linux-kernel - Re: [REGRESSION 5.8] x86/entry: DR0 break-on-write not working

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Date:   Thu, 20 Aug 2020 10:35:39 +0200
From:   peterz@...radead.org
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Andy Lutomirski <luto@...nel.org>, Kyle Huey <me@...ehuey.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Robert O'Callahan <rocallahan@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Brian Gerst <brgerst@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [REGRESSION 5.8] x86/entry: DR0 break-on-write not working

On Wed, Aug 19, 2020 at 10:46:36PM -0500, Josh Poimboeuf wrote:
> On Wed, Aug 19, 2020 at 05:14:18PM -0700, Andy Lutomirski wrote:

> > I'm pretty sure you have the buggy sequence of events right, but for
> > the wrong reason.  There's nothing wrong with scheduling when
> > delivering SIGTRAP, but it's definitely wrong to blindly save and
> > restore DR7 around scheduling and around ptrace invocations.  Remember
> > this is an entry from user mode, so it runs on the user stack.

*groan*, yeah that's broken :/

> Wow, I had no idea user #DB's run on the task stack.  The scheduling
> from #DB blew my mind :-)  What's the purpose of that?

At the very least that vm86 junk needs to I think. Also -RT needs it to
send signals.