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

[Arvind Sankar]

On Fri, Jan 15, 2021 at 03:12:25PM -0500, Arvind Sankar wrote:
> 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@xxxxxxxxx> 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.

Eh, that's an overflow too, isn't it :( Is this option really useful
with kernel code -- this sort of thing is probably done all over the
place?

> 
> 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?