netdev - Re: [PATCH v2 2/2] tun: update file current position

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Date:	Fri, 06 Dec 2013 21:28:38 -0500
From:	Vlad Yasevich <vyasevich@...il.com>
To:	David Miller <davem@...emloft.net>, zwu.kernel@...il.com
CC:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	wuzhy@...ux.vnet.ibm.com, "Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [PATCH v2 2/2] tun: update file current position

On 12/06/2013 12:45 PM, David Miller wrote:
> From: Zhi Yong Wu <zwu.kernel@...il.com>
> Date: Fri,  6 Dec 2013 17:08:50 +0800
> 
>> From: Zhi Yong Wu <wuzhy@...ux.vnet.ibm.com>
>>
>> Signed-off-by: Zhi Yong Wu <wuzhy@...ux.vnet.ibm.com>
> 
> Also applied and queued up for -stable, thanks.
> 
> I noticed in these two cases that that min_t() adjustment of 'ret'
> seems strange.  I can't understand why it's needed.
> 
> If, for example, tun_do_read() really did read more than 'len'
> bytes:
> 
> 1) That would write past the end of the buffer.
> 
> 2) Writing a different value to the ->ki_pos would mean
>    that ->ki_pos is now inaccurate.
> 
> Unless someone can explain why the min_t() is needed, we should remove
> it.

So, back when that code was added, it was actually possible for the
tun_do_read to return a value larger then user specified length, but
the copy would only be done the length bytes.  This was used to signal
MSG_TRUNC.

Specifically we had the following code in tun_put_user()
      len = min_t(int, skb->len, len);

      skb_copy_datagram_const_iovec(skb, 0, iv, total, len);
      total += skb->len;

This has since changed to:
      len = min_t(int, skb->len, len);
      ...
      skb_copy_datagram_const_iovec(skb, vlan_offset, iv, total, len);
      total += len;

So, no it seems impossible to return a value larger then user specified
length, so MSG_TRUNC will never be set.  It probably makes sense to
signal message truncation, but that seems broken right now.

-vlad

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