linux-kernel - Re: [PATCH v2 2/2] arm64: ftrace: fix the comments for ftrace_modify

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Date:	Thu, 3 Dec 2015 21:50:01 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Li Bin <huawei.libin@...wei.com>
Cc:	<will.deacon@....com>, <catalin.marinas@....com>,
	<linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>, <mingo@...hat.com>,
	<guohanjun@...wei.com>, <dingtianhong@...wei.com>
Subject: Re: [PATCH v2 2/2] arm64: ftrace: fix the comments for
 ftrace_modify_code

On Fri, 4 Dec 2015 10:18:39 +0800
Li Bin <huawei.libin@...wei.com> wrote:

> There is no need to worry about module 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.
> 
> Signed-off-by: Li Bin <huawei.libin@...wei.com>
> ---
>  arch/arm64/kernel/ftrace.c |    5 +----
>  1 files changed, 1 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
> index 9669b33..ee91c0c 100644
> --- a/arch/arm64/kernel/ftrace.c
> +++ b/arch/arm64/kernel/ftrace.c
> @@ -29,12 +29,9 @@ static int ftrace_modify_code(unsigned long pc, u32 old, u32 new,
>  
>  	/*
>  	 * Note:
> -	 * Due to modules and __init, code can disappear and change,
> +	 * Due to __init, code can disappear and change,

Init code should not be modified either because it is black listed in
recordmcount.c.

I say just change the comment to be something like:

We are paranoid about modifying text, as if a bug were to happen, it
could cause us to read or write to someplace that could cause harm.
Carefully read and modify the code with aarch64_insn_*() which uses
probe_kernel_*(), and make sure what we read is what we expected it to
be before modifying it.

-- Steve


>  	 * we need to protect against faulting as well as code changing.
>  	 * We do this by aarch64_insn_*() which use the probe_kernel_*().
> -	 *
> -	 * No lock is held here because all the modifications are run
> -	 * through stop_machine().
>  	 */
>  	if (validate) {
>  		if (aarch64_insn_read((void *)pc, &replaced))

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