linux-kernel - Re: [PATCH linux-kselftest/test v1] apparmor: add AppArmor KUnit tests for policy unpack

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Message-ID: <201910311136.BBC4C70B@keescook>
Date:   Thu, 31 Oct 2019 11:40:35 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Brendan Higgins <brendanhiggins@...gle.com>
Cc:     Iurii Zaikin <yzaikin@...gle.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Alan Maguire <alan.maguire@...cle.com>,
        Matthias Maennich <maennich@...gle.com>,
        shuah <shuah@...nel.org>,
        John Johansen <john.johansen@...onical.com>, jmorris@...ei.org,
        serge@...lyn.com, David Gow <davidgow@...gle.com>,
        Theodore Ts'o <tytso@....edu>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-security-module@...r.kernel.org,
        KUnit Development <kunit-dev@...glegroups.com>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        Mike Salvatore <mike.salvatore@...onical.com>
Subject: Re: [PATCH linux-kselftest/test v1] apparmor: add AppArmor KUnit
 tests for policy unpack

On Thu, Oct 31, 2019 at 02:33:32AM -0700, Brendan Higgins wrote:
> 2) One of the layers in your program is too think, and you should
> introduce a new layer with a new public interface that you can test
> through.
> 
> I think the second point here is problematic with how C is written in
> the kernel. We don't really have any concept of public vs. private
> inside the kernel outside of static vs. not static, which is much more
> restricted.

I don't find "2" to be a convincing argument (as you hint a bit at in
the next paragraph)_. There are lots of things code is depending on
(especially given the kernel's coding style guides about breaking up
large functions into little ones), that you want to test to make sure
is working correctly that has no public exposure, and you want to test
those helper's corner cases which might be hard to (currently) reach via
the higher level public APIs.

-- 
Kees Cook