linux-kernel - eh_frame confusion

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Message-ID: <3b00b45f-74b5-13e3-9a98-c3d6b3bb7286@rasmusvillemoes.dk>
Date:   Mon, 2 Mar 2020 11:56:05 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     LKML <linux-kernel@...r.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: eh_frame confusion

I'm building a ppc32 kernel, and noticed that after upgrading from gcc-7
to gcc-8 all object files now end up having .eh_frame section. For
vmlinux, that's not a problem, because they all get discarded in
arch/powerpc/kernel/vmlinux.lds.S . However, they stick around in
modules, which doesn't seem to be useful - given that everything worked
just fine with gcc-7, and I don't see anything in the module loader that
handles .eh_frame.

The reason I care is that my target has a rather tight rootfs budget,
and the .eh_frame section seem to occupy 10-30% of the file size
(obviously very depending on the particular module).

Comparing the .foo.o.cmd files, I don't see change in options that might
explain this (there's a bunch of new -Wno-*, and the -mspe=no spelling
is apparently no longer supported in gcc-8). Both before and after, there's

-fno-dwarf2-cfi-asm

about which gcc's documentation says

'-fno-dwarf2-cfi-asm'
     Emit DWARF unwind info as compiler generated '.eh_frame' section
     instead of using GAS '.cfi_*' directives.

Looking into where that comes from got me even more confused, because
both arm and unicore32 say

# Never generate .eh_frame
KBUILD_CFLAGS           += $(call cc-option,-fno-dwarf2-cfi-asm)

while the ppc32 case at hand says

# FIXME: the module load should be taught about the additional relocs
# generated by this.
# revert to pre-gcc-4.4 behaviour of .eh_frame

but prior to gcc-8, .eh_frame didn't seem to get generated anyway.

Can .eh_frame sections be discarded for modules (on ppc32 at least), or
is there some magic that makes them necessary when building with gcc-8?

Rasmus