Re: [RFC PATCH 6/9] arm64/kprobes: Make kprobe_fault_handler() noinstr

[Mark Rutland]

On Mon, Jul 27, 2026 at 12:25:43PM +0000, Hongyan Xia wrote:
> From: Hongyan Xia <hongyan.xia@xxxxxxxxxxxxx>
> 
> kprobe_fault_handler() is reached from do_page_fault() when a fault
> happens while a kprobe single-step is armed. Everything it calls (the
> save/restore helpers, kprobes_restore_local_irqflag(),
> reset_current_kprobe(), register access) is noinstr after the previous
> patch, and being called from an instrumentable caller into a noinstr
> callee is fine.

I think it's fine to mark this as noinstr.

I don't think we need kprobe_fault_handler() in the first place given we
don't permit kprobes to be placed on instructions with an extable entry.
Any instruction which can be probled cannot legitimately cause a fault.

That and kprobes_restore_local_irqflag() can't do the right thing w.r.t
DAIF, since it only fiddles with the value in the pt_regs, and doesn't
touch the live inherited DAIF value. If we need to fiddle with the live
DAIF value, we must do that much earlier in the entry-common code.

Mark.

> Signed-off-by: Hongyan Xia <hongyan.xia@xxxxxxxxxxxxx>
> ---
>  arch/arm64/kernel/probes/kprobes.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
> index e9fa66fa4217..4172998d48d9 100644
> --- a/arch/arm64/kernel/probes/kprobes.c
> +++ b/arch/arm64/kernel/probes/kprobes.c
> @@ -310,7 +310,7 @@ post_kprobe_handler(struct kprobe *cur, struct kprobe_ctlblk *kcb, struct pt_reg
>  	reset_current_kprobe();
>  }
>  
> -int __kprobes kprobe_fault_handler(struct pt_regs *regs, unsigned int fsr)
> +int noinstr kprobe_fault_handler(struct pt_regs *regs, unsigned int fsr)
>  {
>  	struct kprobe *cur = kprobe_running();
>  	struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();
> -- 
> 2.47.3
>