linux-kernel - Re: [PATCH] x86/mm/fault: Streamline the fault error

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Message-ID: <CALCETrUc7Y+WFkS1=ECXgtLoUXb3HPXsF+x+TCpiuuh3sXFjfA@mail.gmail.com>
Date:   Thu, 6 Dec 2018 11:06:50 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        "Christopherson, Sean J" <sean.j.christopherson@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Rik van Riel <riel@...riel.com>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: [PATCH] x86/mm/fault: Streamline the fault error_code decoder
 some more

On Thu, Dec 6, 2018 at 10:15 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Wed, Dec 5, 2018 at 11:34 PM Ingo Molnar <mingo@...nel.org> wrote:
> >
> > Yeah, so I don't like the overly long 'SUPERVISOR' and the somewhat
> > inconsistent, sporadic handling of negatives. Here's our error code bits:
> >
> > /*
> >  * Page fault error code bits:
> >  *
> >  *   bit 0 ==    0: no page found       1: protection fault
> >  *   bit 1 ==    0: read access         1: write access
> >  *   bit 2 ==    0: kernel-mode access  1: user-mode access
>
> No. Really not at all.
>
> Bit 2 is *not* "kernel vs user".  Never has been. Never will be.
>
> It's a single bit that mixes up *three* different cases:
>
>  - regular user mode access (value: 1)
>
>  - regular CPL0 access (value: 0)
>
>  - CPU system access (value: 0)
>
> and that third case really is important and relevant. And importantly,
> it can happen from user space.
>
> In fact, these days we possibly have a fourth case:
>
>  - kernel access using wruss (value: 1)

Indeed.

>
> and I'd rather see just the numbers (which you have to know to decode)
> than see the simplified AND WRONG decoding of those numbers.

That's why the very next line in the OOPS explains this.

>
> Please don't ever confuse the fault U/S bit with "user vs kernel".
> It's just not true, and people should be very very aware of it now
> being true.
>
> If you care whether a page fault happened in user mode or not, you
> have to look at the register state (ie "user_mode(regs)").

The code we're arguing over was part of a patch set to fix this
confusion in the page fault code for real.

>
> Please call the U/S bit something else than "user" or "kernel".

Dunno.  I kind of like following the SDM.

How do you like the attached patch?

View attachment "fault.patch" of type "text/x-patch" (2498 bytes)