linux-kernel - Re: [PATCH 3/3] selftests/livepatch: Test of the API for specifying functions to search for on a stack

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Date:   Thu, 25 Nov 2021 15:39:29 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Miroslav Benes <mbenes@...e.cz>
Cc:     jpoimboe@...hat.com, jikos@...nel.org, joe.lawrence@...hat.com,
        peterz@...radead.org, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org, shuah@...nel.org,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 3/3] selftests/livepatch: Test of the API for specifying
 functions to search for on a stack

On Fri 2021-11-19 10:03:27, Miroslav Benes wrote:
> Add a test for the API which allows the user to specify functions which
> are then searched for on any tasks's stack during a transition process.
> 
> --- /dev/null
> +++ b/lib/livepatch/test_klp_funcstack_mod.c
> @@ -0,0 +1,72 @@
> +// SPDX-License-Identifier: GPL-2.0
> +// Copyright (C) 2021 Miroslav Benes <mbenes@...e.cz>
> +
> +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
> +
> +#include <linux/module.h>
> +#include <linux/kernel.h>
> +#include <linux/debugfs.h>
> +#include <linux/delay.h>
> +
> +static int sleep_length = 10000;
> +module_param(sleep_length, int, 0644);
> +MODULE_PARM_DESC(sleep_length, "length of sleep in seconds (default=10)");
> +
> +static noinline void child_function(void)
> +{
> +	pr_info("%s enter\n", __func__);
> +	msleep(sleep_length);

The hardcoded sleep is not ideal. It might be too low or non-necessary high.

If I get it correctly, we are trying to achieve here the same as
busymod_work_func() in test_klp_callbacks_busy.c.

The approach with debugfs is an interesting trick. Though, I slightly
prefer using the scheduled work. The workqueue API looks less tricky
to me than sysfs/debugfs API. Also it does not block the module
in the init() callback[*]. But I might be biased.

Anyway, it might make sense to use the same trick in both situations.
It would make it easier to maintain the test modules.

[*] There is actually a race in the workqueue approach. The module
init() callback should wait until the work is really scheduled
and sleeping. It might be achieved by similar hand-shake like
with @block_transition variable. Or completion API might be
even more elegant.


> +	pr_info("%s exit\n", __func__);
> +}
> +
> +static noinline void child2_function(void)
> +{
> +	pr_info("%s\n", __func__);
> +}
> +
> +static noinline void parent_function(void)
> +{
> +	pr_info("%s enter\n", __func__);
> +	child_function();
> +	child2_function();

This would deserve some explanation what we try to simulate here
and how it is achieved. It is not easy for me even with the background
that I have freshly in my mind.

Also I think about more descriptive names ;-)

What about something like this (using workqueue work and completion):

/*
 * Simulate part of the caller code that is in another .elf section
 * and is reached via jump. It this was really the case then the stack
 * unwinder might not be able to detect that the process is sleeping
 * in the caller.
 */
static void simulate_jump_part(void)
{
	pr_info("%s enter\n", __func__);

	/* Stay in the jump part unless told to leave. */
	wait_for_completion(finish_jump);

	pr_info("%s exit\n", __func__);
}

/*
 * Simulate modified part of the caller code. It should never get
 * livepatched when the caller is sleeping in the just_part().
 */
static void simulate_modified_part(void)
{
	pr_info("%s\n", __func__);
}

static void test_not_on_stack_func_work(struct work_struct *work)
{
	pr_info("%s enter\n", __func__);

	/* Simulation ready */
	complete(work_started);

	simulate_jump_part();
	simulate_modified_part();

	pr_info("%s exit\n", __func__);
}

static int test_klp_no_on_stack_init(void)
{
	pr_info("%s\n", __func__);

	schedule_work(&work);
	wait_for_completion(&work_started);

	return 0;
}

static void test_not_on_stack_exit(void)
{
	complete(&finish_jump);
	flush_work(&work);
	pr_info("%s\n", __func__);
}

module_init(test_klp_not_on_stack_init);
module_exit(test_klp_not_on_stack_exit);

> +	pr_info("%s exit\n", __func__);
> +}
> +

Best Regards,
Petr