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

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Date:   Mon, 5 Feb 2018 21:30:48 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Andi Kleen <ak@...ux.intel.com>
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 5, 2018 at 5:22 PM, Andi Kleen <ak@...ux.intel.com> wrote:
> 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.

I think it's a bit different from my first interpretation, this is what I see
in the output of 'objdump -Dr vmlinux' after the final LTO link:

b1da26f0 <amba_console.lto_priv.21118>:
b1da26f0:       74747941        cmnmi   r9, r4, ror r4
b1da26f4:       4d000000        andeq   r0, r0, sp, asr #32
        ...
b1da2700:       b0bdfa08        ldmeq   sl!, {r4, r5, r7, r8, sl, fp,
ip, sp, pc}^
b1da2704:       00000000        andeq   r0, r0, r0
b1da2708:       b0bcc790        strhls  fp, [r7], #192  ; 0xc0
b1da270c:       00000000        andeq   r0, r0, r0
b1da2710:       b1a2bb18        ldmne   fp!, {r0, r4, r5, r7, r9, sp, pc}
b1da2714:       00000000        andeq   r0, r0, r0
b1da2718:       0001ffff                        ; <UNDEFINED>
instruction: 0xffff0100
b1da271c:       00000000        andeq   r0, r0, r0
b1da2720:       b1da0938        stmdacc r9, {r0, r4, r5, r7, r9, fp, ip, lr, pc}
b1da2724:       00000000        andeq   r0, r0, r0


b1da2728 <amba_console.lto_priv.21119>:
b1da2728:       74747941        cmnmi   r9, r4, ror r4
b1da272c:       4d410000        andeq   r4, r0, sp, asr #2
        ...
b1da2738:       b0be163c        ldccc   14, cr11, [r6], {176}   ; 0xb0
b1da273c:       00000000        andeq   r0, r0, r0
b1da2740:       b0bcc790        strhls  fp, [r7], #192  ; 0xc0
b1da2744:       00000000        andeq   r0, r0, r0
b1da2748:       b1a2b8b8        ldmlt   r8!, {r0, r4, r5, r7, r9, sp, pc}
b1da274c:       b1a2e6d8        stmiale r6!, {r0, r4, r5, r7, r9, sp, pc}^
b1da2750:       0011ffff                        ; <UNDEFINED>
instruction: 0xffff1100
b1da2754:       00000000        andeq   r0, r0, r0
b1da2758:       b1d9fed8        ldmle   lr!, {r0, r4, r5, r7, r8, fp,
ip, lr, pc}^
b1da275c:       00000000        andeq   r0, r0, r0

I got a warning from scripts/mod/modpost about vmlinux.o before it,
which has relocations from each of these two into an init function.

My interpretation now is that LTO adds the .lto_priv.21118 suffix for
the two structures to disambiguate them, as one comes from
amba-pl010.c and the other one is from amba-pl011.c.
This means that the whitelist in modpost no longer works, as it
checks the name against the "*_console" glob string.

Almost all other symbols are not affected because the names of the
static symbols are unique, and LTO gets to drop the suffix.

         Arnd