Re: [PATCH] x86: efi: avoid BUILD_BUG_ON() for non-constant p4d_index
From: Arvind Sankar <hidden>
Date: 2021-01-15 19:08:52
Also in:
lkml, platform-driver-x86
On Thu, Jan 07, 2021 at 11:34:15PM +0100, Arnd Bergmann wrote:
quoted hunk ↗ jump to hunk
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.