Re: [PATCH 3/3] arm64/kprobe: Optimize the performance of patching single-step slot

[Mark Rutland]

On Fri, Sep 23, 2022 at 04:46:58PM +0800, Liao Chang wrote:
> Single-step slot would not be used until kprobe is enabled, that means
> no race condition occurs on it under SMP, hence it is safe to pacth ss
> slot without stopping machine.

I think this is correct, but this depends on a couple of subtleties,
importantly:

* That the I-cache maintenance for these instructions is complete *before* the
  kprobe BRK is written (and aarch64_insn_patch_text_nosync() ensures this, but
  just omits causing a Context-Synchronization-Event on all CPUS).

* That the kprobe BRK results in an exception (and consequently a
  Context-Synchronoization-Event), which ensures that the CPU will fetch the
  single-step slot instructions *after* this, ensuring that the new
  instructions are used.

It would be good if we could call that out explicitly.

Thanks,
Mark.

> Since I and D caches are coherent within single-step slot from
> aarch64_insn_patch_text_nosync(), hence no need to do it again via
> flush_icache_range().
> 
> Acked-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> Signed-off-by: Liao Chang <liaochang1@xxxxxxxxxx>
> ---
>  arch/arm64/kernel/probes/kprobes.c | 7 ++-----
>  1 file changed, 2 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
> index d1d182320245..29b98bc12833 100644
> --- a/arch/arm64/kernel/probes/kprobes.c
> +++ b/arch/arm64/kernel/probes/kprobes.c
> @@ -44,13 +44,10 @@ post_kprobe_handler(struct kprobe *, struct kprobe_ctlblk *, struct pt_regs *);
>  static void __kprobes arch_prepare_ss_slot(struct kprobe *p)
>  {
>  	kprobe_opcode_t *addr = p->ainsn.api.insn;
> -	void *addrs[] = {addr, addr + 1};
> -	u32 insns[] = {p->opcode, BRK64_OPCODE_KPROBES_SS};
>  
>  	/* prepare insn slot */
> -	aarch64_insn_patch_text(addrs, insns, 2);
> -
> -	flush_icache_range((uintptr_t)addr, (uintptr_t)(addr + MAX_INSN_SIZE));
> +	aarch64_insn_patch_text_nosync(addr, p->opcode);
> +	aarch64_insn_patch_text_nosync(addr + 1, BRK64_OPCODE_KPROBES_SS);
>  
>  	/*
>  	 * Needs restoring of return address after stepping xol.
> -- 
> 2.17.1
>