[PATCH 2/8] ARM: ftrace: use ADD not POP to counter PUSH at entry

[Nick Desaulniers]

On Tue, Jan 25, 2022 at 7:37 AM Ard Biesheuvel <ardb at kernel.org> wrote:
>
> The compiler emitted hook used for ftrace consists of a PUSH {LR} to
> preserve the link register, followed by a branch-and-link (BL) to
> __gnu_mount_nc. Dynamic ftrace patches away the latter to turn the
> combined sequence into a NOP, using a POP {LR} instruction.
>
> This is not necessary, since the link register does not get clobbered in
> this case, and simply adding #4 to the stack pointer is sufficient, and
> avoids a memory access that may take a few cycles to resolve depending
> on the micro-architecture.

Reviewed-by: Nick Desaulniers <ndesaulniers at google.com>

>
> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
> ---
>  arch/arm/kernel/entry-ftrace.S | 2 +-
>  arch/arm/kernel/ftrace.c       | 4 ++--
>  2 files changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm/kernel/entry-ftrace.S b/arch/arm/kernel/entry-ftrace.S
> index f4886fb6e9ba..dca12a09322a 100644
> --- a/arch/arm/kernel/entry-ftrace.S
> +++ b/arch/arm/kernel/entry-ftrace.S
> @@ -27,7 +27,7 @@
>   * allows it to be clobbered in subroutines and doesn't use it to hold
>   * parameters.)
>   *
> - * When using dynamic ftrace, we patch out the mcount call by a "pop {lr}"
> + * When using dynamic ftrace, we patch out the mcount call by a "add sp, #4"
>   * instead of the __gnu_mcount_nc call (see arch/arm/kernel/ftrace.c).
>   */
>
> diff --git a/arch/arm/kernel/ftrace.c b/arch/arm/kernel/ftrace.c
> index a006585e1c09..db72d3a6522d 100644
> --- a/arch/arm/kernel/ftrace.c
> +++ b/arch/arm/kernel/ftrace.c
> @@ -25,9 +25,9 @@
>  #include <asm/patch.h>
>
>  #ifdef CONFIG_THUMB2_KERNEL
> -#define        NOP             0xf85deb04      /* pop.w {lr} */
> +#define        NOP             0xf10d0d04      /* add.w sp, sp, #4 */
>  #else
> -#define        NOP             0xe8bd4000      /* pop {lr} */
> +#define        NOP             0xe28dd004      /* add   sp, sp, #4 */
>  #endif
>
>  #ifdef CONFIG_DYNAMIC_FTRACE
> --
> 2.30.2
>


-- 
Thanks,
~Nick Desaulniers