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

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Message-ID: <CABCJKufCaOXOUF43a-PQshO8aEsMNhZ2EiyGMSOp9ZGn57G=pg@mail.gmail.com>
Date:   Wed, 6 Oct 2021 16:14:17 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>, X86 ML <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,
        LKML <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 Wed, Oct 6, 2021 at 2:24 PM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>
> On Wed, Oct 06, 2021 at 05:10:16PM -0400, Steven Rostedt wrote:
> > On Wed, 6 Oct 2021 13:43:35 -0700
> > Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> >
> > > On Wed, Oct 06, 2021 at 10:45:41AM -0700, Sami Tolvanen wrote:
> > > > On Wed, Oct 6, 2021 at 9:58 AM Steven Rostedt <rostedt@...dmis.org> wrote:
> > > > >
> > > > > On Wed, 6 Oct 2021 09:31:04 -0700
> > > > > Sami Tolvanen <samitolvanen@...gle.com> wrote:
> > > > >
> > > > > > > > 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.
> > > > > >
> > > > > > Any thoughts on how to make this less confusing?
> > > > >
> > > > >  "Not called by C code, but injected by the compiler."
> > > > >
> > > > > ?
> > > >
> > > > Sure, sounds good to me. I'll update this in v5.
> > >
> > > "injected by the compiler" sounds even more confusing.  It almost sounds
> > > like those functions are generated by GCC, which they are most
> > > definitely not.
> > >
> >
> > Heh, I was thinking of the locations that are injected (mcount / fentry) as
> > these are just replacements for them. Those injections are added by GCC.
> >
> > So, continuing the bikeshedding, what about "not called by C code, but are
> > trampolines injected as calls replacing the nops at the start of
> > functions added by the compiler." ?
>
> I'm not quite sure what that means, but I'll allow it ;-)

Alright, I'll go with the updated version then. I'll also rename
DECLARE_ASM_FUNC_SYMBOL() to DECLARE_NOT_CALLED_FROM_C() unless
someone has strong objections about that.

Sami