Re: [PATCH -tip v6 26/27] Documentation: kprobes: Add how to change the execution path

[Masami Hiramatsu <mhiramat@kernel.org>]

On Tue, 19 Jun 2018 09:31:49 -0700
Randy Dunlap <rdunlap@infradead.org> wrote:

> On 06/19/2018 09:16 AM, Masami Hiramatsu wrote:
> > Add a section that explaining how to change the execution
> > path with kprobes and warnings for some arch.
> > 
> > Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
> > Cc: Jonathan Corbet <corbet@lwn.net>
> > Cc: linux-doc@vger.kernel.org
> > ---
> >  Documentation/kprobes.txt |   20 ++++++++++++++++++++
> >  1 file changed, 20 insertions(+)
> > 
> > diff --git a/Documentation/kprobes.txt b/Documentation/kprobes.txt
> > index 3e9e99ea751b..8a98eed1521b 100644
> > --- a/Documentation/kprobes.txt
> > +++ b/Documentation/kprobes.txt
> > @@ -80,6 +80,26 @@ After the instruction is single-stepped, Kprobes executes the
> >  "post_handler," if any, that is associated with the kprobe.
> >  Execution then continues with the instruction following the probepoint.
> >  
> 
> Hi,
> I have a few small suggestions...

Hi Randy,

Thank you for your suggestions!
All of those are good to me. I'll fix it:)

Thank you!

> 
> 
> > +Changing Execution Path
> > +-----------------------
> > +
> > +Since the kprobes can probe into a running kernel code, it can change
> 
>    Since kprobes can probe into running kernel code, it can change
> 
> > +the register set, including instruction pointer. This operation
> > +requires maximum attention, such as keeping the stack frame, recovering
> > +execution path etc. Since it is operated on running kernel and need deep
> 
>                        Since it operates on a running kernel and needs deep
> 
> > +knowladge of the archtecture and concurrent computing, you can easily
> 
>    knowledge of the architecture
> 
> > +shot your foot.
> 
>    shoot
> 
> > +
> > +If you change the instruction pointer (and set up other related
> > +registers) in pre_handler, you must return !0 so that the kprobes
> 
>                                                  so that kprobes
> 
> > +stops single stepping and just returns to given address.
> 
>                                           to the given address.
> 
> > +This also means post_handler should not be called anymore.
> > +
> > +Note that this operation may be harder on some architectures which
> > +use TOC (Table of Contents) for function call, since you have to
> > +setup new TOC for your function in your module, and recover old
> 
>    setup a new TOC for your function in your module, and recover the old
> 
> > +one after back from it.
> 
>    one after returning from it.
> 
> > +
> >  Return Probes
> >  -------------
> 
> 
> -- 
> ~Randy


-- 
Masami Hiramatsu <mhiramat@kernel.org>