From: "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>
To: Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
"linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>,
Rasmus Villemoes <linux@rasmusvillemoes.dk>,
Michael Ellerman <mpe@ellerman.id.au>
Subject: eh_frame confusion
Date: Mon, 02 Mar 2020 22:39:06 +0530 [thread overview]
Message-ID: <1583168442.ovqnxu16tp.naveen@linux.ibm.com> (raw)
In-Reply-To: <3b00b45f-74b5-13e3-9a98-c3d6b3bb7286@rasmusvillemoes.dk>
Rasmus Villemoes wrote:
> 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
Michael opened a task to look into this recently and I had spent some
time last week on this. The original commit/discussion adding
-fno-dwarf2-cfi-asm refers to R_PPC64_REL32 relocations not being
handled by our module loader:
http://lkml.kernel.org/r/20090224065112.GA6690@bombadil.infradead.org
However, that is now handled thanks to commit 9f751b82b491d:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=9f751b82b491d
I did a test build and a simple module loaded fine, so I think
-fno-dwarf2-cfi-asm is not required anymore, unless Michael has seen
some breakages with it. Michael?
>
> 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?
As Segher points out, it looks like we need to add
-fno-asynchronous-unwind-tables. Most other architectures seem to use
that too.
- Naveen
next prev parent reply other threads:[~2020-03-02 17:09 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-02 10:56 eh_frame confusion Rasmus Villemoes
2020-03-02 12:44 ` Segher Boessenkool
2020-03-02 17:17 ` Naveen N. Rao
2020-03-02 17:09 ` Naveen N. Rao [this message]
2020-03-02 17:32 ` Naveen N. Rao
2020-03-03 9:50 ` Rasmus Villemoes
2020-03-05 12:47 ` Naveen N. Rao
2020-03-03 10:28 ` Michael Ellerman
2020-03-05 12:41 ` Naveen N. Rao
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