linux-kernel - Re: [PATCH -tip] kprobes/x86: Remove unneeded casting from immediate value

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Message-ID: <b79052ee-07d2-a6b9-03cb-b33f61985fe2@gmail.com>
Date:   Fri, 10 Feb 2023 02:01:54 +0200
From:   Nadav Amit <nadav.amit@...il.com>
To:     "Masami Hiramatsu (Google)" <mhiramat@...nel.org>, x86@...nel.org
Cc:     Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH -tip] kprobes/x86: Remove unneeded casting from immediate
 value


On 2/9/23 5:36 PM, Masami Hiramatsu (Google) wrote:
> From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
>
> Remove unneeded casting from immediate value assignments for relative
> jump offset. Since the immediate values in the 'insn' data structure are
> assigned from immediate bytes as a signed value to insn.immediate.value
> by insn_field_set(). Thus, if we need to access that value as a signed
> value (in this kprobe's case), we don't need to cast it.
> This is a kind of clean up (should not change behavior) follows Nadav's
> bugfix.
>
> Link: https://lore.kernel.org/all/20230208071708.4048-1-namit@vmware.com/
>
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> ---
>   arch/x86/kernel/kprobes/core.c |   16 ++++++++--------
>   1 file changed, 8 insertions(+), 8 deletions(-)
>
> diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
> index 695873c0f50b..2440e736d0e4 100644
> --- a/arch/x86/kernel/kprobes/core.c
> +++ b/arch/x86/kernel/kprobes/core.c
> @@ -607,19 +607,19 @@ static int prepare_emulation(struct kprobe *p, struct insn *insn)
>   	case 0xe8:	/* near call relative */
>   		p->ainsn.emulate_op = kprobe_emulate_call;
>   		if (insn->immediate.nbytes == 2)
> -			p->ainsn.rel32 = *(s16 *)&insn->immediate.value;
> +			p->ainsn.rel32 = insn->immediate.value;
>   		else
> -			p->ainsn.rel32 = *(s32 *)&insn->immediate.value;
> +			p->ainsn.rel32 = insn->immediate.value;

Hmm.. I don't get it. What the purpose of keeping the duplicated code 
(after your change)?