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Date: Wed, 14 Nov 2018 15:00:53 -0800
From: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [RFC PATCH 1/1] vmalloc: add test driver to analyse vmalloc
 allocator
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To: Michal Hocko <mhocko@kernel.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 Wed, 14 Nov 2018 16:17:37 +0100 Michal Hocko <mhocko@kernel.org> wrote:

> 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.

That's rather overdoing things, I think.  If someone uses a
dont-use-this symbol then they get to own both pieces when it breaks.

We could simply do

#define EXPORT_SYMBOL_SELFTEST EXPORT_SYMBOL_GPL

then write a script which checks the tree for usages of the
thus-tagged symbols outside tools/testing and lib/ (?)

> If that is not viable for some reason then kallsyms_lookup_name is a
> dirty-but-usable workaround.

Well yes.  It adds a dependency on CONFIG_KALLSYMS and will cause
silent breakage if __vmalloc_node_range gets renamed, has its arguments
changed, etc.