netdev - Re: [PATCH v3 2/3] packet: fill the gap of TPACKET

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Date:	Mon, 16 Dec 2013 21:41:38 +0900
From:	Atzm Watanabe <atzm@...atosphere.co.jp>
To:	Daniel Borkmann <dborkman@...hat.com>,
	David Laight <David.Laight@...LAB.COM>
Cc:	netdev@...r.kernel.org,
	Stephen Hemminger <stephen@...workplumber.org>,
	Ben Hutchings <bhutchings@...arflare.com>,
	David Miller <davem@...emloft.net>
Subject: Re: [PATCH v3 2/3] packet: fill the gap of TPACKET_ALIGNMENT with zeros

At Mon, 16 Dec 2013 11:22:35 +0100,
Daniel Borkmann wrote:
> 
> On 12/16/2013 11:16 AM, David Laight wrote:
> >>> +		memset(h.h2->tp_padding, 0,
> >>> +		       hdrlen - offsetof(struct tpacket2_hdr, tp_padding));
> >
> > What is wrong with 'sizeof h.h2->tp_padding' ?
> > The compiler will probably inline the memset into a couple of word
> > sized writes of zero - probably not measurable.
> > Not zeroing them might be leaking kernel memory contents (depends
> > where the memory came from - might just be stale packet data).
> 
> The ring buffer memory we're operating on comes from mmap(2) btw.

Thank you for comments.
In struct tpacket2_hdr, it seems that a padding member was really
zeroing to fix information leak on commit
13fcb7bd322164c67926ffe272846d4860196dc6 ("af_packet: prevent information leak").
So next time I'll try to zero tp_padding using sizof(h.h2->tp_padding)
David proposed.  If you have any thoughts on this please share it with me.


Thank you again!
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