linux-kernel - Re: ftrace/kprobes: Warning when insmod two modules

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Date:	Wed, 23 Apr 2014 11:37:47 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Takao Indoh <indou.takao@...fujitsu.com>, rusty@...tcorp.com.au,
	fweisbec@...il.com, mingo@...hat.com, ananth@...ibm.com,
	anil.s.keshavamurthy@...el.com, davem@...emloft.net,
	linux-kernel@...r.kernel.org
Subject: Re: ftrace/kprobes: Warning when insmod two modules

(2014/04/23 10:56), Steven Rostedt wrote:
> On Wed, 23 Apr 2014 10:26:00 +0900
> Masami Hiramatsu <masami.hiramatsu.pt@...achi.com> wrote:
> 
> 
>> Agreed. That should be done in a protected (critical) region,
>> and the region must be protected by correct lock. It seems that
>> the ftrace_lock is not a correct one.
> 
> The setting of RO to RW done by ftrace before doing the normal
> modification is under the ftrace_lock mutex. Why wouldn't that be the
> correct lock?

Hmm, Ok. I checked that currently ftrace is the only user of
set_all_modules_text_rw(), so until another user appears,
ftrace_lock mutex can work.  (and also, we need a comment
on the top of such functions, about by what it is protected. )

> The issue today is with the loading of a module and ftrace
> expecting its code to be RW. Here's the current race:
> 
> 
> 	CPU 1				CPU 2
> 	-----				-----
>   load_module()
>    module->state = MODULE_STATE_COMING
> 
> 				register_ftrace_function()
> 				 mutex_lock(&ftrace_lock);
> 				 ftrace_startup()
> 				  update_ftrace_function();
> 				   ftrace_arch_code_modify_prepare()
> 				    set_all_module_text_rw();
> 				   <enables-ftrace>
> 				    ftrace_arch_code_modify_post_process()
> 				     set_all_module_text_ro();
> 
> 				[ here all module text is set to RO,
> 				  including the module that is
> 				  loading!! ]
> 
>    blocking_notifier_call_chain(MODULE_STATE_COMING);
>     ftrace_init_module()
> 
> 
>      [ tries to modify code, but it's RO, and fails! ]
> 
> One solution is to add a way to set a single module text to ro and rw,
> and then we can encapsulate ftrace_init_module() under ftrace_lock
> mutex and have the ftrace_init_module() set the text to RW and then
> back to RO, and this will keep ftrace from having issues with the
> loaded module.

It sounds nicer solution, less side-effect.

> Now, if text poke does something similar, we need to make another mutex
> that covers modifying text. Don't we have one already?

We have the text_mutex already :).

> The worry I have here, and why I still prefer the simple split state of
> MODULE_STATE_COMING, is that once you add another mutex, we now have to
> fight mutex ordering. Not to mention where else things might do this :-p

I see, however, we should take care of it, at least comment level.

Thank you,

-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Research Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@...achi.com


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