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Date:   Tue, 2 Mar 2021 09:41:31 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Masami Hiramatsu <mhiramat@kernel.org>
Cc:     Andy Lutomirski <luto@kernel.org>, Ingo Molnar <mingo@kernel.org>,
        Oleg Nesterov <oleg@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>,
        "David S . Miller" <davem@davemloft.net>, X86 ML <x86@kernel.org>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [RFC PATCH 1/1] x86/kprobes: Use int3 instead of debug trap for
 single-step
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References: <161460768474.430263.18425867006584111900.stgit@devnote2>
 <161460769556.430263.12936080446789384938.stgit@devnote2>
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On Mon, Mar 01, 2021 at 11:08:15PM +0900, Masami Hiramatsu wrote:
> Use int3 instead of debug trap exception for single-stepping the
> probed instructions. Some instructions which change the ip
> registers or modify IF flags are emulated because those are not
> able to be single-stepped by int3 or may allow the interrupt
> while single-stepping.
> 
> This actually changes the kprobes behavior.
> 
> - kprobes can not probe following instructions; int3, iret,
>   far jmp/call which get absolute address as immediate,
>   indirect far jmp/call, indirect near jmp/call with addressing
>   by memory (register-based indirect jmp/call are OK), and
>   vmcall/vmlaunch/vmresume/vmxoff.
> 
> - If the kprobe post_handler doesn't set before registering,
>   it may not be called in some case even if you set it afterwards.
>   (IOW, kprobe booster is enabled at registration, user can not
>    change it)
> 
> But both are rare issue, unsupported instructions will not be
> used in the kernel (or rarely used), and post_handlers are
> rarely used (I don't see it except for the test code).
> 
> Suggested-by: Andy Lutomirski <luto@kernel.org>
> Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>

Very nice!

Aside of a few nits:

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>