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

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Date:   Wed, 6 Oct 2021 09:02:49 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>, x86@...nel.org,
        Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH v4 06/15] ftrace: Use an opaque type for functions not
 callable from C

On Tue, 5 Oct 2021 20:29:45 -0700
Josh Poimboeuf <jpoimboe@...hat.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@...gle.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