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Date:   Thu, 25 Mar 2021 07:56:54 +0900
From:   Masami Hiramatsu <mhiramat@kernel.org>
To:     Colin Ian King <colin.king@canonical.com>
Cc:     Muhammad Usama Anjum <musamaanjum@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        linux-kernel@vger.kernel.org, kernel-janitors@vger.kernel.org,
        dan.carpenter@oracle.com
Subject: Re: [PATCH] x86/kprobes: Remove dead code
Message-Id: <20210325075654.98e4a89c1ce5d25f47b87b78@kernel.org>
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        <5fcd5b04-0fab-3672-c5d2-6e8f73f93bbc@canonical.com>
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On Wed, 24 Mar 2021 17:50:16 +0000
Colin Ian King <colin.king@canonical.com> wrote:

> 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@gmail.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.

Right, it must be moved under the case 0xf0. Something like this.

case 0xf0:
	if (opcde == 0xff) {
		/* indirect jmp is boostable */
		return X86_MODRM_REG(insn->modrm.bytes[0]) == 4;
	}

Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>