linux-kernel - Re: [PATCH] vsprintf: Add test for restricted kernel pointers

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Message-ID: <aWpwMyFEfpCNN297@pathway.suse.cz>
Date: Fri, 16 Jan 2026 18:06:59 +0100
From: Petr Mladek <pmladek@...e.com>
To: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
Cc: Steven Rostedt <rostedt@...dmis.org>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vsprintf: Add test for restricted kernel pointers

On Wed 2026-01-14 08:27:34, Thomas Weißschuh wrote:
> Fill out the tests for restricted kernel pointers, using the %pK format.
> This test can only be executed when built into the kernel, as modules
> do not have access to the kptr_restrict knob.

I think that we could export "kptr_restrict" like we did
with no_hash_pointers. AFAIK, it has been exported just because
of the test module as well.

> Please note that changes to the kptr_restrict sysctl from the kernel
> commandline are only applied *after* the boot-time KUnit tests run.

This is another motivation to export the symbol. Otherwise, it is
really hard to test the non-default variants.

BTW: I have recently heard about EXPORT_SYMBOL_NS(). It would allow
     to export the symbol only for some specific modules.

     I am not sure how exactly it works. I wonder if there already
     exists a namespace for KUnit tests.

     It would be nice to use it, even for "no_hash_pointers"...


> --- a/lib/tests/printf_kunit.c
> +++ b/lib/tests/printf_kunit.c
> @@ -316,7 +316,31 @@ symbol_ptr(struct kunit *kunittest)
>  static void
>  kernel_ptr(struct kunit *kunittest)
>  {
> -	/* We can't test this without access to kptr_restrict. */
> +#ifdef MODULE
> +	kunit_skip(kunittest, "cannot access kptr_restrict from test module");
> +	return;
> +#endif
> +
> +	switch (kptr_restrict) {
> +	case 0:
> +		if (no_hash_pointers) {
> +			test(PTR_STR, "%pK", PTR);
> +		} else {
> +			char buf[PLAIN_BUF_SIZE];
> +
> +			plain_hash_to_buffer(kunittest, PTR, buf, PLAIN_BUF_SIZE);
> +			/* %pK behaves the same as hashing */
> +			test(buf, "%pK", PTR);
> +		}
> +		break;
> +	case 1:
> +		test(PTR_STR, "%pK", PTR);

Hmm, the behavior of %pK depends on capabilities of the caller.
The above code would work when the test is built in.
But we might need to check the capabilities when using test module.
If I get it correctly, the module loaded requires CAP_SYS_MODULE
while %pK behavior depends on CAP_SYSLOG...

> +		break;
> +	case 2:
> +	default:
> +		test(ZEROS "00000000", "%pK", PTR);
> +		break;
> +	}
>  }
>  
>  static void

Anyway, thanks for working on this.

Best Regards,
Petr