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

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Date:	Thu, 16 Nov 2006 23:34:14 +0100
From:	"Jesper Juhl" <jesper.juhl@...il.com>
To:	netdev@...r.kernel.org
Cc:	"David Miller" <davem@...emloft.net>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: IPv4: ip_options_compile() how can we avoid blowing up on a NULL skb???

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@...il.com>
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