linux-kernel - Re: [RFC PATCH 3/3] objtool/mcount: Add powerpc specific functions

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Date:   Mon, 28 Mar 2022 22:14:38 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        "aik@...abs.ru" <aik@...abs.ru>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "naveen.n.rao@...ux.vnet.ibm.com" <naveen.n.rao@...ux.vnet.ibm.com>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        Sathvika Vasireddy <sv@...ux.ibm.com>
Subject: Re: [RFC PATCH 3/3] objtool/mcount: Add powerpc specific functions

On Mon, Mar 28, 2022 at 12:59:20PM -0700, Josh Poimboeuf wrote:

> I'm not making any major changes to the code, just shuffling things
> around to make the interface more modular.  I hope to have something
> soon (this week).  Peter recently added a big feature (Intel IBT) which
> is already in -next.

Hit Linus' tree yesterday :-)

> Some years ago Kamalesh Babulal had a prototype of objtool for ppc64le
> which did the full stack validation.  I'm not sure what ever became of
> that.

I've also heard chatter about s390.

> FWIW, there have been some objtool patches for arm64 stack validation,
> but the arm64 maintainers have been hesitant to get on board with
> objtool, as it brings a certain maintenance burden.  Especially for the
> full stack validation and ORC unwinder.  But if you only want inline
> static calls and/or mcount then it'd probably be much easier to
> maintain.

IIRC the major stumbling block for arm64 is the whole jump-table thing.
Either they need to rely on compiler plugins to provide objtool that
data (yuck, since we support at least 2 different compilers), disable
jump-tables (yuck, for that limits code-gen just to please a tool) or
use DWARF (yuck, because build times).

There was a little talk about an impromptu 'abi' to communicate
jump-table details to objtool without going full on DWARF, but that
seems to have hit a dead end again.