linux-kernel - Re: [PATCH 2/2] x86/unwind: add hardcoded ORC entry for NULL

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Date:   Fri, 1 Mar 2019 10:29:11 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        syzbot <syzbot+ca95b2b7aef9e7cbd6ab@...kaller.appspotmail.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        linux-kbuild@...r.kernel.org
Subject: Re: [PATCH 2/2] x86/unwind: add hardcoded ORC entry for NULL

On Fri, Mar 01, 2019 at 10:24:18AM -0600, Josh Poimboeuf wrote:
> > Is there a reason why the top-level Makefile only sets
> > -fno-optimize-sibling-calls if CONFIG_FRAME_POINTER is set?
> > I suspect that this is just a historical thing, because reliable
> > unwinding didn't work without frame pointers until ORC came along.
> > I'm not quite sure how best to express "don't do tail optimization if
> > either frame pointers are used or ORC is used" in a Makefile, and
> > whether we want an indirection through Kconfig for that, so I'm not
> > doing anything about it in this series.
> > Can someone send a patch to deal with it properly?
> 
> Why would sibling calls be a problem for ORC?  Once a function does a
> sibling call, it has effectively returned and shouldn't show up on the
> stack trace anyway.

Answering my own question, I guess some people might find it confusing
to have a caller skipped in the stack trace.  But nobody has ever
complained about it.

It's not a problem for livepatch since we only care about the return
path.

-- 
Josh