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Peter Anvin" , linux-efi@vger.kernel.org, platform-driver-x86@vger.kernel.org, linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com Subject: Re: [PATCH] x86: efi: avoid BUILD_BUG_ON() for non-constant p4d_index Message-ID: References: <20210107223424.4135538-1-arnd@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 > > > > 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 > > --- > > 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? 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.