char*)&(skb->nh.iph[1]);
               iph = optptr - sizeof(struct iphdr);
       }

...

I don't see how that avoids blowing up if we get passed a NULL skb.

> From the line
   struct rtable *rt = skb ? (struct rtable*)skb->dst : NULL;
it is clear that we /may/ get passed a null skb.

Then a bit further down in the  if (!opt)  bit we dereference 'skb' :
   opt = &(IPCB(skb)->opt);
and we also may dereference it in the  else  part :
   optptr = opt->is_data ? opt->__data : (unsigned char*)&(skb->nh.iph[1]);

So if 'skb' is NULL, the only route I see that doesn't cause a NULL
pointer deref is if  (opt != NULL)  and at the same time
(opt->is_data != NULL)  .   Is that guaranteed in any way?  If now,
how come we don't blow up regularly?


--
Jesper Juhl <jesper.juhl@gmail.com>
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