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From: Knut Omang <knut.omang@oracle.com>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-doc@vger.kernel.org, linux-kbuild@vger.kernel.org,
	Shuah Khan <shuah@kernel.org>, Jonathan Corbet <corbet@lwn.net>,
	Masahiro Yamada <yamada.masahiro@socionext.com>,
	Michal Marek <michal.lkml@markovi.net>,
	Shreyans Devendra Doshi <0xinfosect0r@gmail.com>,
	Alan Maguire <alan.maguire@oracle.com>,
	Brendan Higgins <brendanhiggins@google.com>,
	Kevin Hilman <khilman@baylibre.com>,
	Hidenori Yamaji <hidenori.yamaji@sony.com>,
	Frank Rowand <frowand.list@gmail.com>,
	Timothy Bird <Tim.Bird@sony.com>,
	Luis Chamberlain <mcgrof@kernel.org>,
	"Theodore Ts'o" <tytso@mit.edu>, Daniel Vetter <daniel@ffwll.ch>,
	Stephen Boyd <sboyd@kernel.org>
Subject: Re: [RFC 06/19] ktf: A simple debugfs interface to test results
Date: Thu, 15 Aug 2019 12:35:26 +0200
Message-ID: <9629068a41a160de0145a18dd22924bce70f37fe.camel@oracle.com> (raw)
In-Reply-To: <20190815084921.GE3512@kroah.com>

On Thu, 2019-08-15 at 10:49 +0200, Greg Kroah-Hartman wrote:
> 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!

Might be a case for a KTF selftest to play with the timing to increase the chance :)
Wasn't able to make it crash with these simple, short files.

I notice I had set the .owner field correctly myself in that driver 
code I referred to, so that's a "copy regression".

> > 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.

Ok, will remove it :-)

I tried in vain to find the commit from Al Viro that made the code safe,
to identify which kernels that are safe from this issue,
but he has a **lot** of commits, do you have a clue for what/where to look?

It will be good to have a mention/comment on this for future reference, 
like the earliest kernel version where this is safe.

Maybe we can even get rid of some more of the remaining of these too..
(I notice there's 65 cases of 'if (!try_module_get(THIS_MODULE))'
right now)

Thanks!
Knut

> 
> thanks,
> 
> greg k-h


  reply index

Thread overview: 44+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-13  6:09 [RFC 00/19] Integration of Kernel Test Framework (KTF) into the kernel tree Knut Omang
2019-08-13  6:09 ` [RFC 01/19] kbuild: Fixes to rules for host-cshlib and host-cxxshlib Knut Omang
2019-08-13 14:01   ` Masahiro Yamada
2019-08-13 16:19     ` Knut Omang
2019-08-14  2:02       ` Masahiro Yamada
2019-08-14  5:50         ` Knut Omang
2019-08-14  5:52         ` Knut Omang
2019-08-14 12:52           ` Knut Omang
2019-08-21  1:47             ` Masahiro Yamada
2019-08-21  4:03               ` Knut Omang
2019-08-13  6:09 ` [RFC 02/19] ktf: Introduce the main part of the kernel side of ktf Knut Omang
2019-09-09  1:23   ` Brendan Higgins
2019-09-10  6:15     ` Knut Omang
2019-08-13  6:09 ` [RFC 03/19] ktf: Introduce a generic netlink protocol for test result communication Knut Omang
2019-09-09  1:28   ` Brendan Higgins
2019-09-10  6:30     ` Knut Omang
2019-08-13  6:09 ` [RFC 04/19] ktf: An implementation of a generic associative array container Knut Omang
2019-08-13  6:09 ` [RFC 05/19] ktf: Implementation of ktf support for overriding function entry and return Knut Omang
2019-08-13  6:09 ` [RFC 06/19] ktf: A simple debugfs interface to test results Knut Omang
2019-08-13  8:21   ` Greg Kroah-Hartman
2019-08-14 17:17     ` Knut Omang
2019-08-15  8:49       ` Greg Kroah-Hartman
2019-08-15 10:35         ` Knut Omang [this message]
2019-08-15 10:52           ` Greg Kroah-Hartman
2019-08-13  6:09 ` [RFC 07/19] ktf: Simple coverage support Knut Omang
2019-08-13  6:09 ` [RFC 08/19] ktf: Configurable context support for network info setup Knut Omang
2019-08-13  6:09 ` [RFC 09/19] ktf: resolve: A helper utility to aid in exposing private kernel symbols to KTF tests Knut Omang
2019-08-13  6:09 ` [RFC 10/19] ktf: Add documentation for Kernel Test Framework (KTF) Knut Omang
2019-08-13  6:09 ` [RFC 11/19] ktf: Add a small test suite with a few tests to test KTF itself Knut Omang
2019-08-13  6:09 ` [RFC 12/19] ktf: Main part of user land library for executing tests Knut Omang
2019-08-13  6:09 ` [RFC 13/19] ktf: Integration logic for running ktf tests from googletest Knut Omang
2019-08-13  6:09 ` [RFC 14/19] ktf: Internal debugging facilities Knut Omang
2019-08-13  6:09 ` [RFC 15/19] ktf: Some simple examples Knut Omang
2019-08-13  6:09 ` [RFC 16/19] ktf: Some user applications to run tests Knut Omang
2019-08-13  6:09 ` [RFC 17/19] ktf: Toplevel ktf Makefile/makefile includes and scripts to run from kselftest Knut Omang
2019-08-13  6:09 ` [RFC 18/19] kselftests: Enable building ktf Knut Omang
2019-08-13  6:09 ` [RFC 19/19] Documentation/dev-tools: Add index entry for KTF documentation Knut Omang
2019-08-13  8:10 ` [RFC 00/19] Integration of Kernel Test Framework (KTF) into the kernel tree Brendan Higgins
2019-08-13  8:17 ` Brendan Higgins
2019-08-13 11:29   ` Knut Omang
2019-08-13 17:50     ` Brendan Higgins
2019-08-13  8:23 ` Greg Kroah-Hartman
2019-08-13  9:51   ` Knut Omang
2019-08-13 17:02     ` Brendan Higgins

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