Re: [RFC PATCH 4/9] arm64/kprobes: Make the single-step machinery noinstr

[Mark Rutland <mark.rutland@arm.com>]

On Mon, Jul 27, 2026 at 12:25:39PM +0000, Hongyan Xia wrote:
> From: Hongyan Xia <hongyan.xia@transsion.com>
> 
> Convert the core of the arm64 kprobe single-step flow to noinstr:
> save_previous_kprobe(), restore_previous_kprobe(), set_current_kprobe(),
> kprobes_save_local_irqflag(), kprobes_restore_local_irqflag(),
> setup_singlestep(), reenter_kprobe() and post_kprobe_handler().
> 
> These functions only touch per-cpu state, pt_regs and DAIF, except for:
> 
> - kprobes_inc_nmissed_count(), which is generic and instrumentable; add
>   an arm64-local wrapper that calls it bounded by
>   instrumentation_begin()/end();

Why is it safe to call kprobes_inc_nmissed_count() if it can be
instrumented?

How does that work for other architectures?

> - the instruction simulation path in setup_singlestep(), whose decode
>   handlers are instrumentable; bound arch_simulate_insn() with an
>   instrumentation window;

I don't think that's sufficient. If we can instrument that code, then we
can have unbounded recursion, unless I'm missing something?

I think we need to fix that code to be noinstr safe.

Ada Cc'd was looking into making the insn code generally noinstr-safe,
but that's a big job. Maybe it's possible to clean up the subset that's
necessary for arch_simulate_insn()?

> - the unrecoverable-reentry pr_warn()/dump_kprobe()/BUG() path and the
>   default WARN_ON(), both bounded with instrumentation windows.

That sounds fine.

Mark.

