On Fri, 15 Jan 2021 at 21:27, Arvind Sankar <nived...@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 <a...@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 <a...@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,