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From:   Andy Lutomirski <luto@amacapital.net>
Date:   Wed, 10 Oct 2018 14:13:22 -0700
Message-ID: <CALCETrUZMmTDOh5-Mzyj1rk7Q82DgdQLinOfbzF4Sb7OC2aYMQ@mail.gmail.com>
Subject: Re: [POC][RFC][PATCH 1/2] jump_function: Addition of new feature "jump_function"
To:     Josh Poimboeuf <jpoimboe@redhat.com>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Matthew Helsley <mhelsley@vmware.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        David Woodhouse <dwmw2@infradead.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Jason Baron <jbaron@akamai.com>, Jiri Kosina <jkosina@suse.cz>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Andrew Lutomirski <luto@kernel.org>
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On Wed, Oct 10, 2018 at 11:17 AM Josh Poimboeuf <jpoimboe@redhat.com> wrote:
>
> On Wed, Oct 10, 2018 at 01:16:05PM -0500, Josh Poimboeuf wrote:
> > On Wed, Oct 10, 2018 at 11:03:43AM -0700, Andy Lutomirski wrote:
> > > > +#define DECLARE_STATIC_CALL(tramp, func)                               \
> > > > +       extern typeof(func) tramp;                                      \
> > > > +       static void __used __section(.discard.static_call_tramps)       \
> > > > +               *__static_call_tramp_##tramp = tramp
> > > > +
> > >
> > > Confused.  What's the __static_call_tramp_##tramp variable for?  And
> > > why is a DECLARE_ macro defining a variable?
> >
> > This is the magic needed for objtool to find all the call sites.
> >
> > The variable itself isn't needed, but the .discard.static_call_tramps
> > entry is.  Objtool reads that section to find out which function call
> > sites are targeted to a static call trampoline.
>
> To clarify: objtool reads that section to find out which functions are
> really static call trampolines.  Then it annotates all the instructions
> which call/jmp to those trampolines.  Those annotations are then read by
> the kernel.
>

Ah, right, and objtool runs on a per-object basis so it has no other
way to know what symbols are actually static calls.

There's another way to skin this cat, though:

extern typeof(func) __static_call_trampoline_##tramp;
#define tramp __static_call_trampoline_##tramp

And objtool could recognize it by name.  But, of course, you can't put
a #define in a macro.  But maybe there's a way to hack it up with a
static inline?

Anyway, your way is probably fine with a few caveats:

 - It won't really work if the call comes from a .S file.
 - There should probably be a comment to help de-confuse future people
like me :)