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

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Date:   Fri, 1 Mar 2019 17:55:03 +0100
From:   Jann Horn <jannh@...gle.com>
To:     Josh Poimboeuf <jpoimboe@...hat.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>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        kernel list <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 1, 2019 at 5:29 PM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>
> 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.

Yeah, that's my concern. I understand that it's irrelevant for tooling
that wants to understand what context a function is running in, but it
might matter to a human trying to understand how a function was
reached. In a theoretical worst case, a stack trace might skip
directly from do_syscall_64() into some random helper function that
received a bad pointer, and that might make debugging harder.