linux-kernel - Re: [RFC PATCH 1/1] vmalloc: add test driver to analyse vmalloc allocator

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Message-ID: <20181114151737.GA23419@dhcp22.suse.cz>
Date:   Wed, 14 Nov 2018 16:17:37 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     "Uladzislau Rezki (Sony)" <urezki@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Shuah Khan <shuah@...nel.org>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC PATCH 1/1] vmalloc: add test driver to analyse vmalloc
 allocator

On Tue 13-11-18 14:10:46, Andrew Morton wrote:
[...]
> > +static int vmalloc_test_init(void)
> > +{
> > +	__my_vmalloc_node_range =
> > +		(void *) kallsyms_lookup_name("__vmalloc_node_range");
> > +
> > +	if (__my_vmalloc_node_range)
> > +		do_concurrent_test();
> > +
> > +	return -EAGAIN; /* Fail will directly unload the module */
> > +}
> 
> It's unclear why this module needs access to the internal
> __vmalloc_node_range().  Please fully explain this in the changelog.
> 
> Then, let's just export the thing.  (I expect this module needs a
> Kconfig dependency on CONFIG_KALLSYMS, btw).  A suitable way of doing
> that would be
> 
> /* Exported for lib/test_vmalloc.c.  Please do not use elsewhere */
> EXPORT_SYMBOL_GPL(__vmalloc_node_range);

There was a previous discussion that testing for internal infrastructure
is useful quite often and such a testing module needs an access to such
an internal infrastructure. Exporting those symbols via standard
EXPORT_SYMBOL_GPL is far from optimal because we can be pretty much sure
an abuse will arise sooner than later. I was proposing
EXPORT_SYMBOL_SELFTEST that would link only against testing modules.

If that is not viable for some reason then kallsyms_lookup_name is a
dirty-but-usable workaround.
-- 
Michal Hocko
SUSE Labs