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

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Message-ID: <20141106211754.GA32000@kroah.com>
Date:	Thu, 6 Nov 2014 13:17:54 -0800
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Måns Rullgård <mans@...sr.com>
Cc:	Christian Riesch <christian.riesch@...cron.at>,
	Jiri Slaby <jslaby@...e.cz>, linux-kernel@...r.kernel.org,
	Peter Hurley <peter@...leysoftware.com>, stable@...r.kernel.org
Subject: Re: [PATCH] n_tty: Add memory barrier to fix race condition in
 receive path

On Thu, Nov 06, 2014 at 09:01:36PM +0000, Måns Rullgård wrote:
> Greg Kroah-Hartman <gregkh@...uxfoundation.org> writes:
> 
> > 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.
> >
> > 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?

thanks,

greg k-h
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