Re: [PATCH 1/2] uprobes/x86: Add support to emulate nop5 instruction

[Oleg Nesterov]

On 04/09, Jiri Olsa wrote:
>
> > Just it looks a bit strange to me. Even if we do not have a use-case
> > for other nops, why we can't emulate them all just for consistency?
>
> we can, I went with nop5 just for simplicity, if you think
> having all nops support is better, let's do that

Well... Let me repeat, I am not really arguing and I do not want to delay
your next changes. We can always cleanup this code later. Please see below.

> I checked and compact process executes 64bit nops just fine,
> so we should be ok there

OK. Then, for your original patch:

Acked-by: Oleg Nesterov <oleg@xxxxxxxxxx>

I'd only ask to define is_nop5_insn/emulate_nop5_insn regardless of
CONFIG_X86_64.  I understand that we have no reason to emulate nop5
on the 32-bit kernel, but at the same time I don't see any reason to
complicate this code to explicitly "nack" nop5 in this case.

As for the new version below:

> --- a/arch/x86/kernel/uprobes.c
> +++ b/arch/x86/kernel/uprobes.c
> @@ -840,12 +840,16 @@ 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) {
> +		if (!memcmp(insn->kaddr, x86_nops[i], i))
> +			goto setup;
> +	}

Well, yes, I'd personally obviously prefer this version ;) Just because
it looks a bit more clear/consistent to me. But this is subjective.

And,

> -	case 0x90:	/* prefix* + nop; same as jmp with .offs = 0 */
> -		goto setup;

No, this is wrong. Please see my reply to myself,
https://lore.kernel.org/all/20250409114950.GB32748@xxxxxxxxxx/

This way we can no longer emulate, say, "rep; nop". Exactly because
either way memcmp(x86_nops[i]) checks the whole instruction.

Probably we don't really care, but still this patch shouldn't add any
"regression".

So, let me repeat. Up to you. Whatever you prefer. I just tried to
understand your patch.

You have my ACK in any case.

Oleg.