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

Randy Dunlap
2018-06-19T16:31:58Z

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...

> +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