linux-kernel - Re: [PATCH] x86/kprobes: Fix 1 byte conditional jump target

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Date:   Mon, 6 Feb 2023 10:42:31 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Nadav Amit <nadav.amit@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Nadav Amit <namit@...are.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] x86/kprobes: Fix 1 byte conditional jump target

On 2/4/23 13:08, Nadav Amit wrote:
> --- a/arch/x86/kernel/kprobes/core.c
> +++ b/arch/x86/kernel/kprobes/core.c
> @@ -625,7 +625,7 @@ static int prepare_emulation(struct kprobe *p, struct insn *insn)
>  		/* 1 byte conditional jump */
>  		p->ainsn.emulate_op = kprobe_emulate_jcc;
>  		p->ainsn.jcc.type = opcode & 0xf;
> -		p->ainsn.rel32 = *(char *)insn->immediate.bytes;
> +		p->ainsn.rel32 = *(s8 *)&insn->immediate.value;
>  		break;

This new code is at least consistent with what the other code in that
function does with 1-byte immediates.  But, I'm curious what the point
is about going through the 's8' type.

What's wrong with:

	p->ainsn.rel32 = insn->immediate.value;

?  Am I missing something subtle?