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Date:   Sat, 11 May 2019 09:52:54 +0900
From:   Masami Hiramatsu <mhiramat@kernel.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Josh Poimboeuf <jpoimboe@redhat.com>, linux-kernel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.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>,
        Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        Jiri Kosina <jikos@kernel.org>,
        Miroslav Benes <mbenes@suse.cz>,
        Petr Mladek <pmladek@suse.com>,
        Joe Lawrence <joe.lawrence@redhat.com>,
        Shuah Khan <shuah@kernel.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Mimi Zohar <zohar@linux.ibm.com>,
        Juergen Gross <jgross@suse.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Nayna Jain <nayna@linux.ibm.com>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        Joerg Roedel <jroedel@suse.de>, linux-kselftest@vger.kernel.org
Subject: Re: [PATCH 2/4] x86/kprobes: Fix frame pointer annotations
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On Fri, 10 May 2019 14:31:31 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> On Fri, May 10, 2019 at 01:58:31PM +0900, Masami Hiramatsu wrote:
> > On Thu, 9 May 2019 19:14:16 +0200
> > Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > > Ideally also the optimized kprobe trampoline, but I've not managed to
> > > fully comprehend that one.
> > 
> > As you pointed in other reply, save/restore can be a macro, but
> > each trampoline code is slightly different. Optprobe template has
> > below parts
> > 
> > (jumped from probed address)
> > [store regs]
> > [setup function arguments (pt_regs and probed address)]
> > [handler call]
> > [restore regs]
> > [execute copied instruction]
> 
>  instruction_s_ ?

Yes.

> 
> The JMP to this trampoline is likely 5 bytes and could have clobbered
> multiple instructions, we'd then have to place them all here, and
> 
> > [jump back to probed address]
> 
> jump to after whatever instructions were clobbered by the JMP.

Right!

> > Note that there is a limitation that if it is optiomized probe, user
> > handler can not change regs->ip. (we can not use "ret" after executed
> > a copied instruction, which must run on same stack)
> 
> Changing regs->ip in this case is going to be massively dodgy indeed :-)
> But so would changing much else; changing stack layout would also be
> somewhat tricky.

Yes, so the stack must be same after [restore regs].

Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>