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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 6 Dec 2018 10:15:08 -0800
Message-ID: <CAHk-=wgrpUQyux2ZTJj8FUEdHcEoM0KUNrN3KCc+bBhi0fRhaQ@mail.gmail.com>
Subject: Re: [PATCH] x86/mm/fault: Streamline the fault error_code decoder
 some more
To:     Ingo Molnar <mingo@kernel.org>
Cc:     sean.j.christopherson@intel.com, dave.hansen@linux.intel.com,
        Andrew Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, bp@alien8.de,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Peter Anvin <hpa@zytor.com>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Rik van Riel <riel@surriel.com>, yu-cheng.yu@intel.com
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On Wed, Dec 5, 2018 at 11:34 PM Ingo Molnar <mingo@kernel.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)

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.

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)").

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

           Linus