linux-kernel - Re: [PATCH v1 07/22] powerpc/ftrace: Use patch

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Message-ID: <b7e8ffec-e13f-a423-4baf-b13cb8b5ad41@csgroup.eu>
Date:   Wed, 4 May 2022 12:01:49 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Steven Rostedt <rostedt@...dmis.org>,
        "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
CC:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Ingo Molnar <mingo@...hat.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Mackerras <paulus@...ba.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH v1 07/22] powerpc/ftrace: Use patch_instruction() return
 directly



Le 18/04/2022 à 21:44, Steven Rostedt a écrit :
> On Mon, 18 Apr 2022 11:51:16 +0530
> "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com> wrote:
> 
>>> --- a/arch/powerpc/kernel/trace/ftrace.c
>>> +++ b/arch/powerpc/kernel/trace/ftrace.c
>>> @@ -78,10 +78,7 @@ ftrace_modify_code(unsigned long ip, ppc_inst_t old, ppc_inst_t new)
>>>   	}
>>>
>>>   	/* replace the text with the new text */
>>> -	if (patch_instruction((u32 *)ip, new))
>>> -		return -EPERM;
>>> -
>>> -	return 0;
>>> +	return patch_instruction((u32 *)ip, new);
>>
>> I think the reason we were returning -EPERM is so that ftrace_bug() can
> 
> That is correct.
> 
>> throw the right error message. That will change due to this patch,
>> though I'm not sure how much it matters. -EFAULT and -EPERM seem to
>> print almost the same error message.
> 
> In these cases it helps to know the type of failure, as the way to debug it
> is different.
> 
> -EFAULT: It failed to read it the location. This means that the memory is
> likely not even mapped in, or the pointer is way off.
> 
> -EINVAL: Means that what was read did not match what was expected (the code
> was already updated, pointing to the wrong location, or simply the
> calculation of what to expect is incorrect).
> 
> -EPERM: Means the write failed. What was read was expected, but the
> permissions to write have not been updated properly.
> 
> Differentiating the three is crucial to looking at where the issue lies
> when an ftrace_bug() triggers.
> 


Apparently no caller really care about the value returned by 
patch_instruction(), the ones who check the return value just check that 
it's not 0.

So the most performant would be to have patch_instruction() return 
-EPERM instead of -EFAULT in case of failure.

Christophe