linux-kernel - Re: [PATCH] ftrace: Avoid double-free of dyn

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Date:	Tue, 24 Mar 2009 22:35:31 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Zhaolei <zhaolei@...fujitsu.com>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ftrace: Avoid double-free of dyn_ftrace

On Wed, Mar 25, 2009 at 10:25:47AM +0800, Zhaolei wrote:
> >> > 
> >> > I'm wondering, did you trigger this in practice (if yes, how?), 
> >> > or have you found it via code review?
> >> Hello, Ingo
> >> 
> >> It is found via code review.
> > 
> > Hmm, could you explain this more. I'm thinking that this scenario should 
> > not happen, and if it does, it should probably be a bug.
> > 
> > Because when we call ftrace_free_rec we change the rec->ip to point to the 
> > next record in the chain. Something is very wrong if rec->ip >= s && 
> > rec->ip < e and the record is already free.
> Hello, Steven
> 
> Thanks for your comment.
> I got your meaning, and I agree that if rec->ip >= s && rec->ip < e,
> this record is not freed.
> But IMHO, "if rec->ip >= s && rec->ip < e" is used to select records in the module,
> and function of ignore "freed record" is only its side-effect.
> So, add a special judgement to avoid "freed record" is not a bad idea.
> And I also agree your suggestion of add a WARN_ON, because this should not happened.

Hi Zhaolei,

Great! Feel free to send another patch ;-)

Note, use FTRACE_WARN_ON() macro. This way it shuts down ftrace if it is hit and 
helps to avoid further damage later.

-- Steve

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