Re: [PATCHv2 perf/core 1/2] uprobes/x86: Add support to emulate nop instructions

[Andrii Nakryiko]

On Fri, Apr 11, 2025 at 5:18 AM Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
>
> Adding support to emulate all nop instructions as the original uprobe
> instruction.
>
> This change speeds up uprobe on top of all nop instructions and is a
> preparation for usdt probe optimization, that will be done on top of
> nop5 instruction.
>
> With this change the usdt probe on top of nop5 won't take the performance
> hit compared to usdt probe on top of standard nop instruction.
>
> Suggested-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> Suggested-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> ---
> v2 changes:
> - follow Adndrii/Oleg's suggestion and emulate all the nops
>
>  arch/x86/kernel/uprobes.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
> index 9194695662b2..262960189a1c 100644
> --- a/arch/x86/kernel/uprobes.c
> +++ b/arch/x86/kernel/uprobes.c
> @@ -840,6 +840,12 @@ static int branch_setup_xol_ops(struct arch_uprobe *auprobe, struct insn *insn)
>         insn_byte_t p;
>         int i;
>
> +       /* x86_nops[i]; same as jmp with .offs = 0 */
> +       for (i = 1; i <= ASM_NOP_MAX; ++i) {

i <= ASM_NOP_MAX && i <= insn->length

?

otherwise what prevents us from reading past the actual instruction bytes?


or, actually, shouldn't we just check memcmp(x86_nops[insn->length])
if insn->length < ASM_NOP_MAX ?


> +               if (!memcmp(insn->kaddr, x86_nops[i], i))
> +                       goto setup;
> +       }
> +
>         switch (opc1) {
>         case 0xeb:      /* jmp 8 */
>         case 0xe9:      /* jmp 32 */
> --
> 2.49.0
>