linux-kernel - Re: [PATCH v7 2/3] arm64: implement ftrace with regs

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Message-ID: <198550d8-78d4-6e30-0179-b5e07dd140f8@arm.com>
Date:   Thu, 7 Feb 2019 10:33:50 +0000
From:   Julien Thierry <julien.thierry@....com>
To:     Torsten Duwe <duwe@....de>
Cc:     Mark Rutland <mark.rutland@....com>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        AKASHI Takahiro <takahiro.akashi@...aro.org>,
        Amit Daniel Kachhap <amit.kachhap@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org
Subject: Re: [PATCH v7 2/3] arm64: implement ftrace with regs



On 06/02/2019 15:05, Torsten Duwe wrote:
> On Wed, Feb 06, 2019 at 08:59:44AM +0000, Julien Thierry wrote:
>> Hi Torsten,
>>
>> On 18/01/2019 16:39, Torsten Duwe wrote:
>>
>>> --- a/arch/arm64/kernel/ftrace.c
>>> +++ b/arch/arm64/kernel/ftrace.c
>>> @@ -133,17 +163,45 @@ int ftrace_make_call(struct dyn_ftrace *
>>>  	return ftrace_modify_code(pc, old, new, true);
>>>  }
>>>  
>>> +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
>>> +int ftrace_modify_call(struct dyn_ftrace *rec, unsigned long old_addr,
>>> +			unsigned long addr)
>>> +{
>>> +	unsigned long pc = rec->ip + REC_IP_BRANCH_OFFSET;
>>> +	u32 old, new;
>>> +
>>> +	old = aarch64_insn_gen_branch_imm(pc, old_addr, true);
>>> +	new = aarch64_insn_gen_branch_imm(pc, addr, true);
>>> +
>>> +	return ftrace_modify_code(pc, old, new, true);
>>> +}
>>> +#endif
>>> +
>>>  /*
>>>   * Turn off the call to ftrace_caller() in instrumented function
>>>   */
>>>  int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec,
>>>  		    unsigned long addr)
>>>  {
>>> -	unsigned long pc = rec->ip;
>>> +	unsigned long pc = rec->ip + REC_IP_BRANCH_OFFSET;
>>
>> Sorry to come back on this patch again, but I was looking at the ftrace
>> code a bit, and I see that when processing the ftrace call locations,
>> ftrace calls ftrace_call_adjust() on every ip registered as mcount
>> caller (or in our case patchable entries). This ftrace_call_adjust() is
>> arch specific, so I was thinking we could place the offset in here once
>> and for all so we don't have to worry about it in the future.
> 
> Now that you mention it - yes indeed that's the correct facility to fix
> the deviating address, as Steve has also confirmed. I had totally forgotten
> about this hook.
> 
>> Also, I'm unsure whether it would be safe, but we could patch the "mov
>> x9, lr" there as well. In theory, this would be called at init time
>> (before secondary CPUs are brought up) and when loading a module (so I'd
>> expect no-one is executing that code *yet*.
>>
>> If this is possible, I think it would make things a bit cleaner.
> 
> This is in fact very tempting, but it will introduce a nasty side effect
> to ftrace_call_adjust. Is there any obvious documentation that specifies
> guarantees about ftrace_call_adjust being called exactly once for each site?
> 

I don't see really much documentation on that function. As far as I can
tell it is only called once for each site (and if it didn't, we'd always
be placing the same instruction, but I agree it wouldn't be nice). It
could depend on how far you can expand the notion of "adjusting" :) .

Steven, do you have an opinion on whether it would be acceptable to
modify function entry code in ftrace_call_adjust() ?

Thanks,

-- 
Julien Thierry