Re: [PATCH RFCv3 10/23] uprobes/x86: Add support to emulate nop5 instruction

[Andrii Nakryiko]

On Thu, Mar 20, 2025 at 4:43 AM Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
>
> Adding support to emulate nop5 as the original uprobe instruction.
>
> Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> ---
>  arch/x86/kernel/uprobes.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
>

This optimization is independent from the sys_uprobe, right? Maybe
send it as a stand-alone patch and let's land it sooner?
Also, how hard would it be to do the same for other nopX instructions?


> diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
> index 5ee2cce4c63e..1661e0ab2a3d 100644
> --- a/arch/x86/kernel/uprobes.c
> +++ b/arch/x86/kernel/uprobes.c
> @@ -308,6 +308,11 @@ static int uprobe_init_insn(struct arch_uprobe *auprobe, struct insn *insn, bool
>         return -ENOTSUPP;
>  }
>
> +static int is_nop5_insn(uprobe_opcode_t *insn)
> +{
> +       return !memcmp(insn, x86_nops[5], 5);
> +}
> +
>  #ifdef CONFIG_X86_64
>
>  asm (
> @@ -865,6 +870,11 @@ void arch_uprobe_clear_state(struct mm_struct *mm)
>         hlist_for_each_entry_safe(tramp, n, &state->head_tramps, node)
>                 destroy_uprobe_trampoline(tramp);
>  }
> +
> +static bool emulate_nop5_insn(struct arch_uprobe *auprobe)
> +{
> +       return is_nop5_insn((uprobe_opcode_t *) &auprobe->insn);
> +}
>  #else /* 32-bit: */
>  /*
>   * No RIP-relative addressing on 32-bit
> @@ -878,6 +888,10 @@ static void riprel_pre_xol(struct arch_uprobe *auprobe, struct pt_regs *regs)
>  static void riprel_post_xol(struct arch_uprobe *auprobe, struct pt_regs *regs)
>  {
>  }
> +static bool emulate_nop5_insn(struct arch_uprobe *auprobe)
> +{
> +       return false;
> +}
>  #endif /* CONFIG_X86_64 */
>
>  struct uprobe_xol_ops {
> @@ -1109,6 +1123,8 @@ static int branch_setup_xol_ops(struct arch_uprobe *auprobe, struct insn *insn)
>                 break;
>
>         case 0x0f:
> +               if (emulate_nop5_insn(auprobe))
> +                       goto setup;
>                 if (insn->opcode.nbytes != 2)
>                         return -ENOSYS;
>                 /*
> --
> 2.49.0
>