linux-kernel - Re: [PATCH v2 2/5] metag: ftrace: fix the comments for ftrace_modify

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Date:	Wed, 23 Dec 2015 17:39:05 +0000
From:	James Hogan <james.hogan@...tec.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	Li Bin <huawei.libin@...wei.com>, <mingo@...hat.com>,
	<linux-kernel@...r.kernel.org>, <linux-metag@...r.kernel.org>
Subject: Re: [PATCH v2 2/5] metag: ftrace: fix the comments for
 ftrace_modify_code

Hi Steven,

On Wed, Dec 23, 2015 at 12:33:46PM -0500, Steven Rostedt wrote:
> On Sun, 6 Dec 2015 10:02:55 +0800
> Li Bin <huawei.libin@...wei.com> wrote:
> 
> > There is no need to worry about module and __init text disappearing
> > case, because that ftrace has a module notifier that is called when
> > a module is being unloaded and before the text goes away and this
> > code grabs the ftrace_lock mutex and removes the module functions
> > from the ftrace list, such that it will no longer do any
> > modifications to that module's text, the update to make functions
> > be traced or not is done under the ftrace_lock mutex as well.
> > And by now, __init section codes should not been modified
> > by ftrace, because it is black listed in recordmcount.c and
> > ignored by ftrace.
> > 
> > Cc: James Hogan <james.hogan@...tec.com>
> > Cc: linux-metag@...r.kernel.org
> > Suggested-by: Steven Rostedt <rostedt@...dmis.org>
> > Signed-off-by: Li Bin <huawei.libin@...wei.com>
> 
> Hi, can I get a metag maintainer to ack this. Or you can take the patch
> too.

Yep, sorry for the delay,

Acked-by: James Hogan <james.hogan@...tec.com>

Cheers
James

> 
> -- Steve
> 
> > ---
> >  arch/metag/kernel/ftrace.c |   11 +++++------
> >  1 files changed, 5 insertions(+), 6 deletions(-)
> > 
> > diff --git a/arch/metag/kernel/ftrace.c b/arch/metag/kernel/ftrace.c
> > index ed1d685..ac8c039 100644
> > --- a/arch/metag/kernel/ftrace.c
> > +++ b/arch/metag/kernel/ftrace.c
> > @@ -54,12 +54,11 @@ static int ftrace_modify_code(unsigned long pc, unsigned char *old_code,
> >  	unsigned char replaced[MCOUNT_INSN_SIZE];
> >  
> >  	/*
> > -	 * Note: Due to modules and __init, code can
> > -	 *  disappear and change, we need to protect against faulting
> > -	 *  as well as code changing.
> > -	 *
> > -	 * No real locking needed, this code is run through
> > -	 * kstop_machine.
> > +	 * Note:
> > +	 * We are paranoid about modifying text, as if a bug was to happen, it
> > +	 * could cause us to read or write to someplace that could cause harm.
> > +	 * Carefully read and modify the code with probe_kernel_*(), and make
> > +	 * sure what we read is what we expected it to be before modifying it.
> >  	 */
> >  
> >  	/* read the text we want to modify */
> 

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