linux-kernel - Re: [PATCH 0/3][RFC] ftrace: track dynamic ftrace update failures

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Date:	Tue, 3 Jun 2008 09:43:05 +0530
From:	"Abhishek Sagar" <sagar.abhishek@...il.com>
To:	"Steven Rostedt" <rostedt@...dmis.org>
Cc:	"Ingo Molnar" <mingo@...e.hu>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/3][RFC] ftrace: track dynamic ftrace update failures

On Mon, Jun 2, 2008 at 11:26 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> Hi Abhishek,

Hi Steven,

Thanks for reviewing my recent patches.

> Have you also taken into account adding and removing of modules? This can
> cause some funny behaviours with ftrace that can be rather dangerous.

I've tried to accommodate them (fixed the wrong use of
kernel_text_address). This makes
init and module function records eligible to be freed. Only core
kernel function update failures now get tracked/remembered.

There might still be a need to find a way to track external modifiers
of mcount call-sites such as (but not limited to) kprobes. Consider
this (rather long) contrived scenario:

1. Function foo gets recorded by ftrace.
2. Mcount call site of foo is patched with a NOP by ftraced.
3. foo gets 'enabled' by some ftrace filter rule. NOP changes to a
call to the function tracer.
4. A kprobe is registered on the mcount call-site. It places a trap
generating instruction at the
mcount call site and stores the call-to-function-tracer instruction
elsewhere internally.
5. Tracing is disabled by end user.
6. Disabling of foo fails because Kprobe had placed a trap at the
mcount call site.
7. foo still keeps getting traced because Kprobes will keep
single-stepping the call-to-function-tracer instruction each time the
mcount call site is executed.

Not sure what to do with such cases...

--
Regards,
Abhishek Sagar
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