linux-kernel - Re: [PATCH 03/14] x86/entry/64: Fix unwind hints in register clearing code

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Date:   Thu, 12 Mar 2020 14:57:14 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Vince Weaver <vincent.weaver@...ne.edu>,
        Dave Jones <dsj@...com>, Jann Horn <jannh@...gle.com>,
        Miroslav Benes <mbenes@...e.cz>,
        Andy Lutomirski <luto@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 03/14] x86/entry/64: Fix unwind hints in register
 clearing code

On Thu, Mar 12, 2020 at 12:29:29PM -0700, Andy Lutomirski wrote:
> > On Mar 12, 2020, at 10:31 AM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> > 
> > The PUSH_AND_CLEAR_REGS macro zeroes each register immediately after
> > pushing it.  If an NMI or exception hits after a register is cleared,
> > but before the UNWIND_HINT_REGS annotation, the ORC unwinder will
> > wrongly think the previous value of the register was zero.  This can
> > confuse the unwinding process and cause it to exit early.
> > 
> > Because ORC is simpler than DWARF, there are a limited number of unwind
> > annotation states, so it's not possible to add an individual unwind hint
> > after each push/clear combination.  Instead, the register clearing
> > instructions need to be consolidated and moved to after the
> > UNWIND_HINT_REGS annotation.
> 
> I don’t suppose you know how bad t he performance hit is on a non-PTI machine?

Hm, what does it have to do with PTI?  Should I run a syscall
microbenchmark?

-- 
Josh