linux-hardening - Re: [PATCH v6 8/8] ARM: make get_current() and __my_cpu_offset() __always

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Date:   Tue, 25 Jan 2022 12:48:29 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     linux@...linux.org.uk, linux-arm-kernel@...ts.infradead.org,
        linux-hardening@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
        Kees Cook <keescook@...omium.org>,
        Keith Packard <keithpac@...zon.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Marc Zyngier <maz@...nel.org>
Subject: Re: [PATCH v6 8/8] ARM: make get_current() and __my_cpu_offset() __always_inline

On Tue, Jan 25, 2022 at 1:15 AM Ard Biesheuvel <ardb@...nel.org> wrote:
>
> The get_current() and __my_cpu_offset() accessors evaluate to only a
> single instruction emitted inline, but due to the size of the asm string
> that is created for SMP+v6 configurations, the compiler assumes
> otherwise, and may emit the functions out of line instead.
>
> So use __always_inline to avoid this.
>
> Signed-off-by: Ard Biesheuvel <ardb@...nel.org>

Alternatively, you could use the inline qualifier on the asm stmt.
i.e. `asm inline ("my asm string")`. Only supported since gcc-8.3+ and
all kernel-supported versions of clang though. See `asm_inline` in
include/linux/compiler_types.h.
Either way,
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

> ---
>  arch/arm/include/asm/current.h | 2 +-
>  arch/arm/include/asm/percpu.h  | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm/include/asm/current.h b/arch/arm/include/asm/current.h
> index 131a89bbec6b..1e1178bf176d 100644
> --- a/arch/arm/include/asm/current.h
> +++ b/arch/arm/include/asm/current.h
> @@ -14,7 +14,7 @@ struct task_struct;
>
>  extern struct task_struct *__current;
>
> -static inline __attribute_const__ struct task_struct *get_current(void)
> +static __always_inline __attribute_const__ struct task_struct *get_current(void)
>  {
>         struct task_struct *cur;
>
> diff --git a/arch/arm/include/asm/percpu.h b/arch/arm/include/asm/percpu.h
> index a09034ae45a1..7545c87c251f 100644
> --- a/arch/arm/include/asm/percpu.h
> +++ b/arch/arm/include/asm/percpu.h
> @@ -25,7 +25,7 @@ static inline void set_my_cpu_offset(unsigned long off)
>         asm volatile("mcr p15, 0, %0, c13, c0, 4" : : "r" (off) : "memory");
>  }
>
> -static inline unsigned long __my_cpu_offset(void)
> +static __always_inline unsigned long __my_cpu_offset(void)
>  {
>         unsigned long off;
>
> --
> 2.30.2
>


-- 
Thanks,
~Nick Desaulniers