linux-kernel - Re: [PATCH v3 3/3] selftest: Taint kernel when test module loaded

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Message-ID: <CABVgOSkhr6xfgXZWW6UdbU4MpAvqsDOsqVP0THsH0gVB949HjA@mail.gmail.com>
Date:   Sat, 14 May 2022 16:34:23 +0800
From:   David Gow <davidgow@...gle.com>
To:     Luis Chamberlain <mcgrof@...nel.org>
Cc:     Brendan Higgins <brendanhiggins@...gle.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Jonathan Corbet <corbet@....net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Shuah Khan <skhan@...uxfoundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        "Guilherme G . Piccoli" <gpiccoli@...lia.com>,
        Sebastian Reichel <sre@...nel.org>,
        John Ogness <john.ogness@...utronix.de>,
        Joe Fradley <joefradley@...gle.com>,
        Daniel Latypov <dlatypov@...gle.com>,
        KUnit Development <kunit-dev@...glegroups.com>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Lucas De Marchi <lucas.demarchi@...el.com>,
        Aaron Tomlin <atomlin@...hat.com>,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org
Subject: Re: [PATCH v3 3/3] selftest: Taint kernel when test module loaded

On Fri, May 13, 2022 at 11:38 PM Luis Chamberlain <mcgrof@...nel.org> wrote:
>
> On Fri, May 13, 2022 at 04:32:13PM +0800, David Gow wrote:
> > Make any kselftest test module (using the kselftest_module framework)
> > taint the kernel with TAINT_TEST on module load.
> >
> > Note that several selftests use kernel modules which are not based on
> > the kselftest_module framework, and so will not automatically taint the
> > kernel. These modules will have to be manually modified if they should
> > taint the kernel this way.
> >
> > Similarly, selftests which do not load modules into the kernel generally
> > should not taint the kernel (or possibly should only do so on failure),
> > as it's assumed that testing from user-space should be safe. Regardless,
> > they can write to /proc/sys/kernel/tainted if required.
> >
> > Signed-off-by: David Gow <davidgow@...gle.com>
>
> Not all selftest modules use KSTM_MODULE_LOADERS() so I'd like to see a
> modpost target as well, otherwise this just covers a sliver of
> selftests.
>

My personal feeling is that the ideal way of solving this is actually
to port those modules which aren't using KSTM_MODULE_LOADERS() (or
KUnit, or some other system) to do so, or to otherwise manually tag
them as selftests and/or make them taint the kernel.

That being said, we can gain a bit my making the module-loading
helpers in kselftest/module.sh manually taint the kernel with
/proc/sys/kernel/tainted, which will catch quite a few of them (even
if tainting from userspace before they're loaded is suboptimal).

I've also started experimenting with a "test" MODULE_INFO field, which
modpost would add with the -t option. That still requires sprinkling
MODULE_INFO() everwhere, or the '-t' option to a bunch of makefiles,
or doing something more drastic to set it automatically for modules in
a given directory / makefile. Or the staging thing of checking the
directory / prefix in modpost.

I'll play around some more and have something to show in v4. (If we
have a MODULE_INFO field, we should use it for KUnit modules, but we'd
still have to taint the kernel manually for built-in tests anyway, so
it'd be redundant...)

-- David