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Date: Wed, 14 Nov 2018 16:17:37 +0100
From: Michal Hocko <mhocko@kernel.org>
Subject: Re: [RFC PATCH 1/1] vmalloc: add test driver to analyse vmalloc
 allocator
Message-ID: <20181114151737.GA23419@dhcp22.suse.cz>
References: <20181113151629.14826-1-urezki@gmail.com>
 <20181113151629.14826-2-urezki@gmail.com>
 <20181113141046.f62f5bd88d4ebc663b0ac100@linux-foundation.org>
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To: Andrew Morton <akpm@linux-foundation.org>
Cc: "Uladzislau Rezki (Sony)" <urezki@gmail.com>, Kees Cook <keescook@chromium.org>, Shuah Khan <shuah@kernel.org>, linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>, Matthew Wilcox <willy@infradead.org>, Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>, Thomas Gleixner <tglx@linutronix.de>

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