linux-kernel - Re: [PATCH v2 1/3] kprobes/x86: use instruction_pointer and instruction_pointer

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Message-ID: <20190820132110.GP2332@hirez.programming.kicks-ass.net>
Date:   Tue, 20 Aug 2019 15:21:10 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jisheng Zhang <Jisheng.Zhang@...aptics.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Jonathan Corbet <corbet@....net>,
        Will Deacon <will@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        "David S. Miller" <davem@...emloft.net>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2 1/3] kprobes/x86: use instruction_pointer and
 instruction_pointer_set

On Tue, Aug 20, 2019 at 09:02:59AM +0000, Jisheng Zhang wrote:
> In v2, actually, the arm64 version's kprobe_ftrace_handler() is the same
> as x86's, the only difference is comment, e.g
> 
> /* Kprobe handler expects regs->ip = ip + 1 as breakpoint hit */
> 
> while in arm64
> 
> /* Kprobe handler expects regs->pc = ip + 1 as breakpoint hit */

What's weird; I thought ARM has fixed sized instructions and they are
all 4 bytes? So how does a single byte offset make sense for ARM?