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

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Message-ID: <20250709144706-efda2e7c-c3e4-4905-91ad-7553c46ed2e2@linutronix.de>
Date: Wed, 9 Jul 2025 14:53:29 +0200
From: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
To: Petr Mladek <pmladek@...e.com>
Cc: Arnd Bergmann <arnd@...db.de>, Nathan Chancellor <nathan@...nel.org>, 
	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 Wed, Jul 09, 2025 at 01:36:14PM +0200, Petr Mladek wrote:
> 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.

When the cpumask is allocated on the stack, free_cpumask_var() is a no-op.
So while the stack address would be leaked to another thread,
it should be fine as nothing is ever done with it.
For more clarity it could also be gated explicitly:

if (IS_ENABLED(CONFIG_CPUMASK_OFFSTACK)) {
	err = kunit_add_action_or_reset(test, prbtest_cpumask_cleanup, test_cpus);
	KUNIT_ASSERT_EQ(test, err, 0);
}

<snip>