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

[Måns Rullgård]

Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> writes:

> On Thu, Nov 06, 2014 at 10:12:54PM +0000, Måns Rullgård wrote:
>> Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> writes:
>> 
>> > On Thu, Nov 06, 2014 at 09:38:59PM +0000, Måns Rullgård wrote:
>> >> Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> writes:
>> >> 
>> >> > On Thu, Nov 06, 2014 at 09:01:36PM +0000, Måns Rullgård wrote:
>> >> >> Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> writes:
>> >> >> 
>> >> >> > On Thu, Nov 06, 2014 at 08:49:01PM +0000, Måns Rullgård wrote:
>> >> >> >> Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> 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.

Within the confines of a single CPU + memory, barriers are never needed.
The moment another CPU or master-capable peripheral enters the mix,
proper ordering must be enforced somehow.

If the buffer is already protected by a lock of some kind, this will
provide the necessary barriers, so nothing further is necessary.  If
it's a lock-less design, there will need to be barriers somewhere.

>From the patch context, I can't tell which category this case falls
into, and I'm far too lazy to read the entire file.

-- 
Måns Rullgård
mans@xxxxxxxxx
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