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

[Sumit Garg]

Hi Mark, Sami,

On Tue, 5 Oct 2021 at 21:05, Mark Rutland <mark.rutland@xxxxxxx> wrote:
>
> On Tue, Oct 05, 2021 at 08:20:02AM -0700, Sami Tolvanen wrote:
> > 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?
>
> I don't believe it's possible to disable explicitly, since
> DYNAMIC_FTRACE_WITH_REGS isn't user selectable, and is def bool y,
> depending on HAVE_DYNAMIC_FTRACE_WITH_REGS.
>

Ah, I see.

> Sumit, have you actually seen a problem, or was this found by
> inspection?
>

Actually I have seen this ftrace problem with the android11-5.4-lts
kernel and AOSP master user-space on db845c. The reason being kernel
v5.4 LTS doesn't support ftrace with -fpatchable-function-entry on
arm64.

With the mainline, I haven't tried to reproduce this issue but it was
rather by inspection that this needs to be fixed as well.

> If this isn't an issue in practice, we could add the funciton_nocfi()
> for consistency, but we should make that clear in the commit message,
> and drop the fixes tag.

Sure, let me drop the fixes tag and update the commit description in
v3 as mainline only enabled CFI_CLANG for arm64 when
"-fpatchable-function-entry" is supported.

-Sumit

>
> Thanks,
> Mark.