linux-kernel - Re: [RFC][PATCH 1/2] x86: Allow breakpoints to emulate call functions

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Date:   Wed, 8 May 2019 12:37:48 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...capital.net>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.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>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        "the arch/x86 maintainers" <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>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>, stable <stable@...r.kernel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [RFC][PATCH 1/2] x86: Allow breakpoints to emulate call
 functions

On Tue, 7 May 2019 21:50:52 -0700
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> > It's been a bane of mine for some time.  
> 
> Guys, I have basically a one-liner patch for your hangups.
> 
> It's called "rename 'sp' to 'user_sp' on x86-32".
> 
> Then we make the 'sp' field on x86-64 be a union, so that you can call
> it user_sp or sp as you wish.
> 
> Yeah, it's really more than one line, because obviously the users will
> need chaning, but honestly, that would be a _real_ cleanup. Make the
> register match what it actually is.

But is it? Sure, it will be a reminder that it's different for x86-32,
but that still doesn't take away the fact that pt_regs on x86_32 is an
anomaly! Where else do we have part of a data structure that can't be
read because it can possibly fault? If regs is a valid pointer, one
would think that simply reading regs->sp (or regs->user_sp) would be no
more dangerous than reading regs->ip.

The difference between entry_32.S from entry_64.S causes it to spill
into C code, making the x86_64 code more difficult to deal with. Sure,
10 to 15 years ago, all your arguments would make sense. But today, who
uses x86_32?  Perhaps you may use it in a VM, but I asked a few
developers when was the last time they used one, they told me 5 to 7
years ago. I only boot x86_32 to test to make sure I didn't break it.

Yes, your diffstat is really nice to the changes to entry_32.S, but at
what cost? To make the x86_64 code more complex? That whole returning
the regs in the int3 handler makes no sense on x86_64, but yet we would
need to do it to handle x86_32. Why burden the architecture of today
and tomorrow with the architecture of yesterday? x86_32 is becoming
more obsolete by the day. It baffles me why we wouldn't want to contain
its complexity in a single file then to spread it out like wildfire
across the generic x86 code.

The &regs->sp hack is just one more rung in the complex learning curve
ladder of becoming a Linux kernel developer.

-- Steve