linux-kernel - Re: [POC][RFC][PATCH 1/2] jump_function: Addition of new feature "jump

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Date:   Wed, 10 Oct 2018 13:17:41 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Matthew Helsley <mhelsley@...are.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jason Baron <jbaron@...mai.com>, Jiri Kosina <jkosina@...e.cz>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Andrew Lutomirski <luto@...nel.org>
Subject: Re: [POC][RFC][PATCH 1/2] jump_function: Addition of new feature
 "jump_function"

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.

-- 
Josh