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@kernel.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Ananth N Mavinakayanahalli <ananth@linux.vnet.ibm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: linux-arch@vger.kernel.org
Cc: linux-doc@vger.kernel.org
Link: https://lore.kernel.org/lkml/152942500680.15209.12374262914863044775.stgit@devbox
Signed-off-by: Ingo Molnar <mingo@kernel.org>

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
 -------------