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

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Message-ID: <f66cbe74-b3ce-e471-ad14-ecb729fe7601@c-s.fr>
Date:   Mon, 24 Apr 2017 16:31:01 +0200
From:   Christophe LEROY <christophe.leroy@....fr>
To:     Michael Ellerman <mpe@...erman.id.au>,
        "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



Le 23/04/2017 à 12:26, Michael Ellerman a écrit :
> 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
>

Not so easy indeed as patch_instruction() is called from many higher 
level functions like patch_branch() which are themselves called from 
other functions like do_features_fixup() which are called during init 
but also when loading a module for instance.
It is therefore not easy to implement it via a raw_patch_instruction() 
as proposed.

So I took another approach, taken from x86: a static bool tells whether 
kernel text has been put in RO yet or not. Until this, 
set_kernel_text_ro/rw() return without doing anything.

As for the locking, I put a spin_lock_irqsave() as I was not sure 
whether patch_instruction() can be called during interrupts or not.

Christophe