linux-kernel - Re: [RFC PATCH 0/2] improve vmalloc allocation

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Date:   Wed, 24 Oct 2018 08:22:52 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Shuah Khan <shuah@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Garnier <thgarnie@...gle.com>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joelaf@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...e.hu>, Tejun Heo <tj@...nel.org>,
        maco@...roid.com
Subject: Re: [RFC PATCH 0/2] improve vmalloc allocation

On Tue 23-10-18 12:30:44, Joel Fernandes wrote:
> On Tue, Oct 23, 2018 at 11:13:36AM -0600, Shuah Khan wrote:
> > On 10/23/2018 11:05 AM, Michal Hocko wrote:
> > > On Tue 23-10-18 08:26:40, Matthew Wilcox wrote:
> > >> On Tue, Oct 23, 2018 at 09:02:56AM -0600, Shuah Khan wrote:
> > > [...]
> > >>> The way it can be handled is by adding a test module under lib. test_kmod,
> > >>> test_sysctl, test_user_copy etc.
> > >>
> > >> The problem is that said module can only invoke functions which are
> > >> exported using EXPORT_SYMBOL.  And there's a cost to exporting them,
> > >> which I don't think we're willing to pay, purely to get test coverage.
> > > 
> > > Yes, I think we do not want to export internal functionality which might
> > > be still interesting for the testing coverage. Maybe we want something
> > > like EXPORT_SYMBOL_KSELFTEST which would allow to link within the
> > > kselftest machinery but it wouldn't allow the same for general modules
> > > and will not give any API promisses.
> > > 
> > 
> > I like this proposal. I think we will open up lot of test opportunities with
> > this approach.
> > 
> > Maybe we can use this stress test as a pilot and see where it takes us.
> 
> I am a bit worried that such an EXPORT_SYMBOL_KSELFTEST mechanism can be abused by
> out-of-tree module writers to call internal functionality.
> 
> How would you prevent that?

There is no way to prevent non-exported symbols abuse by 3rd party
AFAIK. EXPORT_SYMBOL_* is not there to prohibid abuse. It is a mere
signal of what is, well, an exported API.
-- 
Michal Hocko
SUSE Labs