Re: [PATCH RFC 0/3] Static calls

From: Peter Zijlstra <peterz@infradead.org>
To: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Ingo Molnar <mingo@kernel.org>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	the arch/x86 maintainers <x86@kernel.org>,
	Andy Lutomirski <luto@kernel.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Jason Baron <jbaron@akamai.com>, Jiri Kosina <jkosina@suse.cz>,
	David Laight <David.Laight@aculab.com>,
	Borislav Petkov <bp@alien8.de>
Subject: Re: [PATCH RFC 0/3] Static calls
Date: Sun, 11 Nov 2018 15:25:25 +0100	[thread overview]
Message-ID: <20181111142525.GF12766@worktop.psav.com> (raw)
In-Reply-To: <CAKv+Gu_y2vH3eu7rMt7d7Hzum_MvYy7=30sNe1fhnB2QLe5i5A@mail.gmail.com>

On Sun, Nov 11, 2018 at 02:42:55PM +0100, Ard Biesheuvel wrote:
> On 11 November 2018 at 00:20, Peter Zijlstra <peterz@infradead.org> wrote:
> > On Fri, Nov 09, 2018 at 02:50:27PM +0100, Ard Biesheuvel wrote:
> >> On 9 November 2018 at 08:28, Ingo Molnar <mingo@kernel.org> wrote:
> >> >> - I'm not sure about the objtool approach.  Objtool is (currently)
> >> >>   x86-64 only, which means we have to use the "unoptimized" version
> >> >>   everywhere else.  I may experiment with a GCC plugin instead.
> >> >
> >> > I'd prefer the objtool approach. It's a pretty reliable first-principles
> >> > approach while GCC plugin would have to be replicated for Clang and any
> >> > other compilers, etc.
> >> >
> >>
> >> I implemented the GCC plugin approach here for arm64
> >
> > I'm confused; I though we only needed objtool for variable instruction
> > length architectures, because we can't reliably decode our instruction
> > stream. Otherwise we can fairly trivially use the DWARF relocation data,
> > no?
> 
> How would that work? We could build vmlinux with --emit-relocs, filter
> out the static jump/call relocations and resolve the symbol names to
> filter the ones associated with calls to trampolines. But then, we
> have to build the static_call_sites section and reinject it back into
> the image in some way, which is essentially objtool, no?

It's a _much_ simpler tool than objtool, but yes, we need a tool that
reads the relocation stuff and (re)injects it in a new section -- we
don't need it on a vmlinux level, it can be done per TU.

Anyway, a GCC plugin (I still have to have a peek at your thing) sounds
like it should work just fine too.