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Date:   Sat, 22 Apr 2017 16:08:30 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Christophe Leroy <christophe.leroy@....fr>,
        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

"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;
}

cheers

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