linux-kernel - Re: [RFC 06/19] ktf: A simple debugfs interface to test results

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Date:   Thu, 15 Aug 2019 10:49:21 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Knut Omang <knut.omang@...cle.com>
Cc:     linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kbuild@...r.kernel.org,
        Shuah Khan <shuah@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Shreyans Devendra Doshi <0xinfosect0r@...il.com>,
        Alan Maguire <alan.maguire@...cle.com>,
        Brendan Higgins <brendanhiggins@...gle.com>,
        Kevin Hilman <khilman@...libre.com>,
        Hidenori Yamaji <hidenori.yamaji@...y.com>,
        Frank Rowand <frowand.list@...il.com>,
        Timothy Bird <Tim.Bird@...y.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Theodore Ts'o <tytso@....edu>, Daniel Vetter <daniel@...ll.ch>,
        Stephen Boyd <sboyd@...nel.org>
Subject: Re: [RFC 06/19] ktf: A simple debugfs interface to test results

On Wed, Aug 14, 2019 at 07:17:07PM +0200, Knut Omang wrote:
> I notice the discussion and your response here: 
> http://linux-kernel.2935.n7.nabble.com/debugfs-and-module-unloading-td865175.html
> I assume that means that protection against module unload while a debugfs file
> is open is now safe.

It should be, if you set the *owner field of your file_operations
properly.  Try it and see!

> On older kernels, having this code in place is far better than an unprotected 
> debugfs entry/exit - I have tested it extensively in the past :-)

Yes, it seems to work, but again, it really is racy and will fail.
Please don't use it.

> I perfectly agree with you that reducing the hole for a race condition 
> is generally a bad idea, but from the above mail thread 
> it seems that's the only available choice for older kernels?

I have no idea, but please, do not use that pattern of code as it is
racy in all kernels, from all of time.

thanks,

greg k-h