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

From: Torsten Duwe <duwe@lst.de>
To: Julien Thierry <julien.thierry@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Ingo Molnar <mingo@redhat.com>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	Arnd Bergmann <arnd@arndb.de>,
	AKASHI Takahiro <takahiro.akashi@linaro.org>,
	Amit Daniel Kachhap <amit.kachhap@arm.com>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, live-patching@vger.kernel.org
Subject: Re: [PATCH v7 2/3] arm64: implement ftrace with regs
Date: Thu, 7 Feb 2019 13:51:59 +0100	[thread overview]
Message-ID: <20190207125159.GA19818@lst.de> (raw)
In-Reply-To: <198550d8-78d4-6e30-0179-b5e07dd140f8@arm.com>

On Thu, Feb 07, 2019 at 10:33:50AM +0000, Julien Thierry wrote:
> 
> 
> 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" :) .

I've been thinking this over and I'm considering to make an ftrace_modify_code
with verify and warn_once if it fails. Then read the insn back and bug_on
should it not be the lr saver. Any objections?

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

Yes, Steve's vote first.

	Torsten