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

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Date:	Thu, 6 Nov 2014 22:40:34 +0100
From:	Christian Riesch <christian.riesch@...cron.at>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	Måns Rullgård <mans@...sr.com>,
	Jiri Slaby <jslaby@...e.cz>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Peter Hurley <peter@...leysoftware.com>,
	stable <stable@...r.kernel.org>
Subject: Re: [PATCH] n_tty: Add memory barrier to fix race condition in
 receive path

On Thu, Nov 6, 2014 at 9:56 PM, Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
> On Thu, Nov 06, 2014 at 08:49:01PM +0000, Måns Rullgård wrote:
>> Greg Kroah-Hartman <gregkh@...uxfoundation.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.

Oh, I didn't know that, thanks a lot for this!

> Anyway, because of this, no need for the wmb() calls, just rearrange the
> logic and all should be good, right?

I came up with the wmb() stuff after getting scared from reading
Documentation/memory-barriers.txt (which I didn't understand) and
Documentation/circular-buffers.txt (which I understood partly). But it
is actually a circular buffer and circular-buffers.txt says I need
memory barriers for circular buffers. Though I do not know how
function calls fit into this picture.

> Christian, can you test that
> instead?

Sure, but it will probably not happen before Monday. Thanks a lot for your help!

Christian
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