linux-kernel - Re: [PATCH 3/3] powerpc/mm: Implement CONFIG_DEBUG

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Date:   Wed, 19 Apr 2017 20:36:43 +0200
From:   christophe leroy <christophe.leroy@....fr>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Scott Wood <oss@...error.net>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] powerpc/mm: Implement CONFIG_DEBUG_RODATA on PPC32



Le 19/04/2017 à 16:22, Christophe LEROY a écrit :
>
>
> Le 19/04/2017 à 16:01, Michael Ellerman a écrit :
>> Christophe Leroy <christophe.leroy@....fr> writes:
>>
>>> diff --git a/arch/powerpc/kernel/ftrace.c b/arch/powerpc/kernel/ftrace.c
>>> index 32509de6ce4c..4af81fb23653 100644
>>> --- a/arch/powerpc/kernel/ftrace.c
>>> +++ b/arch/powerpc/kernel/ftrace.c
>>> @@ -526,7 +526,9 @@ void ftrace_replace_code(int enable)
>>>   */
>>>  void arch_ftrace_update_code(int command)
>>>  {
>>> +    set_kernel_text_rw();
>>>      ftrace_modify_all_code(command);
>>> +    set_kernel_text_ro();
>>>  }
>>
>> I'm not sure that's the right place for that.
>
>
> I took arch/arm/ as model. It does the following. Is that wrong ?
>
>
> static int __ftrace_modify_code(void *data)
> {
>     int *command = data;
>
>     set_kernel_text_rw();
>     ftrace_modify_all_code(*command);
>     set_kernel_text_ro();
>
>     return 0;
> }
>
> void arch_ftrace_update_code(int command)
> {
>     stop_machine(__ftrace_modify_code, &command, NULL);
> }
>
>
>
>> If you look at other arches they hook into ftrace_modify_code(), where
>> you have the address that's being modified.
>>
>
> Ok, I will look at other arches.

Could you provide more details on what you have seen on other arches ? I 
didn't notice anything related in any other arches' ftrace_modify_code()

What I could find however is, in x86, the use of 
ftrace_arch_code_modify_prepare() and 
ftrace_arch_code_modify_post_process ():

int ftrace_arch_code_modify_prepare(void)
{
	set_kernel_text_rw();
	set_all_modules_text_rw();
	return 0;
}

int ftrace_arch_code_modify_post_process(void)
{
	set_all_modules_text_ro();
	set_kernel_text_ro();
	return 0;
}


Those functions are also used on arm but only for modules:

int ftrace_arch_code_modify_prepare(void)
{
	set_all_modules_text_rw();
	return 0;
}

int ftrace_arch_code_modify_post_process(void)
{
	set_all_modules_text_ro();
	/* Make sure any TLB misses during machine stop are cleared. */
	flush_tlb_all();
	return 0;
}


Should I apply the x86 approach, or something else ?

Christophe

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