linux-kernel - Re: [PATCH 1/2] modpost: don't add warnings for LTO-generated symbols

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Message-ID: <20180205162211.GM30338@tassilo.jf.intel.com>
Date:   Mon, 5 Feb 2018 08:22:11 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Jessica Yu <jeyu@...nel.org>, Kees Cook <keescook@...omium.org>,
        Nicolas Pitre <nico@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Wanlong Gao <wanlong.gao@...il.com>,
        Luis de Bethencourt <luisbg@...nel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>
Subject: Re: [PATCH 1/2] modpost: don't add warnings for LTO-generated symbols

On Mon, Feb 05, 2018 at 04:59:32PM +0100, Arnd Bergmann wrote:
> On Mon, Feb 5, 2018 at 4:50 PM, Andi Kleen <ak@...ux.intel.com> wrote:
> > On Mon, Feb 05, 2018 at 04:12:52PM +0100, Arnd Bergmann wrote:
> >> WARNING: vmlinux.o(.data+0x12e0): Section mismatch in reference from the variable pfkey_net_ops.lto_priv.2992 to the function .init.text:pfkey_net_init.lto_priv.2977()
> >> The variable pfkey_net_ops.lto_priv.2992 references
> >> the function __init pfkey_net_init.lto_priv.2977()
> >> If the reference is valid then annotate the
> >> variable with __init* or __refdata (see linux/init.h) or name the variable:
> >> *_template, *_timer, *_sht, *_ops, *_probe, *_probe_one, *_console
> >
> > A better fix would be to ensure modpost always runs on already LD_FINALed
> > objects, so it never sees LTO.
> >
> > Otherwise you would need to teach modpost about the LTO symbol table
> > and some other magic. I did that once, but it turned out to be very
> > ugly.
> 
> I'm not sure I understand what that means. Do you mean that with LD_FINAL,
> those symbol references are completely eliminated so we don't need to worry
> about them any more?

Yes there should not be any .lto_priv references after LDFINAL, it will
be just like any other ELF object. We also need to do the same
thing before objtool.

> 
> I got about a dozen section mismatch errors with LTO in cases where the
> calling function gets a specialized version of a structure, and my oneline
> patch above addresses them all.

Not sure I follow here. .lto_priv should be anything LTO, not just
some specific optimizations.

-Andi