netdev - Re: [Patch net] igb: pass the correct maxlen for eth_get

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Message-ID: <CAHA+R7MJv7tbHwU_ECbJunXVJ8uo=eQv+Xmw42oR5sKEde35RQ@mail.gmail.com>
Date:	Wed, 22 Apr 2015 16:23:12 -0700
From:	Cong Wang <cwang@...pensource.com>
To:	Alexander Duyck <alexander.duyck@...il.com>
Cc:	Cong Wang <xiyou.wangcong@...il.com>,
	netdev <netdev@...r.kernel.org>,
	intel-wired-lan@...ts.osuosl.org,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>
Subject: Re: [Patch net] igb: pass the correct maxlen for eth_get_headlen()

On Wed, Apr 22, 2015 at 3:34 PM, Alexander Duyck
<alexander.duyck@...il.com> wrote:
> On 04/22/2015 02:56 PM, Cong Wang wrote:
>> On Wed, Apr 22, 2015 at 2:42 PM, Alexander Duyck
>> <alexander.duyck@...il.com> wrote:
>>> On 04/22/2015 01:33 PM, Cong Wang wrote:
>>>> First, make sure you don't miss the TSIP case right above:
>>>>
>>>> The frag starting pointer and its size are advanced by:
>>>>
>>>> skb_frag_size_sub(frag, IGB_TS_HDR_LEN);
>>>> ...
>>>> va += IGB_TS_HDR_LEN;
>>>>
>>>> even though we unlikely pull header longer than
>>>> IGB_RX_HDR_LEN - IGB_TS_HDR_LEN either.
>>> So I believe this is a possible bug, one heck of a corner case to get
>>> into though.  It requires timestamp in packet, size 240 - 256, and a
>>> malformed header.
>>>
>>> The proper fix would probably be to pull the timestamp out of the packet
>>> before we add it to the frame.  I'll submit a patch to address this.
>>>
>>
>> Huh? Doesn't my patch already fix this? skb_frag_size() is always
>> up to date. Or you mean another different problem?
>
> Your patch has other issues.  I am still NAKing your patch, but there is
> an issue with igb that you have pointed out.  The proper fix would be to
> deal with the timestamp before we add the page fragment to the skb.
>

If the first frag is always 2K, then this is not a problem either.
IGB_TS_HDR_LEN + IGB_RX_HDR_LEN < 2K.


>>
>>>> Second, the check you mentioned above is:
>>>>
>>>> if ((size <= IGB_RX_HDR_LEN) && !skb_is_nonlinear(skb))
>>>>
>>>> skb is nonlinear _after_ the first igb_add_rx_frag(), a second
>>>> igb_add_rx_frag() is possible since igb_is_non_eop() could
>>>> return true.
>>> I'm not sure this part makes any sense.  We pull the data out of the
>>> first fragment always.  If skb_is_nonlinear is set then we should have
>>> at least 2K - 16B in the case of igb.  We will never have a second
>>> fragment without at least 2K of data being given in the first.
>> Apparently my igb knowledge isn't enough to verify this, I just did
>> logical analysis.
>
> The logic with igb is that if skb_is_nonlinear it means that a page has
> already been added.  The way the hardware works is that it will break a
> frame up into 2K segments until the entire frame is written.  So the
> only way to get a second page fragment is if the first has used the
> entire 2K buffer it was given.

So the first frag is always 2K, at least more than IGB_RX_HDR_LEN
then we are fine even for TSIP case.

The code looks correct to me now, except it is suspicious skb->len
is not updated after skb_copy_to_linear_data() while skb->tail is
advanced already. I need to think more before submitting a patch.
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