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

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Date:   Thu,  7 Jan 2021 23:34:15 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Ard Biesheuvel <ardb@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Arnd Bergmann <arnd@...db.de>, Darren Hart <dvhart@...radead.org>,
        Andy Shevchenko <andy@...radead.org>,
        "H. Peter Anvin" <hpa@...or.com>, linux-efi@...r.kernel.org,
        platform-driver-x86@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: [PATCH] x86: efi: avoid BUILD_BUG_ON() for non-constant p4d_index

From: Arnd Bergmann <arnd@...db.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@...db.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