linux-kernel - Re: [PATCH v2 3/5] powerpc: ftrace: fix the comments for ftrace_modify

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Date:	Wed, 23 Dec 2015 12:34:59 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Li Bin <huawei.libin@...wei.com>
Cc:	<mingo@...hat.com>, <linux-kernel@...r.kernel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	<linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH v2 3/5] powerpc: ftrace: fix the comments for
 ftrace_modify_code

On Sun, 6 Dec 2015 10:02:56 +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: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> Cc: Paul Mackerras <paulus@...ba.org>
> Cc: Michael Ellerman <mpe@...erman.id.au>
> Cc: linuxppc-dev@...ts.ozlabs.org
> Suggested-by: Steven Rostedt <rostedt@...dmis.org>
> Signed-off-by: Li Bin <huawei.libin@...wei.com>

Hi, can I get a POWERPC maintainer to ack this, or you can take it too.

-- Steve

> ---
>  arch/powerpc/kernel/ftrace.c |   12 +++++-------
>  1 files changed, 5 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/ftrace.c b/arch/powerpc/kernel/ftrace.c
> index 44d4d8e..83e3c88 100644
> --- a/arch/powerpc/kernel/ftrace.c
> +++ b/arch/powerpc/kernel/ftrace.c
> @@ -47,13 +47,11 @@ ftrace_modify_code(unsigned long ip, unsigned int old, unsigned int new)
>  	unsigned int replaced;
>  
>  	/*
> -	 * Note: Due to modules and __init, code can
> -	 *  disappear and change, we need to protect against faulting
> -	 *  as well as code changing. We do this by using the
> -	 *  probe_kernel_* functions.
> -	 *
> -	 * No real locking needed, this code is run through
> -	 * kstop_machine, or before SMP starts.
> +	 * 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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