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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