> The __kprobes attribute (notrace + .kprobes.text) is replaced by
> noinstr, which is a strict superset for these functions.
> 
> Signed-off-by: Hongyan Xia <hongyan.xia@transsion.com>
> ---
>  arch/arm64/kernel/probes/kprobes.c | 39 ++++++++++++++++++------------
>  1 file changed, 24 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
> index 4e0efad5caf2..1b12341b2af3 100644
> --- a/arch/arm64/kernel/probes/kprobes.c
> +++ b/arch/arm64/kernel/probes/kprobes.c
> @@ -14,6 +14,7 @@
>  #include <linux/extable.h>
>  #include <linux/kasan.h>
>  #include <linux/kernel.h>
> +#include <linux/instrumentation.h>
>  #include <linux/kprobes.h>
>  #include <linux/sched/debug.h>
>  #include <linux/set_memory.h>
> @@ -39,7 +40,7 @@
>  DEFINE_PER_CPU(struct kprobe *, current_kprobe) = NULL;
>  DEFINE_PER_CPU(struct kprobe_ctlblk, kprobe_ctlblk);
>  
> -static void __kprobes
> +static void noinstr
>  post_kprobe_handler(struct kprobe *, struct kprobe_ctlblk *, struct pt_regs *);
>  
>  void *alloc_insn_page(void)
> @@ -170,7 +171,7 @@ void __kprobes arch_remove_kprobe(struct kprobe *p)
>  	}
>  }
>  
> -static void __kprobes save_previous_kprobe(struct kprobe_ctlblk *kcb)
> +static void noinstr save_previous_kprobe(struct kprobe_ctlblk *kcb)
>  {
>  	kcb->prev_kprobe.kp = kprobe_running();
>  	kcb->prev_kprobe.status = kcb->kprobe_status;
> @@ -184,7 +185,7 @@ static void __kprobes save_previous_kprobe(struct kprobe_ctlblk *kcb)
>  	kcb->prev_kprobe.saved_irqflag = kcb->saved_irqflag;
>  }
>  
> -static void __kprobes restore_previous_kprobe(struct kprobe_ctlblk *kcb)
> +static void noinstr restore_previous_kprobe(struct kprobe_ctlblk *kcb)
>  {
>  	__this_cpu_write(current_kprobe, kcb->prev_kprobe.kp);
>  	kcb->kprobe_status = kcb->prev_kprobe.status;
> @@ -197,7 +198,7 @@ static void __kprobes restore_previous_kprobe(struct kprobe_ctlblk *kcb)
>  	kcb->saved_irqflag = kcb->prev_kprobe.saved_irqflag;
>  }
>  
> -static void __kprobes set_current_kprobe(struct kprobe *p)
> +static void noinstr set_current_kprobe(struct kprobe *p)
>  {
>  	__this_cpu_write(current_kprobe, p);
>  }
> @@ -207,23 +208,23 @@ static void __kprobes set_current_kprobe(struct kprobe *p)
>   * simple and avoid nesting exceptions. Interrupts do have to be disabled since
>   * the kprobe state is per-CPU and doesn't get migrated.
>   */
> -static void __kprobes kprobes_save_local_irqflag(struct kprobe_ctlblk *kcb,
> -						struct pt_regs *regs)
> +static void noinstr kprobes_save_local_irqflag(struct kprobe_ctlblk *kcb,
> +					       struct pt_regs *regs)
>  {
>  	kcb->saved_irqflag = regs->pstate & DAIF_MASK;
>  	regs->pstate |= DAIF_MASK;
>  }
>  
> -static void __kprobes kprobes_restore_local_irqflag(struct kprobe_ctlblk *kcb,
> -						struct pt_regs *regs)
> +static void noinstr kprobes_restore_local_irqflag(struct kprobe_ctlblk *kcb,
> +						  struct pt_regs *regs)
>  {
>  	regs->pstate &= ~DAIF_MASK;
>  	regs->pstate |= kcb->saved_irqflag;
>  }
>  
> -static void __kprobes setup_singlestep(struct kprobe *p,
> -				       struct pt_regs *regs,
> -				       struct kprobe_ctlblk *kcb, int reenter)
> +static void noinstr setup_singlestep(struct kprobe *p,
> +				     struct pt_regs *regs,
> +				     struct kprobe_ctlblk *kcb, int reenter)
>  {
>  	unsigned long slot;
>  
> @@ -244,13 +245,15 @@ static void __kprobes setup_singlestep(struct kprobe *p,
>  		instruction_pointer_set(regs, slot);
>  	} else {
>  		/* insn simulation */
> +		instrumentation_begin();
>  		arch_simulate_insn(p, regs);
> +		instrumentation_end();
>  	}
>  }
>  
> -static int __kprobes reenter_kprobe(struct kprobe *p,
> -				    struct pt_regs *regs,
> -				    struct kprobe_ctlblk *kcb)
> +static int noinstr reenter_kprobe(struct kprobe *p,
> +				  struct pt_regs *regs,
> +				  struct kprobe_ctlblk *kcb)
>  {
>  	switch (kcb->kprobe_status) {
>  	case KPROBE_HIT_SSDONE:
> @@ -262,23 +265,29 @@ static int __kprobes reenter_kprobe(struct kprobe *p,
>  		 * recoverable one-level reentry, so handle it in the same way as
>  		 * reentry from KPROBE_HIT_ACTIVE or KPROBE_HIT_SSDONE.
>  		 */
> +		instrumentation_begin();
>  		kprobes_inc_nmissed_count(p);
> +		instrumentation_end();
>  		setup_singlestep(p, regs, kcb, 1);
>  		break;
>  	case KPROBE_REENTER:
> +		instrumentation_begin();
>  		pr_warn("Failed to recover from reentered kprobes.\n");
>  		dump_kprobe(p);
>  		BUG();
> +		instrumentation_end();
>  		break;
>  	default:
> +		instrumentation_begin();
>  		WARN_ON(1);
> +		instrumentation_end();
>  		return 0;
>  	}
>  
>  	return 1;
>  }
>  
> -static void __kprobes
> +static void noinstr
>  post_kprobe_handler(struct kprobe *cur, struct kprobe_ctlblk *kcb, struct pt_regs *regs)
>  {
>  	/* return addr restore if non-branching insn */
> -- 
> 2.47.3
>