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Message-ID: <aJDqmoUNhwWeAlpa@antec>
Date: Mon, 4 Aug 2025 18:15:06 +0100
From: Stafford Horne <shorne@...il.com>
To: Kees Cook <kees@...nel.org>
Cc: linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
x86@...nel.org, linux-alpha@...r.kernel.org,
linux-csky@...r.kernel.org, linux-hexagon@...r.kernel.org,
linux-m68k@...ts.linux-m68k.org, linux-mips@...r.kernel.org,
linux-openrisc@...r.kernel.org, linux-parisc@...r.kernel.org,
linuxppc-dev@...ts.ozlabs.org, linux-riscv@...ts.infradead.org,
linux-s390@...r.kernel.org, linux-sh@...r.kernel.org,
sparclinux@...r.kernel.org, llvm@...ts.linux.dev,
linux-hardening@...r.kernel.org
Subject: Re: [PATCH 10/17] openrisc: Add __attribute_const__ to ffs()-family
implementations
On Mon, Aug 04, 2025 at 09:44:06AM -0700, Kees Cook wrote:
> While tracking down a problem where constant expressions used by
> BUILD_BUG_ON() suddenly stopped working[1], we found that an added static
> initializer was convincing the compiler that it couldn't track the state
> of the prior statically initialized value. Tracing this down found that
> ffs() was used in the initializer macro, but since it wasn't marked with
> __attribute__const__, the compiler had to assume the function might
> change variable states as a side-effect (which is not true for ffs(),
> which provides deterministic math results).
>
> Add missing __attribute_const__ annotations to OpenRISC's implementations of
> ffs(), __ffs(), fls(), and __fls() functions. These are pure mathematical
> functions that always return the same result for the same input with no
> side effects, making them eligible for compiler optimization.
>
> Build tested ARCH=openrisc defconfig with GCC or1k-linux 15.1.0.
THis looks ok to me.
> Link: https://github.com/KSPP/linux/issues/364 [1]
> Signed-off-by: Kees Cook <kees@...nel.org>
Acked-by: Stafford Horne <shorne@...il.com>
> ---
> arch/openrisc/include/asm/bitops/__ffs.h | 2 +-
> arch/openrisc/include/asm/bitops/__fls.h | 2 +-
> arch/openrisc/include/asm/bitops/ffs.h | 2 +-
> arch/openrisc/include/asm/bitops/fls.h | 2 +-
> 4 files changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/arch/openrisc/include/asm/bitops/__ffs.h b/arch/openrisc/include/asm/bitops/__ffs.h
> index 1e224b616fdf..4827b66530b2 100644
> --- a/arch/openrisc/include/asm/bitops/__ffs.h
> +++ b/arch/openrisc/include/asm/bitops/__ffs.h
> @@ -11,7 +11,7 @@
>
> #ifdef CONFIG_OPENRISC_HAVE_INST_FF1
>
> -static inline unsigned long __ffs(unsigned long x)
> +static inline __attribute_const__ unsigned long __ffs(unsigned long x)
> {
> int ret;
>
> diff --git a/arch/openrisc/include/asm/bitops/__fls.h b/arch/openrisc/include/asm/bitops/__fls.h
> index 9658446ad141..637cc76fe4b7 100644
> --- a/arch/openrisc/include/asm/bitops/__fls.h
> +++ b/arch/openrisc/include/asm/bitops/__fls.h
> @@ -11,7 +11,7 @@
>
> #ifdef CONFIG_OPENRISC_HAVE_INST_FL1
>
> -static inline unsigned long __fls(unsigned long x)
> +static inline __attribute_const__ unsigned long __fls(unsigned long x)
> {
> int ret;
>
> diff --git a/arch/openrisc/include/asm/bitops/ffs.h b/arch/openrisc/include/asm/bitops/ffs.h
> index b4c835d6bc84..536a60ab9cc3 100644
> --- a/arch/openrisc/include/asm/bitops/ffs.h
> +++ b/arch/openrisc/include/asm/bitops/ffs.h
> @@ -10,7 +10,7 @@
>
> #ifdef CONFIG_OPENRISC_HAVE_INST_FF1
>
> -static inline int ffs(int x)
> +static inline __attribute_const__ int ffs(int x)
> {
> int ret;
>
> diff --git a/arch/openrisc/include/asm/bitops/fls.h b/arch/openrisc/include/asm/bitops/fls.h
> index 6b77f6556fb9..77da7639bb3e 100644
> --- a/arch/openrisc/include/asm/bitops/fls.h
> +++ b/arch/openrisc/include/asm/bitops/fls.h
> @@ -11,7 +11,7 @@
>
> #ifdef CONFIG_OPENRISC_HAVE_INST_FL1
>
> -static inline int fls(unsigned int x)
> +static inline __attribute_const__ int fls(unsigned int x)
> {
> int ret;
>
> --
> 2.34.1
>
>
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