linux-kernel - Re: [PATCH 3/4] x86/ftrace: make ftrace_int3

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Message-ID: <20190427102657.GF2623@hirez.programming.kicks-ass.net>
Date:   Sat, 27 Apr 2019 12:26:57 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nicolai Stange <nstange@...e.de>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        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>,
        Andy Lutomirski <luto@...nel.org>,
        Joerg Roedel <jroedel@...e.de>, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 3/4] x86/ftrace: make ftrace_int3_handler() not to skip
 fops invocation

On Sat, Apr 27, 2019 at 12:06:38PM +0200, Nicolai Stange wrote:
> ftrace_int3_handler()'s context is different from the interrupted call
> instruction's one, obviously. In order to be able to emulate the call
> within the original context, make ftrace_int3_handler() set its iret
> frame's ->ip to some helper stub. Upon return from the trap, this stub will
> then mimic the call by pushing the the return address onto the stack and
> issuing a jmp to the target address. As describe above, the jmp target
> will be either of ftrace_ops_list_func() or ftrace_regs_caller(). Provide
> one such stub implementation for each of the two cases.

Yuck; I'd much rather we get that static_call() stuff sorted such that
text_poke() and poke_int3_handler() can do CALL emulation.

Given all the back and forth, I think the solution where we shift
pt_regs a bit to allow the emulated PUSH is a viable solution; eg. I
think we collectively hated it least.