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

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Message-ID: <20250409112839.GA32748@redhat.com>
Date: Wed, 9 Apr 2025 13:28:39 +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 04/08, Jiri Olsa wrote:
>
> --- a/arch/x86/kernel/uprobes.c
> +++ b/arch/x86/kernel/uprobes.c
> @@ -608,6 +608,16 @@ static void riprel_post_xol(struct arch_uprobe *auprobe, struct pt_regs *regs)
>  		*sr = utask->autask.saved_scratch_register;
>  	}
>  }
> +
> +static int is_nop5_insn(uprobe_opcode_t *insn)
> +{
> +	return !memcmp(insn, x86_nops[5], 5);
> +}
> +
> +static bool emulate_nop5_insn(struct arch_uprobe *auprobe)
> +{
> +	return is_nop5_insn((uprobe_opcode_t *) &auprobe->insn);
> +}

Why do we need 2 functions? Can't branch_setup_xol_ops() just use
is_nop5_insn(insn->kaddr) ?

>  #else /* 32-bit: */
>  /*
>   * No RIP-relative addressing on 32-bit
> @@ -621,6 +631,10 @@ 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;
> +}

Hmm, why? I mean, why we can't emulate x86_nops[5] if !CONFIG_X86_64 ?

OTOH. What if the kernel is 64-bit, but the probed task is 32-bit and it
uses the 64-bit version of BYTES_NOP5?

Perhaps this is fine, I simply don't know, so let me ask...

> @@ -852,6 +866,8 @@ static int branch_setup_xol_ops(struct arch_uprobe *auprobe, struct insn *insn)
>  		break;
>
>  	case 0x0f:
> +		if (emulate_nop5_insn(auprobe))
> +			goto setup;

I think this will work, but if we want to emulate nop5, then perhaps
we can do the same for other nops?

For the moment, lets forget about compat tasks on a 64-bit kernel, can't
we simply do something like below?

Oleg.
---

diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
index 9194695662b2..76d2cceca6c4 100644
--- 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;
 
 	case 0xe8:	/* call relative */
 		branch_clear_offset(auprobe, insn);