Re: [PATCH v4 06/15] ftrace: Use an opaque type for functions not callable from C

From: Steven Rostedt <rostedt@goodmis.org>
To: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Sami Tolvanen <samitolvanen@google.com>,
	x86@kernel.org, Kees Cook <keescook@chromium.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Nathan Chancellor <nathan@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Sedat Dilek <sedat.dilek@gmail.com>,
	linux-hardening@vger.kernel.org, linux-kernel@vger.kernel.org,
	llvm@lists.linux.dev
Subject: Re: [PATCH v4 06/15] ftrace: Use an opaque type for functions not callable from C
Date: Wed, 6 Oct 2021 09:02:49 -0400	[thread overview]
Message-ID: <20211006090249.248c65b0@gandalf.local.home> (raw)
In-Reply-To: <20211006032945.axlqh3vehgar6adr@treble>

On Tue, 5 Oct 2021 20:29:45 -0700
Josh Poimboeuf <jpoimboe@redhat.com> wrote:

Thanks for Cc'ing me, as I should have been Cc'd on the original patch.

> On Thu, Sep 30, 2021 at 11:05:22AM -0700, Sami Tolvanen wrote:
> > With CONFIG_CFI_CLANG, the compiler changes function references to point
> > to the CFI jump table. As ftrace_call, ftrace_regs_call, and mcount_call
> > are not called from C, use DECLARE_ASM_FUNC_SYMBOL to declare them.

"not called from C" is a bit confusing.

> > 
> > Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
> > ---
> >  include/linux/ftrace.h | 7 ++++---
> >  1 file changed, 4 insertions(+), 3 deletions(-)
> > 
> > diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> > index 832e65f06754..67de28464aeb 100644
> > --- a/include/linux/ftrace.h
> > +++ b/include/linux/ftrace.h
> > @@ -578,9 +578,10 @@ extern void ftrace_replace_code(int enable);
> >  extern int ftrace_update_ftrace_func(ftrace_func_t func);
> >  extern void ftrace_caller(void);
> >  extern void ftrace_regs_caller(void);
> > -extern void ftrace_call(void);
> > -extern void ftrace_regs_call(void);
> > -extern void mcount_call(void);
> > +
> > +DECLARE_ASM_FUNC_SYMBOL(ftrace_call);
> > +DECLARE_ASM_FUNC_SYMBOL(ftrace_regs_call);
> > +DECLARE_ASM_FUNC_SYMBOL(mcount_call);  
> 
> I'm thinking DECLARE_ASM_FUNC_SYMBOL needs a better name. It's not clear
> from reading it why some asm symbols need the macro and others don't.
> 
> I guess it means "an asm text symbol which isn't callable from C code
> (not including alternatives)"?
> 
> DECLARE_UNCALLED_SYMBOL() maybe?
> 

That's even worse ;-) Because "called" is an assembler command in x86, and
it is "called" from assembly (when you look at an objdump, it is most
definitely "called").

Perhaps DECLARE_ASM_INTERNAL_SYMBOL() ?

Or call it "DECLARE_ASM_MCOUNT_SYMBOL()" as "mcount" is the original name
of what a compiler does when passed the -pg option, and that's exactly what
those functions are.

-- Steve