Date:	Fri, 28 Nov 2014 10:08:28 +0000
From:	"Jon Medhurst (Tixy)" <tixy@...aro.org>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Cc:	Wang Nan <wangnan0@...wei.com>, linux@....linux.org.uk,
	will.deacon@....com, taras.kondratiuk@...aro.org,
	ben.dooks@...ethink.co.uk, cl@...ux.com, rabin@....in,
	davem@...emloft.net, lizefan@...wei.com,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: Re: [PATCH v10 2/2] ARM: kprobes: enable OPTPROBES for ARM 32

On Fri, 2014-11-28 at 12:12 +0900, Masami Hiramatsu wrote:
> (2014/11/27 23:36), Jon Medhurst (Tixy) wrote:
[...]
> > I thought it good to see what sort of benefits this code achieves,
> > especially as it could grow quite complex over time, and the cost of
> > that versus the benefit should be considered.
> 
> I don't think it's so complex. It's actually cleanly separated.
> However, ARM tree should have arch/arm/kernel/kprobe/ dir,
> since there are too many kprobe related files under arch/arm/kernel/ ...

Yes, that does seem like a good idea. Or rather a 'probes' directory to
also include uprobes as that shares a lot of code with kprobes.

> 
> >>
> >> Limitations:
> >>  - Currently only kernel compiled with ARM ISA is supported.
> > 
> > Supporting Thumb will be very difficult because I don't believe that
> > putting a branch into an IT block could be made to work, and you can't
> > feasibly know if an instruction is in an IT block other than by first
> > using something like the breakpoint probe method and then when that is
> > hit examine the IT flags to see if they're set. If they aren't you could
> > then change the probe to an optimised probe. Is transforming the probe
> > type like that currently supported by the generic kprobes code?
> 
> Optprobe framework optimizes probes transparently. If it can not be
> optimized, it just do nothing on it.

Yes, but I was saying that with the Thumb ISA, we can't know until the
first time a probe is hit if it is possible to optimise it, so when any
probe is first registered we would have to return an error from
arch_prepare_optimized_kprobe. Then have probe handling code do some
checks when it is first hit, and then trigger the optimising of the
probe if possible. I guess the extra plumbing for that wouldn't be too
hard.

> 
> 
> > Also, the Thumb branch instruction can only jump half as far as the ARM
> > mode one. And being 32-bits when a lot of instructions people will want
> > to probe are 16-bits will be an additional problem, similar as
> > identified below for ARM instructions...
> > 
> > 
> >>
> >>  - Offset between probe point and optinsn slot must not larger than
> >>    32MiB.
> > 
> > 
> > I see that elsewhere [1] people are working on supporting loading kernel
> > modules at locations that are out of the range of a branch instruction,
> > I guess because with multi-platform kernels and general code bloat
> > kernels are getting too big. The same reasons would impact the usability
> > of optimized kprobes as well if they're restricted to the range of a
> > single branch instruction.
> > 
> > [1] http://lists.infradead.org/pipermail/linux-arm-kernel/2014-November/305539.html
> > 
> > 
> >>  Masami Hiramatsu suggests replacing 2 words, it will make
> >>    things complex. Futher patch can make such optimization.
> > 
> > I'm wondering how can we replace 2 words if we can't determine if the
> > second word is the target of a branch instruction?
> 
> on X86, we already have an instruction decoder for finding the
> branch target :).

How do you know where to start decoding the instructions stream from?

>  But yes, it can be impossible in other arch if
> it intensively uses indirect branch.

I don't know if it's 'impossible' on ARM, would need someone with
expertise in formal proofs. Anyway, I for one wouldn't want to have to
try such a thing on ARM unless I was given it as something like a paid
year long research project. :-)

> [...]

>  
> > I initially had some trouble testing this. I tried running the kprobes
> > test code with some printf's added to the code and it seems that only
> > very rarely are optimised probes actually executed. This turned out to
> > be due to the optimization being run as a background task after a delay.
> > So I ended up hacking kernel/kprobes.c to force some calls to
> > wait_for_kprobe_optimizer(). It would be nice to have the test code to
> > robustly cover both optimised and unoptimised cases but that would need
> > some new exported functions from the generic kprobes code, not sure what
> > people think of that idea?
> 
> Hm, did you use ftrace's kprobe events?

Not something I've come across. I'm somewhat ashamed to say that kprobes
is something that I've only worked on from an implementation point of
view, not a user point of view.

> You can actually add kprobes via /sys/kernel/debug/tracing/kprobe_events and
> see what kprobes are optimized via /sys/kernel/debug/kprobes/list.
> 
> For more information, please refer
>  Documentation/trace/kprobetrace.txt
>  Documentation/kprobes.txt

Well, on ARM we decode and emulate the entire instruction set, so when I
came to implement Thumb ISA kprobes I created test code with test cases
to cover every instruction form and combination of argument types, which
required a fair amount of automation, so I created a test framework for
that (arch/arm/kernel/kprobes-test*). I also added test to cover the
existing ARM ISA code at the time and found it mostly broken and had to
fix it

I know comprehensive testing isn't the Linux way, but that was my first
Linux project and I brought my old habits with me. And as you can see
from my testing of these latest patches I've not yet given up those
habits.

-- 
Tixy

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