From: Ard Biesheuvel <ardb@kernel.org>
To: Arvind Sankar <nivedita@alum.mit.edu>
Cc: Arnd Bergmann <arnd@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
X86 ML <x86@kernel.org>,
Nathan Chancellor <natechancellor@gmail.com>,
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-x86@vger.kernel.org,
Linux Kernel Mailing List <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: Sat, 16 Jan 2021 17:34:27 +0100 [thread overview]
Message-ID: <CAMj1kXGZFZciN1_KruCr=g6GANNpRrCLR48b3q13+QfK481C7Q@mail.gmail.com> (raw)
In-Reply-To: <YAH6r3lak/F2wndp@rani.riverdale.lan>
On Fri, 15 Jan 2021 at 21:27, Arvind Sankar <nivedita@alum.mit.edu> wrote:
>
> On Fri, Jan 15, 2021 at 02:07:51PM -0500, Arvind Sankar wrote:
> > On Thu, Jan 07, 2021 at 11:34:15PM +0100, Arnd Bergmann wrote:
> > > From: Arnd Bergmann <arnd@arndb.de>
> > >
> > > When 5-level page tables are enabled, clang triggers a BUILD_BUG_ON():
> > >
> > > x86_64-linux-ld: arch/x86/platform/efi/efi_64.o: in function `efi_sync_low_kernel_mappings':
> > > efi_64.c:(.text+0x22c): undefined reference to `__compiletime_assert_354'
> > >
> > > Use the same method as in commit c65e774fb3f6 ("x86/mm: Make PGDIR_SHIFT
> > > and PTRS_PER_P4D variable") and change it to MAYBE_BUILD_BUG_ON(),
> > > so it only triggers for constant input.
> > >
> > > Link: https://github.com/ClangBuiltLinux/linux/issues/256
> > > Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> > > ---
> > > arch/x86/platform/efi/efi_64.c | 4 ++--
> > > 1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/arch/x86/platform/efi/efi_64.c b/arch/x86/platform/efi/efi_64.c
> > > index e1e8d4e3a213..62bb1616b4a5 100644
> > > --- a/arch/x86/platform/efi/efi_64.c
> > > +++ b/arch/x86/platform/efi/efi_64.c
> > > @@ -137,8 +137,8 @@ void efi_sync_low_kernel_mappings(void)
> > > * As with PGDs, we share all P4D entries apart from the one entry
> > > * that covers the EFI runtime mapping space.
> > > */
> > > - BUILD_BUG_ON(p4d_index(EFI_VA_END) != p4d_index(MODULES_END));
> > > - BUILD_BUG_ON((EFI_VA_START & P4D_MASK) != (EFI_VA_END & P4D_MASK));
> > > + MAYBE_BUILD_BUG_ON(p4d_index(EFI_VA_END) != p4d_index(MODULES_END));
> > > + MAYBE_BUILD_BUG_ON((EFI_VA_START & P4D_MASK) != (EFI_VA_END & P4D_MASK));
> > >
> > > pgd_efi = efi_pgd + pgd_index(EFI_VA_END);
> > > pgd_k = pgd_offset_k(EFI_VA_END);
> > > --
> > > 2.29.2
> > >
> >
> > I think this needs more explanation as to why clang is triggering this.
> > The issue mentions clang not inline p4d_index(), and I guess not
> > performing inter-procedural analysis either?
> >
> > For the second assertion there, everything is always constant AFAICT:
> > EFI_VA_START, EFI_VA_END and P4D_MASK are all constants regardless of
> > CONFIG_5LEVEL.
> >
> > For the first assertion, it isn't technically constant, but if
> > p4d_index() gets inlined, the compiler should be able to see that the
> > two are always equal, even though ptrs_per_p4d is not constant:
> > EFI_VA_END >> 39 == MODULES_END >> 39
> > so the masking with ptrs_per_p4d-1 doesn't matter for the comparison.
> >
> > As a matter of fact, it seems like the four assertions could be combined
> > into:
> > BUILD_BUG_ON((EFI_VA_END & P4D_MASK) != (MODULES_END & P4D_MASK));
> > BUILD_BUG_ON((EFI_VA_START & P4D_MASK) != (EFI_VA_END & P4D_MASK));
> > instead of separately asserting they're the same PGD entry and the same
> > P4D entry.
> >
> > Thanks.
>
> I actually don't quite get the MODULES_END check -- Ard, do you know
> what that's for?
>
Maybe Boris remembers? He wrote the original code for the 'new' EFI
page table layout.
> What we really should be checking is that EFI_VA_START is in the top-most
> PGD entry and the top-most P4D entry, since we only copy PGD/P4D entries
> before EFI_VA_END, but not after EFI_VA_START. So the checks should
> really be
> BUILD_BUG_ON(((EFI_VA_START - 1) & P4D_MASK) != (-1ul & P4D_MASK));
> BUILD_BUG_ON(((EFI_VA_START - 1) & P4D_MASK) != (EFI_VA_END & P4D_MASK));
> imo. I guess that's what using MODULES_END is effectively checking, but
> it would be clearer to check it directly.
This obviously needs a comment, but checking that everything lives in
the top 512 GB of the kernel VA space seems sufficient to me,
next prev parent reply other threads:[~2021-01-16 17:10 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-07 22:34 [PATCH] x86: efi: avoid BUILD_BUG_ON() for non-constant p4d_index Arnd Bergmann
2021-01-07 22:42 ` Nathan Chancellor
2021-01-13 17:51 ` Ard Biesheuvel
2021-01-15 18:23 ` Borislav Petkov
2021-01-15 18:32 ` Nathan Chancellor
2021-01-15 19:07 ` Borislav Petkov
2021-01-15 19:11 ` Arvind Sankar
2021-01-15 19:18 ` Borislav Petkov
2021-01-15 19:54 ` Arnd Bergmann
2021-01-15 20:12 ` Arvind Sankar
2021-01-15 20:32 ` Arvind Sankar
2021-01-15 19:07 ` Arvind Sankar
2021-01-15 20:27 ` Arvind Sankar
2021-01-16 16:34 ` Ard Biesheuvel [this message]
2021-01-18 20:24 ` Borislav Petkov
2021-01-18 21:42 ` Arvind Sankar
2021-01-20 9:33 ` Ard Biesheuvel
2021-01-20 11:44 ` Borislav Petkov
2021-02-03 18:51 ` Nathan Chancellor
2021-02-03 20:29 ` Ard Biesheuvel
2021-02-04 10:51 ` Borislav Petkov
2021-02-04 10:59 ` Ard Biesheuvel
2021-02-04 19:16 ` Nathan Chancellor
2021-02-04 21:43 ` Arvind Sankar
2021-02-04 22:13 ` Borislav Petkov
2021-02-05 0:08 ` Arvind Sankar
2021-02-05 11:39 ` [PATCH] x86/efi: Remove EFI PGD build time checks Borislav Petkov
2021-02-05 11:57 ` Ard Biesheuvel
2021-02-05 18:14 ` Nick Desaulniers
2021-02-05 18:56 ` Nathan Chancellor
2021-02-05 10:34 ` [PATCH] x86: efi: avoid BUILD_BUG_ON() for non-constant p4d_index Borislav Petkov
2021-02-05 18:27 ` Nick Desaulniers
2021-02-05 18:31 ` Nathan Chancellor
2021-01-20 11:26 ` Kirill A. Shutemov
2021-01-20 11:06 ` Kirill A. Shutemov
2021-02-06 12:56 ` [tip: x86/urgent] x86/efi: Remove EFI PGD build time checks tip-bot2 for Borislav Petkov
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