Is ip_hdr() on a sk_buff guaranteed to work?

Question

When dealing with a sk_buff in kernel modules code, I see many code samples use either

(struct iphdr *)skb_network_header(skb) //skb is an instance of (struct sk_buff*)

or the function ip_hdr() which is really the same (calling skb_network_header and casting). Is this safe? If we don't have any assumptions on where that sk_buff came from, is there a check I can do to make sure the network protocol is indeed IP?

Edit: It may be that by the point there can be an sk_buff the only possible network protocol is IP, but I would still like to see a "proof" or explanation for that. What if I implement my own protocol in the Linux kernel for example and the network protocol header may be something else? How can I make sure the network transport protocol is indeed IP?

Answer 1

Answering to myself because at the time I found a fuller answer.

It is possible to make sure the protocol is IP by the 'protocol' field of the sk_buff struct. For example:

if (skb->protocol != htons(ETH_P_IP)) return ERROR_NOT_IP; //ERROR_NOT_IP defined by me

struct iphdr* ip_header = ip_hdr(skb); //Now that we know the protocol we can use ip_hdr()

Answer 2

skb refers to socket buffer, the data structures used to represent and manage packets in the Linux Kernel.
And the implementation of Linux network system is designed to independent of a specific protocol.
Socket buffer in Linux Kernel network system will give you more details.

To your question:
The caller that called skb_network_header() should guarantee he knows the protocal. Proof : you can find out many references to skb_network_header(), including arp_hdr(), ipv6_hdr(), ip_hdr(), etc. The caller that call arp_hdr() to casting network header to aprhdr, will guarantee the protocal is ARP.