Re: [v5][PATCH] livepatch/ppc: Enable livepatching on powerpc

On 09/03/16 03:02, Petr Mladek wrote:

On Tue 2016-03-08 18:33:57, Balbir Singh wrote:

quoted

Changelog v5:
	1. Removed the mini-stack frame created for klp_return_helper.
	   As a result of the mini-stack frame, function with > 8
	   arguments could not be patched
	2. Removed camel casing in the comments

I tested this patch and it fails when I call a patched printk()
from a module.

You might try it with the test patch below. It is a bit twisted
because it calls the patched printk from livepatch_cmdline_proc_show()
that it added by the same patch module. Please, look at
livepatch_cmdline_proc_show(), it does:

	static int count;

	if (!count++)
		trace_printk("%s\n", "this has been live patched");
	else
		printk("%s\n", "this has been live patched");


It means that calls only trace_printk() when called first time.
It calls the patched printk when called second time.


I have tested it the following way:


# booted kernel with the changes below
# applied the patch:
$> modprobe livepatch-sample

# trigger the pached printk()
$>cat /sys/kernel/livepatch/livepatch_sample/enabled
1

# look into both dmesg and trace buffer
$> dmesg | tail -n 1
[  727.537307] patch enabled: 1
$> cat /sys/kernel/debug/tracing/trace | tail -n 1
             cat-3588  [003] ....   727.537448: livepatch_printk: patch enabled: 1

# trigger livepatch_cmdline_proc_show() 1st time
c79:~ # cat /proc/cmdline 
this has been live patched

# the message appeared only in trace buffer
$> dmesg | tail -n 1
[  727.537307] patch enabled: 1
c79:~ # cat /sys/kernel/debug/tracing/trace | tail -n 1
             cat-3511  [000] ....    862.958383:	     livepatch_cmdline_proc_show: this has been live patched


# trigger livepatch_cmdline_proc_show() 2nd time
c79:~ # cat /proc/cmdline 

!!! KABOOM !!!

It is becaused it tried to call the patched printk()?

Yes, the situation is that we restored the r2 for the kernel (from ftrace_caller, it is now kernel_toc),
whereas the LR points to the module. The difference between r2 and r0 > 4GB.

Very good test case. Did it work with v4? I presume it did because we have enough space to save both

Thanks,
Balbir Singh