Re: [PATCH] n_tty: Add memory barrier to fix race condition in receive path

[Peter Hurley <peter@hurleysoftware.com>]

On 11/06/2014 05:31 PM, Greg Kroah-Hartman wrote:
> On Thu, Nov 06, 2014 at 10:12:54PM +0000, Måns Rullgård wrote:
>> Greg Kroah-Hartman <gregkh@linuxfoundation.org> writes:
>>
>>> On Thu, Nov 06, 2014 at 09:38:59PM +0000, Måns Rullgård wrote:
>>>> Greg Kroah-Hartman <gregkh@linuxfoundation.org> writes:
>>>>
>>>>> On Thu, Nov 06, 2014 at 09:01:36PM +0000, Måns Rullgård wrote:
>>>>>> Greg Kroah-Hartman <gregkh@linuxfoundation.org> writes:
>>>>>>
>>>>>>> On Thu, Nov 06, 2014 at 08:49:01PM +0000, Måns Rullgård wrote:
>>>>>>>> Greg Kroah-Hartman <gregkh@linuxfoundation.org> writes:
>>>>>>>>
>>>>>>>>> On Thu, Nov 06, 2014 at 12:39:59PM +0100, Christian Riesch wrote:
>>>>>>>>>> The current implementation of put_tty_queue() causes a race condition
>>>>>>>>>> when re-arranged by the compiler.
>>>>>>>>>>
>>>>>>>>>> On my build with gcc 4.8.3, cross-compiling for ARM, the line
>>>>>>>>>>
>>>>>>>>>> 	*read_buf_addr(ldata, ldata->read_head++) = c;
>>>>>>>>>>
>>>>>>>>>> was re-arranged by the compiler to something like
>>>>>>>>>>
>>>>>>>>>> 	x = ldata->read_head
>>>>>>>>>> 	ldata->read_head++
>>>>>>>>>> 	*read_buf_addr(ldata, x) = c;
>>>>>>>>>>
>>>>>>>>>> which causes a race condition. Invalid data is read if data is read
>>>>>>>>>> before it is actually written to the read buffer.
>>>>>>>>>
>>>>>>>>> Really?  A compiler can rearange things like that and expect things to
>>>>>>>>> actually work?  How is that valid?
>>>>>>>>
>>>>>>>> This is actually required by the C spec.  There is a sequence point
>>>>>>>> before a function call, after the arguments have been evaluated.  Thus
>>>>>>>> all side-effects, such as the post-increment, must be complete before
>>>>>>>> the function is called, just like in the example.
>>>>>>>>
>>>>>>>> There is no "re-arranging" here.  The code is simply wrong.
>>>>>>>
>>>>>>> Ah, ok, time to dig out the C spec...
>>>>>>>
>>>>>>> Anyway, because of this, no need for the wmb() calls, just rearrange the
>>>>>>> logic and all should be good, right?  Christian, can you test that
>>>>>>> instead?
>>>>>>
>>>>>> Weakly ordered SMP systems probably need some kind of barrier.  I didn't
>>>>>> look at it carefully.
>>>>>
>>>>> It shouldn't need a barier, as it is a sequence point with the function
>>>>> call.  Well, it's an inline function, but that "shouldn't" matter here,
>>>>> right?
>>>>
>>>> Sequence points say nothing about the order in which stores become
>>>> visible to other CPUs.  That's why there are barrier instructions.
>>>
>>> Yes, but "order" matters.
>>>
>>> If I write code that does:
>>>
>>> 100	x = ldata->read_head;
>>> 101	&ldata->read_head[x & SOME_VALUE] = y;
>>> 102	ldata->read_head++;
>>>
>>> the compiler can not reorder lines 102 and 101 just because it feels
>>> like it, right?  Or is it time to go spend some reading of the C spec
>>> again...
>>
>> The compiler can't.  The hardware can.  All the hardware promises is
>> that at some unspecified time in the future, both memory locations will
>> have the correct values.  Another CPU might see 'read_head' updated
>> before it sees the corresponding data value.  A wmb() between the writes
>> forces the CPU to complete preceding stores before it begins subsequent
>> ones.
> 
> Yes, sorry, I'm not talking about other CPUs and what they see, I'm
> talking about the local one.  I'm not assuming that this is SMP "safe"
> at all.  If it is supposed to be, then yes, we do have problems, but
> there should be a lock _somewhere_ protecting this.
> 
> Peter's emails seem to be bouncing horridly right now, otherwise he
> would chime in and set me straight as to how this all should be
> working...

Sorry for the bouncing emails; something is wrong with my hosting
because I'm just now seeing these emails but not my inbox mails :/

I need to spend some time looking at this.

Regards,
Peter Hurley