linux-kernel - [RFC PATCH -tip v5 26/27] Documentation: kprobes: Add how to change the execution path

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Message-Id: <152812806959.10068.3756867273565043166.stgit@devbox>
Date:   Tue,  5 Jun 2018 01:01:09 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>
Cc:     Masami Hiramatsu <mhiramat@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        Ananth N Mavinakayanahalli <ananth@...ux.vnet.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-arch@...r.kernel.org, Jonathan Corbet <corbet@....net>,
        linux-doc@...r.kernel.org
Subject: [RFC PATCH -tip v5 26/27] Documentation: kprobes: Add how to change the execution path

Add a section that explaining how to change the execution
path with kprobes and warnings for some arch.

Signed-off-by: Masami Hiramatsu <mhiramat@...nel.org>
Cc: Jonathan Corbet <corbet@....net>
Cc: linux-doc@...r.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.
 
+Changing Execution Path
+-----------------------
+
+Since the kprobes can probe into a 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
+knowladge of the archtecture and concurrent computing, you can easily
+shot your foot.
+
+If you change the instruction pointer (and set up other related
+registers) in pre_handler, you must return !0 so that the kprobes
+stops single stepping and just returns to 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
+one after back from it.
+
 Return Probes
 -------------