Re: [RFC PATCH 5/9] arm64/kprobes: Invoke pre/post handlers inside instrumentation

[Mark Rutland]

On Mon, Jul 27, 2026 at 12:25:41PM +0000, Hongyan Xia wrote:
> From: Hongyan Xia <hongyan.xia@xxxxxxxxxxxxx>
> 
> The kprobe pre_handler() and post_handler() are arbitrary instrumentable
> code and can themselves trace, fault, or hit other kprobes. They are
> the only parts of the kprobe debug exception flow that legitimately
> need instrumentation.

That doesn't explain what this patch changes, or why.

I'm fine with marking all the kprobe functions noinstr, but we don't
need the intrumentation_{begin,end}() gunk.

In general, it's not sound for pre_handler and post_handler to invoke
arbitrary code, given that they could lead to recursion. We just hope
people don't do stupid things with them.

Mark.

> Signed-off-by: Hongyan Xia <hongyan.xia@xxxxxxxxxxxxx>
> ---
>  arch/arm64/include/asm/kprobes.h   |  9 +++------
>  arch/arm64/kernel/probes/kprobes.c | 22 ++++++++++++++++++----
>  2 files changed, 21 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kprobes.h b/arch/arm64/include/asm/kprobes.h
> index 35ce2c94040e..a694f7d34f45 100644
> --- a/arch/arm64/include/asm/kprobes.h
> +++ b/arch/arm64/include/asm/kprobes.h
> @@ -48,11 +48,8 @@ void __kprobes *trampoline_probe_handler(struct pt_regs *regs);
>  
>  #endif /* CONFIG_KPROBES */
>  
> -int __kprobes kprobe_brk_handler(struct pt_regs *regs,
> -				 unsigned long esr);
> -int __kprobes kprobe_ss_brk_handler(struct pt_regs *regs,
> -				 unsigned long esr);
> -int __kprobes kretprobe_brk_handler(struct pt_regs *regs,
> -				 unsigned long esr);
> +int noinstr kprobe_brk_handler(struct pt_regs *regs, unsigned long esr);
> +int noinstr kprobe_ss_brk_handler(struct pt_regs *regs, unsigned long esr);
> +int noinstr kretprobe_brk_handler(struct pt_regs *regs, unsigned long esr);
>  
>  #endif /* _ARM_KPROBES_H */
> diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
> index 1b12341b2af3..e9fa66fa4217 100644
> --- a/arch/arm64/kernel/probes/kprobes.c
> +++ b/arch/arm64/kernel/probes/kprobes.c
> @@ -301,8 +301,11 @@ post_kprobe_handler(struct kprobe *cur, struct kprobe_ctlblk *kcb, struct pt_reg
>  	}
>  	/* call post handler */
>  	kcb->kprobe_status = KPROBE_HIT_SSDONE;
> +
> +	instrumentation_begin();
>  	if (cur->post_handler)
>  		cur->post_handler(cur, regs, 0);
> +	instrumentation_end();
>  
>  	reset_current_kprobe();
>  }
> @@ -359,24 +362,28 @@ int __kprobes kprobe_fault_handler(struct pt_regs *regs, unsigned int fsr)
>  	return 0;
>  }
>  
> -int __kprobes
> +int noinstr
>  kprobe_brk_handler(struct pt_regs *regs, unsigned long esr)
>  {
>  	struct kprobe *p, *cur_kprobe;
>  	struct kprobe_ctlblk *kcb;
>  	unsigned long addr = instruction_pointer(regs);
> +	bool handled;
>  
>  	kcb = get_kprobe_ctlblk();
>  	cur_kprobe = kprobe_running();
>  
> +	instrumentation_begin();
>  	p = get_kprobe((kprobe_opcode_t *) addr);
>  	if (WARN_ON_ONCE(!p)) {
> +		instrumentation_end();
>  		/*
>  		 * Something went wrong. This BRK used an immediate reserved
>  		 * for kprobes, but we couldn't find any corresponding probe.
>  		 */
>  		return DBG_HOOK_ERROR;
>  	}
> +	instrumentation_end();
>  
>  	if (cur_kprobe) {
>  		/* Hit a kprobe inside another kprobe */
> @@ -387,6 +394,10 @@ kprobe_brk_handler(struct pt_regs *regs, unsigned long esr)
>  		set_current_kprobe(p);
>  		kcb->kprobe_status = KPROBE_HIT_ACTIVE;
>  
> +		instrumentation_begin();
> +		handled = p->pre_handler && p->pre_handler(p, regs);
> +		instrumentation_end();
> +
>  		/*
>  		 * If we have no pre-handler or it returned 0, we
>  		 * continue with normal processing.  If we have a
> @@ -394,7 +405,7 @@ kprobe_brk_handler(struct pt_regs *regs, unsigned long esr)
>  		 * modify the execution path and not need to single-step
>  		 * Let's just reset current kprobe and exit.
>  		 */
> -		if (!p->pre_handler || !p->pre_handler(p, regs))
> +		if (!handled)
>  			setup_singlestep(p, regs, kcb, 0);
>  		else
>  			reset_current_kprobe();
> @@ -403,7 +414,7 @@ kprobe_brk_handler(struct pt_regs *regs, unsigned long esr)
>  	return DBG_HOOK_HANDLED;
>  }
>  
> -int __kprobes
> +int noinstr
>  kprobe_ss_brk_handler(struct pt_regs *regs, unsigned long esr)
>  {
>  	struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();
> @@ -422,13 +433,16 @@ kprobe_ss_brk_handler(struct pt_regs *regs, unsigned long esr)
>  	return DBG_HOOK_ERROR;
>  }
>  
> -int __kprobes
> +int noinstr
>  kretprobe_brk_handler(struct pt_regs *regs, unsigned long esr)
>  {
>  	if (regs->pc != (unsigned long)__kretprobe_trampoline)
>  		return DBG_HOOK_ERROR;
>  
> +	instrumentation_begin();
>  	regs->pc = kretprobe_trampoline_handler(regs, (void *)regs->regs[29]);
> +	instrumentation_end();
> +
>  	return DBG_HOOK_HANDLED;
>  }
>  
> -- 
> 2.47.3
>