netdev - Memory alignment issue in a network protocol header

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Date:	Thu, 17 Apr 2008 20:08:41 -0700 (PDT)
From:	Eus <eus@...ber.fsf.org>
To:	Linux Networking Mailing List <netdev@...r.kernel.org>
Subject: Memory alignment issue in a network protocol header

Hi Ho!

I am implementing a networking protocol (http://sourceforge.net/projects/atn).
This networking protocol has the following header:

struct clnphdr {
	__u8 cnf_proto_id;
	__u8 cnf_hdr_len;
	__u8 cnf_vers;
	__u8 cnf_ttl;
	__u8 cnf_flag;
	__be16 cnf_seglen;
	__u8 cnf_cksum_msb;
	__u8 cnf_cksum_lsb;
	__u8 dest_len;
	__u8 dest_addr[20];
	__u8 src_len;
	__u8 src_addr[20];
	__u8 next_part[0];
};

Because of 4-byte memory alignment in i386, there will be 1-byte padding in
`cnf_seglen'.

This alignment prevents me from doing:
        struct clnphdr clnph = (struct clnphdr *) skb->nh.raw;
because ntohs(clnph->cnf_seglen) will return the wrong result.

The wrong result will occur because in `skb->nh.raw' the data is packed (no
hole).

The easiest solution will be to put __attribute__((packed)) in the definition of
struct clnphdr, but this will incur performance penalty, won't this?

So, how should I handle this problem in the proper way as to permit portability
to 64-bit machines as well?

Thank you very much.

Best regards,
Eus

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