linux-kernel - Re: [ 24/71] ring-buffer: Fix race between integrity check and readers

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Date:	Thu, 17 Jan 2013 18:02:14 -0200
From:	Herton Ronaldo Krzesinski <herton.krzesinski@...onical.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	alan@...rguk.ukuu.org.uk, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [ 24/71] ring-buffer: Fix race between integrity check and
 readers

On Tue, Jan 15, 2013 at 02:54:21PM -0800, Greg Kroah-Hartman wrote:
> 3.0-stable review patch.  If anyone has any objections, please let me know.
> 
> ------------------
> 
> From: Steven Rostedt <srostedt@...hat.com>
> 
> commit 9366c1ba13fbc41bdb57702e75ca4382f209c82f upstream.
> 
> The function rb_check_pages() was added to make sure the ring buffer's
> pages were sane. This check is done when the ring buffer size is modified
> as well as when the iterator is released (closing the "trace" file),
> as that was considered a non fast path and a good place to do a sanity
> check.
> 
> The problem is that the check does not have any locks around it.
> If one process were to read the trace file, and another were to read
> the raw binary file, the check could happen while the reader is reading
> the file.
> 
> The issues with this is that the check requires to clear the HEAD page
> before doing the full check and it restores it afterward. But readers
> require the HEAD page to exist before it can read the buffer, otherwise
> it gives a nasty warning and disables the buffer.
> 
> By adding the reader lock around the check, this keeps the race from
> happening.
> 
> Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

Too late now (and harmless), but the commit id and changelog are wrong, the
real commit for this backport is 54f7be5b831254199522523ccab4c3d954bbf576
(ring-buffer: Fix NULL pointer if rb_set_head_page() fails). The same
happened with the patch applied on 3.4 this cycle.

-- 
[]'s
Herton
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