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

[Sumit Garg]

On Tue, 5 Oct 2021 at 16:11, Mark Rutland <mark.rutland@xxxxxxx> wrote:
>
> 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.

Okay, I will add it to the commit description.

>
> > Signed-off-by: Sumit Garg <sumit.garg@xxxxxxxxxx>
>
> This probably wants:
>
> Fixes: 9186ad8e66bab6a1 ("arm64: allow CONFIG_CFI_CLANG to be selected")
>

Okay, I will add it in v2.

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

Thanks,
Sumit

> 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
> >