Re: [PATCH] arm64: ftrace: use function_nocfi for _mcount as well

[Mark Rutland]

On Tue, Oct 05, 2021 at 04:02:08PM +0530, Sumit Garg wrote:
> Commit 800618f955a9 ("arm64: ftrace: use function_nocfi for ftrace_call")
> only fixed address of ftrace_call but address of _mcount needs to be
> fixed as well. Use function_nocfi() to get the actual address of _mcount
> function as with CONFIG_CFI_CLANG, the compiler replaces function pointers
> with jump table addresses which breaks dynamic ftrace as the address of
> _mcount is replaced with the address of _mcount.cfi_jt.

It might be worth noting that where the toolchain implements
patchable-function-entry, we'll use that in preference to regular -pg,
and this problem won't apply. i.e. this won't show with recent versions
of clang.

> Signed-off-by: Sumit Garg <sumit.garg@xxxxxxxxxx>

This probably wants:

Fixes: 9186ad8e66bab6a1 ("arm64: allow CONFIG_CFI_CLANG to be selected")

With that:

Acked-by: Mark Rutland <mark.rutland@xxxxxxx>

Mark.

> ---
>  arch/arm64/include/asm/ftrace.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/ftrace.h b/arch/arm64/include/asm/ftrace.h
> index 91fa4baa1a93..347b0cc68f07 100644
> --- a/arch/arm64/include/asm/ftrace.h
> +++ b/arch/arm64/include/asm/ftrace.h
> @@ -15,7 +15,7 @@
>  #ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
>  #define ARCH_SUPPORTS_FTRACE_OPS 1
>  #else
> -#define MCOUNT_ADDR		((unsigned long)_mcount)
> +#define MCOUNT_ADDR		((unsigned long)function_nocfi(_mcount))
>  #endif
>  
>  /* The BL at the callsite's adjusted rec->ip */
> -- 
> 2.17.1
>