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

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Message-ID: <CAHk-=wi6A9tgw=kkPh5Ywqt687VvsVEjYXVkAnq0jpt0u0tk6g@mail.gmail.com>
Date:   Thu, 2 May 2019 11:43:37 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     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>,
        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>,
        "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 2, 2019 at 11:18 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> We could fix this by not using the common exit path on int3; not sure we
> want to go there, but that is an option.

I don't think it's an option in general, because *some* int3
invocations will need all the usual error return.

But I guess we could make "int3 from kernel space" special.

I'm not sure how much that would help, but it might be worth looking into.

> ARGH; I knew it was too pretty :/ Yes, something like what you suggest
> will be needed, I'll go look at that once my brain recovers a bit from
> staring at entry code all day.

Looks like it works based on your other email.

What would it look like with the "int3-from-kernel is special" modification?

Because *if* we can make the "kernel int3" entirely special, that
would make the "Eww factor" much less of this whole thing.

I forget: is #BP _only_ for the "int3" instruction? I know we have
really nasty cases with #DB (int1) because of "pending exceptions
happen on the first instruction in kernel space", and that makes it
really really nasty to handle with all the stack switch and %cr3
handling etc.

But if "int3 from kernel space" _only_ happens on actual "int3"
instructions, then we really could just special-case that case. We'd
know that %cr3 has been switched, we'd know that we don't need to do
fsgs switching, we'd know we already have a good stack and percpu data
etc set up.

So then special casing #BP would actually allow us to have a simple
and straightforward kernel-int3-only sequence?

And then having that odd stack setup special case would be *much* more
palatable to me.

               Linus