[PATCH v3 5/9] ARM: delay: Turn delay functions into static inlines

[Ard Biesheuvel]

On Mon, 11 Mar 2024 at 10:15, Linus Walleij <linus.walleij at linaro.org> wrote:
>
> The members of the vector table arm_delay_ops are called
> directly using defines, but this is really confusing for
> KCFI. Wrap the calls in static inlines and tag them with
> __nocfi so things start to work.
>
> Without this patch, platforms without a delay timer will
> not boot (sticks in calibrating loop etc).
>
> Signed-off-by: Linus Walleij <linus.walleij at linaro.org>
> ---
>  arch/arm/include/asm/delay.h | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)
>

So what goes wrong without this patch? Is it really that easy to confuse kCFI?

As far as I can tell (and I tried reverting it too), this patch should
not be needed - in general, when all function pointer variables and
the functions themselves are defined in the C domain, the compiler
should be able to figure it out.

Also, these are functions that are used on every system and called
often and in a predictable manner, so they are prime targets for the
kind of attacks that CFI is intended to harden against, so wrapping
them in __nocfi is probably something we should try to avoid.


> diff --git a/arch/arm/include/asm/delay.h b/arch/arm/include/asm/delay.h
> index 1d069e558d8d..7d611b810b6c 100644
> --- a/arch/arm/include/asm/delay.h
> +++ b/arch/arm/include/asm/delay.h
> @@ -55,7 +55,10 @@ extern struct arm_delay_ops {
>         unsigned long ticks_per_jiffy;
>  } arm_delay_ops;
>
> -#define __delay(n)             arm_delay_ops.delay(n)
> +static inline void __nocfi __delay(unsigned long n)
> +{
> +       arm_delay_ops.delay(n);
> +}
>
>  /*
>   * This function intentionally does not exist; if you see references to
> @@ -76,8 +79,15 @@ extern void __bad_udelay(void);
>   * first constant multiplications gets optimized away if the delay is
>   * a constant)
>   */
> -#define __udelay(n)            arm_delay_ops.udelay(n)
> -#define __const_udelay(n)      arm_delay_ops.const_udelay(n)
> +static inline void __nocfi __udelay(unsigned long n)
> +{
> +       arm_delay_ops.udelay(n);
> +}
> +
> +static inline void __nocfi __const_udelay(unsigned long n)
> +{
> +       arm_delay_ops.const_udelay(n);
> +}
>
>  #define udelay(n)                                                      \
>         (__builtin_constant_p(n) ?                                      \
>
> --
> 2.34.1
>