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

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Message-ID: <20210107224246.GA3523037@ubuntu-m3-large-x86>
Date:   Thu, 7 Jan 2021 15:42:46 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Ard Biesheuvel <ardb@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, 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@...r.kernel.org,
        platform-driver-x86@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH] x86: efi: avoid BUILD_BUG_ON() for non-constant p4d_index

On Thu, Jan 07, 2021 at 11:34:15PM +0100, Arnd Bergmann 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>

Reviewed-by: Nathan Chancellor <natechancellor@...il.com>
Tested-by: Nathan Chancellor <natechancellor@...il.com>

> ---
>  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
>