linux-kernel - Re: [PATCH 2/4] x86/kprobes: Fix frame pointer annotations

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Date:   Fri, 10 May 2019 14:31:31 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>, linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Nicolai Stange <nstange@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Petr Mladek <pmladek@...e.com>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Shuah Khan <shuah@...nel.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Juergen Gross <jgross@...e.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nayna Jain <nayna@...ux.ibm.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Joerg Roedel <jroedel@...e.de>, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 2/4] x86/kprobes: Fix frame pointer annotations

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@...radead.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_ ?

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.

> 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.