linux-kernel - Re: [PATCH] x86/kprobes: Remove dead code

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Message-ID: <5fcd5b04-0fab-3672-c5d2-6e8f73f93bbc@canonical.com>
Date:   Wed, 24 Mar 2021 17:50:16 +0000
From:   Colin Ian King <colin.king@...onical.com>
To:     Muhammad Usama Anjum <musamaanjum@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org,
        dan.carpenter@...cle.com
Subject: Re: [PATCH] x86/kprobes: Remove dead code

On 24/03/2021 17:36, Muhammad Usama Anjum wrote:
> The condition in switch statement `opcode & 0xf0` cannot evaluate to
> 0xff. So this case statement will never execute. Remove it.
> 
> Fixes: 6256e668b7 ("x86/kprobes: Use int3 instead of debug trap for single-step")
> Signed-off-by: Muhammad Usama Anjum <musamaanjum@...il.com>
> ---
>  arch/x86/kernel/kprobes/core.c | 3 ---
>  1 file changed, 3 deletions(-)
> 
> diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
> index 89d9f26785c7..3b7bcc077020 100644
> --- a/arch/x86/kernel/kprobes/core.c
> +++ b/arch/x86/kernel/kprobes/core.c
> @@ -177,9 +177,6 @@ int can_boost(struct insn *insn, void *addr)
>  	case 0xf0:
>  		/* clear and set flags are boostable */
>  		return (opcode == 0xf5 || (0xf7 < opcode && opcode < 0xfe));
> -	case 0xff:
> -		/* indirect jmp is boostable */
> -		return X86_MODRM_REG(insn->modrm.bytes[0]) == 4;
>  	default:
>  		/* CS override prefix and call are not boostable */
>  		return (opcode != 0x2e && opcode != 0x9a);
> 

The 0xff case was added with some form of intention to be executed so I
suspect removing it is not an appropriate fix.