linux-kernel - Re: [patch for-3.6] fs, debugfs: fix race in u32_array

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Message-ID: <505C3FEB.7030606@linux.vnet.ibm.com>
Date:	Fri, 21 Sep 2012 15:52:35 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	David Rientjes <rientjes@...gle.com>
CC:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Konrad Rzeszutek Wilk <konrad@...nel.org>,
	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Suzuki Poulose <suzuki@...ibm.com>
Subject: Re: [patch for-3.6] fs, debugfs: fix race in u32_array_read and allocate
 array at open

On 09/21/2012 02:46 PM, David Rientjes wrote:
> u32_array_open() is racy when multiple threads read from a file with a
> seek position of zero, i.e. when two or more simultaneous reads are
> occurring after the non-seekable files are created.  It is possible that
> file->private_data is double-freed because the threads races between
>
> 	kfree(file->private-data);
>
> and
>
> 	file->private_data = NULL;
>
> The fix is to only do format_array_alloc() when the file is opened and
> free it when it is closed.  This means that any thread that holds the
> file open and reads multiple times will see persistent data;

I think you meant we can read data only once. second time onwards we 
don't see any data. (except when fd is forked by child/ races in
threads).

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