Re: [PATCH 3/3] selftests/livepatch: Test of the API for specifying functions to search for on a stack
From: Petr Mladek <pmladek@suse.com>
Date: 2021-11-25 14:41:41
Also in:
live-patching, lkml
On Fri 2021-11-19 10:03:27, Miroslav Benes wrote:
quoted hunk ↗ jump to hunk
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@suse.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