Re: [PATCH v2] arm64: ftrace: use function_nocfi for _mcount as well

[Sami Tolvanen]

Hi Sumit,

On Tue, Oct 5, 2021 at 5:37 AM Sumit Garg <sumit.garg@xxxxxxxxxx> 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.
>
> This problem won't apply where the toolchain implements
> -fpatchable-function-entry as we'll use that in preference to regular -pg,
> i.e. this won't show up with recent versions of clang.
>
> Fixes: 9186ad8e66bab6a1 ("arm64: allow CONFIG_CFI_CLANG to be selected")
> Signed-off-by: Sumit Garg <sumit.garg@xxxxxxxxxx>
> Acked-by: Mark Rutland <mark.rutland@xxxxxxx>
> ---
>
> Changes in v2:
> - Added fixes tag.
> - Extended commit description.
> - Picked up Mark's ack.
>
>  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
>

Clang >= 10 supports -fpatchable-function-entry and CFI requires Clang
12, so I assume this is only an issue if
CONFIG_DYNAMIC_FTRACE_WITH_REGS is explicitly disabled?

Nevertheless, the patch looks good to me.

Reviewed-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>

Sami