linux-kernel - Re: [PATCH v6] arm64: implement ftrace with regs

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Date:   Fri, 4 Jan 2019 23:41:45 +0100
From:   Torsten Duwe <duwe@....de>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Mark Rutland <mark.rutland@....com>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Julien Thierry <julien.thierry@....com>,
        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 v6] arm64: implement ftrace with regs

On Fri, Jan 04, 2019 at 01:06:48PM -0500, Steven Rostedt wrote:
> On Fri, 4 Jan 2019 17:50:18 +0000
> Mark Rutland <mark.rutland@....com> wrote:
> 
> > At Linux Plumbers, I had a conversation with Steve Rostedt, and we came
> > to the conclusion that (withut heavyweight synchronization) patching two
> > NOPs at runtime isn't safe, since a CPU might have executed the first
> > NOP as a NOP before another CPU patches both instructions. So a CPU
> > might execute:
> > 
> > 	NOP
> > 	BL	ftrace_regs_caller
> > 
> > ... rather than the expected:
> > 
> > 	MOV	X9, X30
> > 	BL	ftrace_regs_caller
> > 
> > ... and therefore X9 contains some UNKNOWN value, rather than the
> > original LR value.

I'm perfectly aware of that; an earlier version had barriers, attempting
to avoid just that, which Mark(?) wrote weren't neccessary.

But is this a realistic scenario? All function entries are aligned 8 bytes.
Are there arm64 implementations out there that fetch only 4 bytes and
give a chance to mess with the 2nd 4 bytes? You at arm.com should know, and
I won't be surprised if the answer is a weird "yes". Or maybe it's just
another erratum lurking somewhere...

My point is: those 2 insn will _never_ be split by any alignment
boundary > 8; does that mean anything, have you considered this?

> > I wonder if we could solve that by patching the kernel at build-time, to
> > add the MOV X9, X30 in place of the first NOP. If we were to do that, we
> > could also update the addresses to pooint at the second NOP, simplifying
> > the changes to the runtime code.
> 
> You can also patch it at boot up when there's only one CPU running, and
> interrupts are disabled.

May I remind about possible performance hits? Even the NOPs had a tiny impact
on certain in-order implementations. I'd rather switch between the mov and
a "b +2".

	Torsten