netdev - Re: [PATCH] net: af_packet: Don't initialize vnet

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Message-Id: <20110512.173608.1652572492952866283.davem@davemloft.net>
Date:	Thu, 12 May 2011 17:36:08 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	joe@...ches.com
Cc:	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH] net: af_packet: Don't initialize vnet_hdr

From: Joe Perches <joe@...ches.com>
Date: Thu, 12 May 2011 14:33:20 -0700

> On Thu, 2011-05-12 at 17:26 -0400, David Miller wrote:
>> From: Joe Perches <joe@...ches.com>
>> Date: Thu, 12 May 2011 13:36:10 -0700
>> 
>> > Save a memset, initialize only the portion necessary.
>> > 
>> > packet_snd either gets this structure completely from
>> > memcpy_fromiovec or uses only the hdr_len set to 0,
>> > so don't always initialize the structure to 0.
>> > 
>> > Signed-off-by: Joe Perches <joe@...ches.com>
>> 
>> On ARM this won't be tightly packed, therefore you'll leave
>> uninitialized pieces of padding in this structure and this
>> thing is an "on-the-wire" network header.
>> 
>> I'm not applying this.
> 
> I believe that it's only sent when po->has_vnet_hdr
> is set.  In that case, it's completely filled from
> memcpy_fromiovec.  In the not set po->has_vnet_hdr case,
> only vnet_hdr.hdr_len is accessed by packet_alloc_skb.

I think with the way this code is protecting accesses to vnet_hdr,
it's going to start warning that some parts "may" be use
uninitialized.

It's also slightly ugly to partially initialize these on-stack things.

I would rather see the code rearranged such that this sort of
hackish scheme isn't necessary.

Again, I'm not applying this.
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