Pointer type of _arp in __skb_flow_dissect()

Pointer type of _arp in __skb_flow_dissect()

[Nicolas Iooss]

Hello,

Linux 4.11-rc4 contains the following code in function
__skb_flow_dissect(), file net/core/flow_dissector.c:

	const struct arphdr *arp;
	struct arphdr *_arp;

	arp = __skb_header_pointer(skb, nhoff, sizeof(_arp), data,
				   hlen, &_arp);


Here _arp and arp are both pointers to arphdr structures. In other calls
to __skb_header_pointer(), the buffer argument (_arp here) would have
been a struct instead of a pointer. What makes ARP packets different in
__skb_flow_dissect()?

Thanks,
Nicolas

PS: the code which I am curious about seems to have been introduced in
4.11-rc1 with commit 55733350e5e8 ("flow disector: ARP support")

Re: Pointer type of _arp in __skb_flow_dissect()

[Simon Horman]

Hi Nicolas,

thanks for pointing this out.

On Fri, Mar 31, 2017 at 08:53:10PM +0200, Nicolas Iooss wrote:
> Hello,
> 
> Linux 4.11-rc4 contains the following code in function
> __skb_flow_dissect(), file net/core/flow_dissector.c:
> 
> 	const struct arphdr *arp;
> 	struct arphdr *_arp;

It looks like *_arp should be _arp.
And it looks like this affects net, since v4.11-rc1, and net-next.
I'll test the fix and post a patch unless someone beats me to it.

> 
> 	arp = __skb_header_pointer(skb, nhoff, sizeof(_arp), data,
> 				   hlen, &_arp);
> 
> 
> Here _arp and arp are both pointers to arphdr structures. In other calls
> to __skb_header_pointer(), the buffer argument (_arp here) would have
> been a struct instead of a pointer. What makes ARP packets different in
> __skb_flow_dissect()?
> 
> Thanks,
> Nicolas
> 
> PS: the code which I am curious about seems to have been introduced in
> 4.11-rc1 with commit 55733350e5e8 ("flow disector: ARP support")