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

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Date:   Fri, 15 Jan 2021 15:12:25 -0500
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Borislav Petkov <bp@...en8.de>,
        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 08:54:18PM +0100, Arnd Bergmann wrote:
> 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

That does seem to be an overflow, but that happens at compile-time.
Maybe
	AC(-68, UL) << 30
would be a better definition to avoid overflow.

The real issue might be (ptrs_per_p4d - 1) which can overflow at
run-time, and maybe the added ubsan code makes p4d_index() not worth
inlining according to clang?