linux-kernel - Re: [PATCH v2] arm64: ftrace: use function_nocfi for

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Date:   Fri, 8 Oct 2021 09:05:46 +0100
From:   Will Deacon <will@...nel.org>
To:     Sumit Garg <sumit.garg@...aro.org>
Cc:     Mark Rutland <mark.rutland@....com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>, ben.dai@...soc.com,
        Nathan Chancellor <nathan@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        LKML <linux-kernel@...r.kernel.org>, llvm@...ts.linux.dev
Subject: Re: [PATCH v2] arm64: ftrace: use function_nocfi for _mcount as well

Hi Sumit,

On Wed, Oct 06, 2021 at 11:05:52AM +0530, Sumit Garg wrote:
> On Tue, 5 Oct 2021 at 21:05, Mark Rutland <mark.rutland@....com> wrote:
> > On Tue, Oct 05, 2021 at 08:20:02AM -0700, Sami Tolvanen wrote:
> > > On Tue, Oct 5, 2021 at 5:37 AM Sumit Garg <sumit.garg@...aro.org> wrote:
> > > > 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.

Did you post a v3? Just want to make sure I didn't miss it.

Will