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

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Date:   Mon, 6 May 2019 10:14:28 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        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>
Subject: Re: [RFC][PATCH 1/2] x86: Allow breakpoints to emulate call functions

On Thu, May 02, 2019 at 11:02:40AM -0700, Linus Torvalds wrote:
> On Thu, May 2, 2019 at 9:21 AM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > TL;DR, on x86_32 kernel->kernel IRET frames are only 3 entries and do
> > not include ESP/SS, so not only wasn't regs->sp setup, if you changed it
> > it wouldn't be effective and corrupt random stack state.
> 
> Indeed, the 32-bit case for same-RPL exceptions/iret is entirely
> different, and I'd forgotten about that.
> 
> And honestly, this makes the 32-bit case much worse. Now the entry
> stack modifications of int3 suddenly affect not just the entry, but
> every exit too.
> 
> This is _exactly_ the kind of subtle kernel entry/exit code I wanted
> us to avoid.

I actually love this patch (absent the bugs).  This is already something
that has been sorely needed for years.

The "struct pt_regs is incomplete on x86-32" thing is a monstrosity
which has long been a source of confusion and bugs.  Sure, this patch
adds some complexity to the entry code, but on the other hand it
actually makes it possible to use pt_regs sanely: regs->sp is no longer
uninitialized.  So a class of (very non-obvious) bugs is eliminated.

I don't think it would make sense to make this change for int3 only,
because the benefits are global.

-- 
Josh