Re: IP header alignment

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On Mon, 14 Jan 2008 10:33:52 +0200
"Vladimir Kondratiev" <vladimir.kondratiev@xxxxxxxxx> wrote:

> Following patch made against 2.6.24-rc7
> It warns if IP stack gets frame with IP header not aligned on dword.
> Idea is this will help to identify mis-behaving drivers that do not
> enforce this alignment.
> -----------------------------
> commit d9ff4a47ca7bd902947eb22ed3b01682dce2752f
> Author: Vladimir Kondratiev <vladimir.kondratiev@xxxxxxxxx>
> Date:   Mon Jan 14 10:08:09 2008 +0200
> 
>     [NET] warn on non-aligned IP header
> 
>     IP stack silently assumes that IP header is aligned on dword.
>     While IP stack may be modified to overcome this requirement, it will
>     impose performance penalty. Better off to force drivers to properly align
>     data in received frame.
> 
>     This patch warns once, hoping that frame will be handled properly any way.
>     Drop frame in this case would be unappropriate - this will break some
>     wireless drivers.
> 
> diff --git a/net/ipv4/ip_input.c b/net/ipv4/ip_input.c
> index 168c871..88c8369 100644
> --- a/net/ipv4/ip_input.c
> +++ b/net/ipv4/ip_input.c
> @@ -400,7 +400,23 @@ int ip_rcv(struct sk_buff *skb, struct net_device
> *dev, struct packet_type *pt,
>          goto inhdr_error;
> 
>      iph = ip_hdr(skb);
> -
> +        /**
> +         * IP stack silently assumes that IP header aligned on dword.
> +     * If it is not:
> +     * 1. extra time for non-aligned access will be paid for every
> +     * architecture;
> +     * 2. various things will mis-behave on hardware that do not
> +     * support non-aligned access.
> +     * Driver code should be fixed.
> +         */
> +        if (unlikely((u32)iph % 4)) {
> +        static int __warned;
> +        if (unlikely(!__warned)) {
> +            __warned = 1;
> +            dev_warn(&orig_dev->dev,"Non aligned IP header from [%s]\n",
> +                 orig_dev->name);
> +        }
> +        }
>      /*
>       *    RFC1122: 3.1.2.2 MUST silently discard any IP frame that
> fails the checksum.
>       *

No.  There is hardware like Yukon-EC used in mac mini where the cost
of the CPU unaligned access is trivial and the cost of copying every
frame to shut up the stupid warning would be high.




-- 
Stephen Hemminger <stephen.hemminger@xxxxxxxxxx>
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