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Date:   Tue, 7 May 2019 14:41:31 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        Nicolai Stange <nstange@suse.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        the arch/x86 maintainers <x86@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Jiri Kosina <jikos@kernel.org>,
        Miroslav Benes <mbenes@suse.cz>,
        Petr Mladek <pmladek@suse.com>,
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        Shuah Khan <shuah@kernel.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
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        Mimi Zohar <zohar@linux.ibm.com>,
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        <linux-kselftest@vger.kernel.org>, stable <stable@vger.kernel.org>,
        Masami Hiramatsu <mhiramat@kernel.org>
Subject: Re: [RFC][PATCH 1/2] x86: Allow breakpoints to emulate call functions
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On Tue, May 07, 2019 at 08:27:16AM -0400, Steven Rostedt wrote:
> On Tue, 7 May 2019 11:27:31 +0200
> Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > FWIW, both these trampolines assume a kprobe will not
> > int3_emulate_{push/call}(), for both bitnesses.
> > 
> > But then; I'm thinking kprobes should be inspection only and not modify
> > things. So that might just be good enough.
> 
> I believe there are kprobe calls that do modify things. Note, they can
> modify regs->ip. 

The kprobe pre_handler as used by kretprobes does, and that is indeed
handled by the trampolines.

> Kprobes sets the FTRACE_OPS_FL_IPMODIFY flag, thus
> they can never be put at the same location that is being live patched.

OK, so do we want to allow kprobes that also modify regs->sp ? Because
then we need to change these trampolines a bit.

I'd prefer not to allow kprobes this.