linux-kernel - Re: [PATCH] x86: efi: avoid BUILD_BUG_ON() for non-constant p4d

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Message-ID: <CAK8P3a39vtF8GvRVQtEYssc+GvX-75j9-4pwXj4qhc7LK2RgNw@mail.gmail.com>
Date:   Fri, 15 Jan 2021 20:54:18 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Arvind Sankar <nivedita@...m.mit.edu>,
        Nathan Chancellor <natechancellor@...il.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        Darren Hart <dvhart@...radead.org>,
        Andy Shevchenko <andy@...radead.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        linux-efi <linux-efi@...r.kernel.org>,
        Platform Driver <platform-driver-x86@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] x86: efi: avoid BUILD_BUG_ON() for non-constant p4d_index

On Fri, Jan 15, 2021 at 8:18 PM Borislav Petkov <bp@...en8.de> wrote:
>
> On Fri, Jan 15, 2021 at 02:11:25PM -0500, Arvind Sankar wrote:
> > That's how build-time assertions work: they are _supposed_ to be
> > optimized away completely when the assertion is true. If they're
> > _not_ optimized away, the build will fail.
>
> Yah, that I know, thanks.
>
> If gcc really inlines p4d_index() and does a lot more aggressive
> optimization to determine that the condition is false and thus optimize
> everything away (and clang doesn't), then that would explain the
> observation.

One difference is that gcc does not have
-fsanitize=unsigned-integer-overflow at all, and I don't see the
assertion without that on clang either, so it's possible that clang
behaves as designed here.

The description is:
    -fsanitize=unsigned-integer-overflow: Unsigned integer overflow, where
     the result of an unsigned integer computation cannot be represented in
     its type. Unlike signed integer overflow, this is not undefined behavior,
     but it is often unintentional. This sanitizer does not check for
lossy implicit
     conversions performed before such a computation (see
    -fsanitize=implicit-conversion).

The "-68 * ((1UL) << 30" computation does overflow an unsigned long
as intended, right? Maybe this is enough for the ubsan code in clang to
just disable some of the optimization steps that the assertion relies on.

        Arnd