Re: [PATCH] x86: efi: avoid BUILD_BUG_ON() for non-constant p4d_index

From: Arvind Sankar <nivedita@alum.mit.edu>
To: Arnd Bergmann <arnd@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>,
	Arvind Sankar <nivedita@alum.mit.edu>,
	Nathan Chancellor <natechancellor@gmail.com>,
	Ard Biesheuvel <ardb@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	the arch/x86 maintainers <x86@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Arnd Bergmann <arnd@arndb.de>, Darren Hart <dvhart@infradead.org>,
	Andy Shevchenko <andy@infradead.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	linux-efi <linux-efi@vger.kernel.org>,
	Platform Driver <platform-driver-x86@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>
Subject: Re: [PATCH] x86: efi: avoid BUILD_BUG_ON() for non-constant p4d_index
Date: Fri, 15 Jan 2021 15:12:25 -0500	[thread overview]
Message-ID: <YAH3KY0I2AWudkc9@rani.riverdale.lan> (raw)
In-Reply-To: <CAK8P3a39vtF8GvRVQtEYssc+GvX-75j9-4pwXj4qhc7LK2RgNw@mail.gmail.com>

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@alien8.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?