Re: [PATCH v2 1/2] arm64: implement FTRACE_WITH_REGS

[Petr Mladek <pmladek@suse.com>]

On Fri 2016-07-08 17:07:09, Torsten Duwe wrote:
> 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.

Yeah, but I cannot find this kind of check in other architectures.
I checked arch/x86/kernel/ftrace.c, arch/s390/kernel/ftrace.c, and
arch/powerpc/kernel/ftrace.c. These all support ftrace with
regs and livepatching.

Best Regards,
Petr