linux-kernel - Re: [PATCH] x86/kprobes: Use switch-case for 0xFF opcodes in prepare

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Date:   Wed, 30 Nov 2022 22:38:14 +0900
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     Chuang Wang <nashuiliang@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, Mark Brown <broonie@...nel.org>,
        Rob Herring <robh@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alex Deucher <alexander.deucher@....com>,
        "Masami Hiramatsu (Google)" <mhiramat@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/kprobes: Use switch-case for 0xFF opcodes in
 prepare_emulation

On Tue, 29 Nov 2022 16:39:51 +0800
Chuang Wang <nashuiliang@...il.com> wrote:

> For the `FF /digit` opcodes in prepare_emulation, use switch-case
> instead of hand-written code to make the logic easier to understand.
> 
> Signed-off-by: Chuang Wang <nashuiliang@...il.com>

This looks good to me.

Acked-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>

Thank you!

> ---
>  arch/x86/kernel/kprobes/core.c | 20 +++++++++++---------
>  1 file changed, 11 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
> index 66299682b6b7..66ec32b6cd0e 100644
> --- a/arch/x86/kernel/kprobes/core.c
> +++ b/arch/x86/kernel/kprobes/core.c
> @@ -655,17 +655,19 @@ static int prepare_emulation(struct kprobe *p, struct insn *insn)
>  		 * is determined by the MOD/RM byte.
>  		 */
>  		opcode = insn->modrm.bytes[0];
> -		if ((opcode & 0x30) == 0x10) {
> -			if ((opcode & 0x8) == 0x8)
> -				return -EOPNOTSUPP;	/* far call */
> -			/* call absolute, indirect */
> +		switch (X86_MODRM_REG(opcode)) {
> +		case 0b010:	/* FF /2, call near, absolute indirect */
>  			p->ainsn.emulate_op = kprobe_emulate_call_indirect;
> -		} else if ((opcode & 0x30) == 0x20) {
> -			if ((opcode & 0x8) == 0x8)
> -				return -EOPNOTSUPP;	/* far jmp */
> -			/* jmp near absolute indirect */
> +			break;
> +		case 0b100:	/* FF /4, jmp near, absolute indirect */
>  			p->ainsn.emulate_op = kprobe_emulate_jmp_indirect;
> -		} else
> +			break;
> +		case 0b011:	/* FF /3, call far, absolute indirect */
> +		case 0b101:	/* FF /5, jmp far, absolute indirect */
> +			return -EOPNOTSUPP;
> +		}
> +
> +		if (!p->ainsn.emulate_op)
>  			break;
>  
>  		if (insn->addr_bytes != sizeof(unsigned long))
> -- 
> 2.37.2
> 


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>