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 <[email protected]> > > > > 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 <[email protected]> > > --- > > 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.
Back in the days BUILD_BUG_ON() false-triggered on GCC-4.8 as well. -- Kirill A. Shutemov
