netdev - Re: [PATCH] net: skb content must be visible for lockless skb

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Date:   Mon, 1 Aug 2022 21:37:05 +0300
From:   Kirill Tkhai <tkhai@...ru>
To:     Paolo Abeni <pabeni@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH] net: skb content must be visible for lockless skb_peek()
 and its variations

On 01.08.2022 12:59, Paolo Abeni wrote:
> On Mon, 2022-08-01 at 10:00 +0300, Kirill Tkhai wrote:
>> On 01.08.2022 09:52, Paolo Abeni wrote:
>>> On Sun, 2022-07-31 at 23:39 +0300, Kirill Tkhai wrote:
>>>> From: Kirill Tkhai <tkhai@...ru>
>>>>
>>>> Currently, there are no barriers, and skb->xxx update may become invisible on cpu2.
>>>> In the below example var2 may point to intial_val0 instead of expected var1:
>>>>
>>>> [cpu1]					[cpu2]
>>>> skb->xxx = initial_val0;
>>>> ...
>>>> skb->xxx = var1;			skb = READ_ONCE(prev_skb->next);
>>>> <no barrier>				<no barrier>
>>>> WRITE_ONCE(prev_skb->next, skb);	var2 = skb->xxx;
>>>>
>>>> This patch adds barriers and fixes the problem. Note, that __skb_peek() is not patched,
>>>> since it's a lowlevel function, and a caller has to understand the things it does (and
>>>> also __skb_peek() is used under queue lock in some places).
>>>>
>>>> Signed-off-by: Kirill Tkhai <tkhai@...ru>
>>>> ---
>>>> Hi, David, Eric and other developers,
>>>>
>>>> picking unix sockets code I found this problem, 
>>>
>>> Could you please report exactly how/where the problem maifests (e.g.
>>> the involved call paths/time sequence)?
>>
>> I didn't get why call paths in the patch description are not enough for you. Please, explain
>> what you want.
> 
> You mentioned the unix socket, so I expect to see something alike (I'm
> totally making up the symbols lists just to give an example): 
> 
> CPU0					CPU1
> unix_stream_read_generic()		unix_stream_sendmsg()
> skb_peek()				skb_queue_tail(other->sk_receive_queue)
> 
> plus some wording on how the critical race is reached, if not
> completely obvious.

Ah, you mean specific functions. Yes, this example and the rest of places, skb_peek{,tail} are
used without queue lock, e.g., unix_stream_data_wait().
 
>>  
>>>> and for me it looks like it exists. If there
>>>> are arguments that everything is OK and it's expected, please, explain.
>>>
>>> I don't see why such barriers are needed for the locked peek/tail
>>> variants, as the spin_lock pair implies a full memory barrier.
>>
>> This is for lockless skb_peek() calls and the patch is called in that way :). For locked skb_peek()
>> this is not needed. 
> 
> But you are also unconditioanlly adding barriers to the locked
> append/enqueue functions - which would possibly make sense only when
> the latters are paired with lockless read access.
> 
> As Eric said, if any new barrier is needed, we want to apply it only
> where needed, and not to every skb_queue*()/skb_peek() user, so very
> likely a new helper (o a new pair of helpers) will be needed.

Ok, thanks, Paolo.

Kirill