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

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Message-ID: <20250409114950.GB32748@redhat.com>
Date: Wed, 9 Apr 2025 13:49:51 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Jiri Olsa <jolsa@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>,
	Andrii Nakryiko <andrii@...nel.org>, bpf@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
	x86@...nel.org, Song Liu <songliubraving@...com>,
	Yonghong Song <yhs@...com>,
	John Fastabend <john.fastabend@...il.com>,
	Hao Luo <haoluo@...gle.com>, Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Alan Maguire <alan.maguire@...cle.com>
Subject: Re: [PATCH 1/2] uprobes/x86: Add support to emulate nop5 instruction

On 04/09, Oleg Nesterov wrote:
>
> For the moment, lets forget about compat tasks on a 64-bit kernel, can't
> we simply do something like 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;
>  
> +	/* prefix* + nop[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;
> +	}
> +
>  	switch (opc1) {
>  	case 0xeb:	/* jmp 8 */
>  	case 0xe9:	/* jmp 32 */
>  		break;
> -	case 0x90:	/* prefix* + nop; same as jmp with .offs = 0 */
> -		goto setup;

OK, I guess we can't remove this "case 0x90" because of prefixes, please
ignore this part.

Oleg.