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

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Date:   Wed, 13 Jan 2021 18:51:59 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        X86 ML <x86@...nel.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        Darren Hart <dvhart@...radead.org>,
        Andy Shevchenko <andy@...radead.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        linux-efi <linux-efi@...r.kernel.org>,
        platform-driver-x86@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] x86: efi: avoid BUILD_BUG_ON() for non-constant p4d_index

On Thu, 7 Jan 2021 at 23:34, Arnd Bergmann <arnd@...nel.org> wrote:
>
> 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>

Acked-by: Ard Biesheuvel <ardb@...nel.org>

This can go via the x86 tree directly, IMO

> ---
>  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
>