linux-kernel - Re: [PATCH AUTOSEL 4.19 18/21] kprobes: Tell lockdep about kprobe nesting

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Message-Id: <20201110154458.546c220fcae09592cf5282b9@kernel.org>
Date:   Tue, 10 Nov 2020 15:44:58 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Sasha Levin <sashal@...nel.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        "Steven Rostedt (VMware)" <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [PATCH AUTOSEL 4.19 18/21] kprobes: Tell lockdep about kprobe
 nesting

Hi,

On Mon,  9 Nov 2020 22:55:38 -0500
Sasha Levin <sashal@...nel.org> wrote:

> From: "Steven Rostedt (VMware)" <rostedt@...dmis.org>
> 
> [ Upstream commit 645f224e7ba2f4200bf163153d384ceb0de5462e ]
> 
> Since the kprobe handlers have protection that prohibits other handlers from
> executing in other contexts (like if an NMI comes in while processing a
> kprobe, and executes the same kprobe, it will get fail with a "busy"
> return). Lockdep is unaware of this protection. Use lockdep's nesting api to
> differentiate between locks taken in INT3 context and other context to
> suppress the false warnings.
> 
> Link: https://lore.kernel.org/r/20201102160234.fa0ae70915ad9e2b21c08b85@kernel.org
> 

This fixes a lockdep false positive warning comes from commit e03b4a084ea6
("kprobes: Remove NMI context check"). Does anyone report that happen on the
stable kernel?

If not, you do not need this patch for stable kernels.

Thank you,


> Cc: Peter Zijlstra <peterz@...radead.org>
> Acked-by: Masami Hiramatsu <mhiramat@...nel.org>
> Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
> Signed-off-by: Sasha Levin <sashal@...nel.org>
> ---
>  kernel/kprobes.c | 25 +++++++++++++++++++++----
>  1 file changed, 21 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index 2161f519d4812..2ce9053de6ae4 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -1204,7 +1204,13 @@ __acquires(hlist_lock)
>  
>  	*head = &kretprobe_inst_table[hash];
>  	hlist_lock = kretprobe_table_lock_ptr(hash);
> -	raw_spin_lock_irqsave(hlist_lock, *flags);
> +	/*
> +	 * Nested is a workaround that will soon not be needed.
> +	 * There's other protections that make sure the same lock
> +	 * is not taken on the same CPU that lockdep is unaware of.
> +	 * Differentiate when it is taken in NMI context.
> +	 */
> +	raw_spin_lock_irqsave_nested(hlist_lock, *flags, !!in_nmi());
>  }
>  NOKPROBE_SYMBOL(kretprobe_hash_lock);
>  
> @@ -1213,7 +1219,13 @@ static void kretprobe_table_lock(unsigned long hash,
>  __acquires(hlist_lock)
>  {
>  	raw_spinlock_t *hlist_lock = kretprobe_table_lock_ptr(hash);
> -	raw_spin_lock_irqsave(hlist_lock, *flags);
> +	/*
> +	 * Nested is a workaround that will soon not be needed.
> +	 * There's other protections that make sure the same lock
> +	 * is not taken on the same CPU that lockdep is unaware of.
> +	 * Differentiate when it is taken in NMI context.
> +	 */
> +	raw_spin_lock_irqsave_nested(hlist_lock, *flags, !!in_nmi());
>  }
>  NOKPROBE_SYMBOL(kretprobe_table_lock);
>  
> @@ -1884,7 +1896,12 @@ static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs)
>  
>  	/* TODO: consider to only swap the RA after the last pre_handler fired */
>  	hash = hash_ptr(current, KPROBE_HASH_BITS);
> -	raw_spin_lock_irqsave(&rp->lock, flags);
> +	/*
> +	 * Nested is a workaround that will soon not be needed.
> +	 * There's other protections that make sure the same lock
> +	 * is not taken on the same CPU that lockdep is unaware of.
> +	 */
> +	raw_spin_lock_irqsave_nested(&rp->lock, flags, 1);
>  	if (!hlist_empty(&rp->free_instances)) {
>  		ri = hlist_entry(rp->free_instances.first,
>  				struct kretprobe_instance, hlist);
> @@ -1895,7 +1912,7 @@ static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs)
>  		ri->task = current;
>  
>  		if (rp->entry_handler && rp->entry_handler(ri, regs)) {
> -			raw_spin_lock_irqsave(&rp->lock, flags);
> +			raw_spin_lock_irqsave_nested(&rp->lock, flags, 1);
>  			hlist_add_head(&ri->hlist, &rp->free_instances);
>  			raw_spin_unlock_irqrestore(&rp->lock, flags);
>  			return 0;
> -- 
> 2.27.0
> 


-- 
Masami Hiramatsu <mhiramat@...nel.org>