linux-kernel - Re: [PATCH] ftrace/x86: Add a comment to why we take text_mutex in ftrace_arch_code_modify

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Date:   Thu, 27 Jun 2019 20:21:09 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Petr Mladek <pmladek@...e.com>,
        Miroslav Benes <mbenes@...e.cz>, Jessica Yu <jeyu@...nel.org>,
        Jiri Kosina <jikos@...nel.org>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        live-patching@...r.kernel.org,
        Johannes Erdfelt <johannes@...felt.com>,
        Ingo Molnar <mingo@...nel.org>, mhiramat@...nel.org,
        torvalds@...ux-foundation.org
Subject: Re: [PATCH] ftrace/x86: Add a comment to why we take text_mutex in
 ftrace_arch_code_modify_prepare()

On Thu, Jun 27, 2019 at 09:18:19PM -0400, Steven Rostedt wrote:
> 
> From: "Steven Rostedt (VMware)" <rostedt@...dmis.org>
> 
> Taking the text_mutex in ftrace_arch_code_modify_prepare() is to fix a
> race against module loading and live kernel patching that might try to
> change the text permissions while ftrace has it as read/write. This
> really needs to be documented in the code. Add a comment that does such.
> 
> Suggested-by: Josh Poimboeuf <jpoimboe@...hat.com>
> Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
> ---
>  arch/x86/kernel/ftrace.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
> index 33786044d5ac..d7e93b2783fd 100644
> --- a/arch/x86/kernel/ftrace.c
> +++ b/arch/x86/kernel/ftrace.c
> @@ -36,6 +36,11 @@
>  
>  int ftrace_arch_code_modify_prepare(void)
>  {
> +	/*
> +	 * Need to grab text_mutex to prevent a race from module loading
> +	 * and live kernel patching from changing the text permissions while
> +	 * ftrace has it set to "read/write".
> +	 */
>  	mutex_lock(&text_mutex);
>  	set_kernel_text_rw();
>  	set_all_modules_text_rw();
> -- 
> 2.20.1
> 

Reviewed-by: Josh Poimboeuf <jpoimboe@...hat.com>

-- 
Josh