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

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Message-ID: <Pine.LNX.4.58.0806030858500.20107@gandalf.stny.rr.com>
Date:	Tue, 3 Jun 2008 09:00:26 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Abhishek Sagar <sagar.abhishek@...il.com>
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 Tue, 3 Jun 2008, Abhishek Sagar wrote:
>
> > 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...

Can we add an API to kprobes or perhaps a way to register code
modification in ftrace, that ftrace can ignore? Let the two know about
each other, but in a way that they dont need to know the internals of one
another.

-- Steve

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