linux-kernel - Re: [PATCH v2 1/2] arm64: implement FTRACE_WITH

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Date:	Fri, 8 Jul 2016 17:07:09 +0200
From:	Torsten Duwe <duwe@....de>
To:	Petr Mladek <pmladek@...e.com>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>, takahiro.akashi@...aro.org,
	Jungseok Lee <jungseoklee85@...il.com>,
	Arnd Bergmann <arnd@...db.de>,
	Li Bin <huawei.libin@...wei.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>,
	Christopher Li <sparse@...isli.org>,
	Jiri Kosina <jikos@...nel.org>, andrew.wafaa@....com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	live-patching@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-sparse@...r.kernel.org
Subject: Re: [PATCH v2 1/2] arm64: implement FTRACE_WITH_REGS

On Fri, Jul 08, 2016 at 04:58:00PM +0200, Petr Mladek wrote:
> On Mon 2016-06-27 17:17:17, Torsten Duwe wrote:
> > Once gcc is enhanced to optionally generate NOPs at the beginning
> > of each function, like the concept proven in
> > https://gcc.gnu.org/ml/gcc-patches/2016-04/msg01671.html
> > (sans the "fprintf (... pad_size);", which spoils the data structure
> > for kernel use), the generated pads can nicely be used to reroute
> > function calls for tracing/profiling, or live patching.
> > diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
> > index ebecf9a..917065c 100644
> > --- a/arch/arm64/kernel/ftrace.c
> > +++ b/arch/arm64/kernel/ftrace.c
> > @@ -39,6 +39,12 @@ static int ftrace_modify_code(unsigned long pc, u32 old, u32 new,
> >  		if (aarch64_insn_read((void *)pc, &replaced))
> >  			return -EFAULT;
> >  
> > +		/* If we already have what we'll finally want,
> > +		 * report success. This is needed on startup.
> > +		 */
> > +		if (replaced == new)
> > +			return 0;
> 
> This looks strange. I wonder if it actually hides a real bug that we
> modify the code twice or so.

Not at all. All "profilers" we abused so far generate code that needs to
be disabled on boot first. prolog-pad generates nops, initially.

	Torsten