Re: [PATCH] x86: efi: avoid BUILD_BUG_ON() for non-constant p4d_index

[Arvind Sankar]

On Thu, Jan 07, 2021 at 11:34:15PM +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@xxxxxxxx>
> 
> 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@xxxxxxxx>
> ---
>  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.