netdev - Re: [RFC] [NET] [0/2] pskb_expand

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Date:	Wed, 26 Mar 2008 07:55:36 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Hideo AOKI <haoki@...hat.com>
Cc:	netdev <netdev@...r.kernel.org>, David Miller <davem@...emloft.net>
Subject: Re: [RFC] [NET] [0/2] pskb_expand_head() bugfix

On Tue, Mar 25, 2008 at 02:39:04PM -0400, Hideo AOKI wrote:
> Hello,
> 
> This patch set fixes a potential bug in pskb_expand_head().
> 
> Current pskb_expand_head() doesn't change truesize, while it
> reallocates memory. Then, if argument nhead or ntail aren't 0, caller
> must update truesize.
> 
> We had this bug at audit_expand() in January and fixed it as commit
> 406a1d868001423c85a3165288e566e65f424fe6. However, some drivers and
> subsystems still use pskb_expand_head() without updating truesize.

Drivers usually aren't supposed to change truesize so doing
this would actually create bugs.

> In addition, there is another problem to update truesise. Since
> pskb_expand_head() aligns memory size before reallocation, caller
> functions may not update turesize correctly if they just add nhaad
> and ntail to turesize.

That should be fixable by making sure that nhead + ntail is
aligned.

Cheers,
-- 
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Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
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