linux-kernel - Re: [PATCH v2 3/3] powerpc/mm: Implement CONFIG_DEBUG

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Message-ID: <87bmrna08j.fsf@concordia.ellerman.id.au>
Date:   Sun, 23 Apr 2017 20:26:20 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     christophe leroy <christophe.leroy@....fr>,
        "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Scott Wood <oss@...error.net>,
        Paul Mackerras <paulus@...ba.org>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2 3/3] powerpc/mm: Implement CONFIG_DEBUG_RODATA on PPC32

christophe leroy <christophe.leroy@....fr> writes:

> Le 22/04/2017 à 08:08, Michael Ellerman a écrit :
>> "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com> writes:
>>> Excerpts from Christophe Leroy's message of April 21, 2017 18:32:
>>>> diff --git a/arch/powerpc/kernel/ftrace.c
>>>> b/arch/powerpc/kernel/ftrace.c
>>>> index 32509de6ce4c..06d2ac53f471 100644
>>>> --- a/arch/powerpc/kernel/ftrace.c
>>>> +++ b/arch/powerpc/kernel/ftrace.c
>>>> @@ -46,6 +46,7 @@ static int
>>>> @@ -67,10 +68,11 @@ ftrace_modify_code(unsigned long ip, unsigned int old, unsigned int new)
>>>>  	}
>>>>
>>>>  	/* replace the text with the new text */
>>>> -	if (patch_instruction((unsigned int *)ip, new))
>>>> -		return -EPERM;
>>>> +	set_kernel_text_rw(ip);
>>>> +	err = patch_instruction((unsigned int *)ip, new);
>>>> +	set_kernel_text_ro(ip);
>>>
>>> Is there a reason to not put those inside patch_instruction()?
>>
>> Yes and no.
>>
>> patch_instruction() is called quite early from apply_feature_fixups(), I
>> haven't looked closely but I suspect the set_kernel_text_rx() routines
>> won't work that early.
>>
>> But on the other hand patch_instruction() is used by things other than
>> ftrace, like jump labels, so we probably want the rw/ro setting in there
>> so that we don't have to go and fixup jump labels etc.
>>
>> So probably we need a raw_patch_instruction() which does just the
>> patching (what patch_instruction() does now), and can be used early in
>> boot. And then patch_instruction() would have the rw/ro change in it, so
>> that all users of it are OK.
>>
>> eg ~=:
>>
>> int raw_patch_instruction(unsigned int *addr, unsigned int instr)
>> {
>>   ...
>> }
>>
>> int patch_instruction(unsigned int *addr, unsigned int instr)
>> {
>> 	int err;
>>
>> 	set_kernel_text_rw(ip);
>> 	err = raw_patch_instruction((unsigned int *)ip, new);
>> 	set_kernel_text_ro(ip);
>>
>> 	return err;
>> }
>
> Shouldn't we then also have some kind of protection against parallel use 
> of patch_instruction() like a spin_lock_irqsave(), or is it garantied 
> not to happen for other reasons ?
>
> Otherwise, we might end up with one instance setting back the kernel 
> text to RO while the other one has just put it RW and is about to patch 
> the instruction.

Yes it'd need some locking for sure.

"Locking left as an exercise for the reader." ;)

cheers