Re: [PATCH v5 14/21] kprobes: Remove NMI context check

[Steven Rostedt]

[ Peter Z, please take a look a this ]

On Mon, 2 Nov 2020 16:02:34 +0900
Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:

> >From 509b27efef8c7dbf56cab2e812916d6cd778c745 Mon Sep 17 00:00:00 2001  
> From: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Date: Mon, 2 Nov 2020 15:37:28 +0900
> Subject: [PATCH] kprobes: Disable lockdep for kprobe busy area
> 
> Since the code area in between kprobe_busy_begin()/end() prohibits
> other kprobs to call probe handlers, we can avoid inconsitent
> locks there. But lockdep doesn't know that, so it warns rp->lock
> or kretprobe_table_lock.
> 
> To supress those false-positive errors, disable lockdep while
> kprobe_busy is set.
> 
> Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> ---
>  kernel/kprobes.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index 8a12a25fa40d..c7196e583600 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -1295,10 +1295,12 @@ void kprobe_busy_begin(void)
>  	__this_cpu_write(current_kprobe, &kprobe_busy);
>  	kcb = get_kprobe_ctlblk();
>  	kcb->kprobe_status = KPROBE_HIT_ACTIVE;
> +	lockdep_off();
>  }
>  
>  void kprobe_busy_end(void)
>  {
> +	lockdep_on();
>  	__this_cpu_write(current_kprobe, NULL);
>  	preempt_enable();
>  }
> -- 

No, this is not the correct workaround (too big of a hammer). You could do
the following: