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

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Date:	Thu, 06 Nov 2014 20:49:01 +0000
From:	Måns Rullgård <mans@...sr.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
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

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.

-- 
Måns Rullgård
mans@...sr.com
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