linux-hardening - Re: [PATCH 2/3] printk: kunit: support offstack cpumask

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Message-ID: <aG5ULpdSoAR6nF5R@pathway.suse.cz>
Date: Wed, 9 Jul 2025 13:36:14 +0200
From: Petr Mladek <pmladek@...e.com>
To: Arnd Bergmann <arnd@...db.de>
Cc: Nathan Chancellor <nathan@...nel.org>,
	Thomas Weißschuh <thomas.weissschuh@...utronix.de>,
	John Ogness <john.ogness@...utronix.de>,
	Dan Carpenter <dan.carpenter@...aro.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	Kees Cook <kees@...nel.org>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	David Gow <davidgow@...gle.com>, Arnd Bergmann <arnd@...nel.org>,
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH 2/3] printk: kunit: support offstack cpumask

On Tue 2025-07-08 16:48:47, Arnd Bergmann wrote:
> On Tue, Jul 8, 2025, at 16:24, Petr Mladek wrote:
> > On Wed 2025-07-02 13:28:35, Nathan Chancellor wrote:
> >> On Wed, Jul 02, 2025 at 11:51:56AM +0200, Petr Mladek wrote:
> >
> > Thanks a lot for the nice report.
> >
> > The problem is how cpumask_var_t is defined in include/linux/cpumask_types.h:
> >
> > #ifdef CONFIG_CPUMASK_OFFSTACK
> > typedef struct cpumask *cpumask_var_t;
> > #else
> > typedef struct cpumask cpumask_var_t[1];
> > #endif /* CONFIG_CPUMASK_OFFSTACK */
> >
> > And KUNIT_DEFINE_ACTION_WRAPPER() expect that the 3rd parameter
> > is a pointer.
> >
> > I am going to solve this by adding a wrapper over free_cpumask_var()
> > which would work with a pointer to cpumask_var_t.
> 
> I'm not familiar enough with the cleanup mechanism of kunit,
> but can't you just move the mask allocation outside of
> test_readerwriter()?

The only solution would be global variable.

test_readerwriter() is the top-level function passed
to KUnit framework via:

	KUNIT_CASE_SLOW(test_readerwriter),

And it seems that the clean is even done in a separate process.

I see the following:

   KUNIT_CASE_SLOW() sets .run_case()

The callback is called via via:

  + kunit_try_run_case()
   +  kunit_run_case_internal()
     + test_case->run_case()

And kunit_try_run_case() is called via:

    + kunit_run_case_catch_errors()
      + kunit_try_catch_run()
	+ kthread_create()
	     -> kunit_try_run_case() in the new thread

The clean up is called from the same kunit_run_case_catch_errors()
in another thread

      + kunit_try_catch_run()
	+ kthread_create()
	    -> kunit_try_run_case_cleanup() in another new thread

> > + */
> > +static void prbtest_free_cpumask_var(cpumask_var_t *mask)
> > +{
> > +	free_cpumask_var(*mask);
> > +}
> 
> Or you could pass this as a cpumask_t pointer instead,
> which should do the right thing without the indirection.

Nice trick. I am going to try it.

> >  	KUNIT_ASSERT_TRUE(test, alloc_cpumask_var(&test_cpus, GFP_KERNEL));
> > -	err = kunit_add_action_or_reset(test, prbtest_cpumask_cleanup, test_cpus);
> > +	err = kunit_add_action_or_reset(test, prbtest_cpumask_cleanup, &test_cpus);
> 
> In my original version, I did not have the
> KUNIT_ASSERT_TRUE() here, which seems sufficient since this
> is not what you are testing at all, and in normal systems
> this would just be a stack variable.

I think that KUNIT_ASSERT is standard handling of any problem in the
test. At least, I see KUNIT_ASSERT*() after any *malloc*() in
the code samples in Documentation/dev-tools/kunit/usage.rst

Best Regards,
Petr