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

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Date:	Wed, 23 Apr 2014 10:26:00 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Takao Indoh <indou.takao@...fujitsu.com>
Cc:	rusty@...tcorp.com.au, rostedt@...dmis.org, 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

(2014/04/22 17:35), Takao Indoh wrote:
>>> >> But the text is already RO, so it causes panic. We need to call notifier
>>> >> before setting it RO. Or should we unset RO temporarily in
>>> >> ftrace_process_locs()?
>> > 
>> > Perhaps, IMHO, ftrace needs to change the module RW in ftrace_init_module and
>> > makes it RO after modifying the module text.
> Hmm..., I think the same problem occurs if we set module RW in
> ftrace_init_module().
> 
> <insmod module B>
> init_module
>   load_module
>     complete_formation
>       set_section_ro_nx -------------------------------------- (1)
>       set_section_ro_nx -------------------------------------- (2)
>       blocking_notifier_call_chain
>         ftrace_module_notify_enter
>           ftrace_init_module --------------------------------- (3)
>             ftrace_process_locs
>              mutex_lock(&ftrace_lock) ------------------------ (4)
>              ftrace_update_code
>                __ftrace_replace_code
>                  ftrace_make_nop
>                    ftrace_modify_code_direct
>                      do_ftrace_mod_code
>                        probe_kernel_write -------------------- (5)
> 
> 
> The text of module B is set to RO at (1) and (2) by Rusty's patch. And
> even if we change it to RW at (3), it set to RO again by another module
> while module B is waiting at (4).
> 
> So, we need to set module to RW somewhere after get ftrace_lock, maybe
> in ftrace_update_code()?

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.

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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