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

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Date:   Mon, 29 Apr 2019 12:24:23 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andrew Lutomirski <luto@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Nicolai Stange <nstange@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        "the arch/x86 maintainers" <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>,
        Joerg Roedel <jroedel@...e.de>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        live-patching@...r.kernel.org,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH 3/4] x86/ftrace: make ftrace_int3_handler() not to skip
 fops invocation

On Mon, Apr 29, 2019 at 12:13 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
>
>
> On Mon, Apr 29, 2019, 12:02 Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>>
>>
>>
>> If nmi were to break it, it would be a cpu bug.
>
>
> Side note: we *already* depend on sti shadow working in other parts of the kernel, namely sti->iret.
>

Where?  STI; IRET would be nuts.

Before:

commit 4214a16b02971c60960afd675d03544e109e0d75
Author: Andy Lutomirski <luto@...nel.org>
Date:   Thu Apr 2 17:12:12 2015 -0700

    x86/asm/entry/64/compat: Use SYSRETL to return from compat mode SYSENTER

we did sti; sysxit, but, when we discussed this, I don't recall anyone
speaking up in favor of the safely of the old code.

Not to mention that the crash we'll get if we get an NMI and a
rescheduling interrupt in this path will be very, very hard to debug.