linux-kernel - eh_frame confusion

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Date:   Mon, 02 Mar 2020 22:39:06 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>
To:     Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Michael Ellerman <mpe@...erman.id.au>
Subject: eh_frame confusion

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