[PATCH] arm64: avoid BUILD_BUG_ON() in alternative-macros

[Ard Biesheuvel]

On Tue, 20 Sept 2022 at 16:00, Mark Rutland <mark.rutland at arm.com> wrote:
>
> Nathan reports that the build fails when using clang and LTO:
>
> |  In file included from kernel/bounds.c:10:
> |  In file included from ./include/linux/page-flags.h:10:
> |  In file included from ./include/linux/bug.h:5:
> |  In file included from ./arch/arm64/include/asm/bug.h:26:
> |  In file included from ./include/asm-generic/bug.h:5:
> |  In file included from ./include/linux/compiler.h:248:
> |  In file included from ./arch/arm64/include/asm/rwonce.h:11:
> |  ./arch/arm64/include/asm/alternative-macros.h:224:2: error: call to undeclared function 'BUILD_BUG_ON'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
> |          BUILD_BUG_ON(feature >= ARM64_NCAPS);
> |          ^
> |  ./arch/arm64/include/asm/alternative-macros.h:241:2: error: call to undeclared function 'BUILD_BUG_ON'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
> |          BUILD_BUG_ON(feature >= ARM64_NCAPS);
> |          ^
> |  2 errors generated.
>
> ... the problem being that when LTO is enabled, <asm/rwonce.h> includes
> <asm/alternative-macros.h>, and causes a circular include dependency
> through <linux/bug.h>. This manifests as BUILD_BUG_ON() not being
> defined when used within <asm/alternative-macros.h>.
>
> This patch avoids the problem and simplifies the include dependencies by
> using compiletime_assert() instead of BUILD_BUG_ON().
>
> Signed-off-by: Mark Rutland <mark.rutland at arm.com>
> Fixes: 21fb26bfb01ffe0d ("arm64: alternatives: add alternative_has_feature_*()")
> Reported-by: Nathan Chancellor <nathan at kernel.org>
> Tested-by: Nathan Chancellor <nathan at kernel.org>
> Link: http://lore.kernel.org/r/YyigTrxhE3IRPzjs@dev-arch.thelio-3990X
> Cc: Ard Biesheuvel <ardb at kernel.org>
> Cc: Catalin Marinas <catalin.marinas at arm.com>
> Cc: James Morse <james.morse at arm.com>
> Cc: Joey Gouly <joey.gouly at arm.com>
> Cc: Marc Zyngier <maz at kernel.org>
> Cc: Will Deacon <will at kernel.org>

Reviewed-by: Ard Biesheuvel <ardb at kernel.org>

> ---
>  arch/arm64/include/asm/alternative-macros.h | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/include/asm/alternative-macros.h b/arch/arm64/include/asm/alternative-macros.h
> index 4a2a98d6d2227..966767debaa3a 100644
> --- a/arch/arm64/include/asm/alternative-macros.h
> +++ b/arch/arm64/include/asm/alternative-macros.h
> @@ -215,13 +215,13 @@ alternative_endif
>
>  #ifndef __ASSEMBLY__
>
> -#include <linux/bug.h>
>  #include <linux/types.h>
>
>  static __always_inline bool
>  alternative_has_feature_likely(unsigned long feature)
>  {
> -       BUILD_BUG_ON(feature >= ARM64_NCAPS);
> +       compiletime_assert(feature < ARM64_NCAPS,
> +                          "feature must be < ARM64_NCAPS");
>
>         asm_volatile_goto(
>         ALTERNATIVE_CB("b       %l[l_no]", %[feature], alt_cb_patch_nops)
> @@ -238,7 +238,8 @@ alternative_has_feature_likely(unsigned long feature)
>  static __always_inline bool
>  alternative_has_feature_unlikely(unsigned long feature)
>  {
> -       BUILD_BUG_ON(feature >= ARM64_NCAPS);
> +       compiletime_assert(feature < ARM64_NCAPS,
> +                          "feature must be < ARM64_NCAPS");
>
>         asm_volatile_goto(
>         ALTERNATIVE("nop", "b   %l[l_yes]", %[feature])
> --
> 2.30.2
>