Re: eh_frame confusion

Michael Ellerman wrote:

"Naveen N. Rao" [off-list ref] writes:

quoted

Rasmus Villemoes wrote:

quoted

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

I opened that issue purely based on noticing the wart in the Makefile,
not because I'd actually tested it.

quoted

However, that is now handled thanks to commit 9f751b82b491d:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=9f751b82b491d

Haha, written by me, what an idiot.

So the Makefile hack can presumably be dropped, because the module
loader can handle the relocations.

And then maybe we also want to turn off the unwind tables, but that
would be a separate patch.

quoted

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?

No, as I said above it was just reading the Makefile.

Ok, thanks for clarifying. To test, I did 'allmodconfig' builds across 
three environments:
- gcc (Ubuntu 9.2.1-9ubuntu2) 9.2.1 20191008 -- ppc64le
- gcc (SUSE Linux) 7.5.0 -- ppc64le
- gcc (GCC) 8.2.1 20181215 (Red Hat 8.2.1-6) -- ppc64 (BE)

Then, used the below command to list all relocations in the modules:
$ find . -name '*.ko' | xargs -n 1 readelf -Wr  | grep -v "Relocation " | grep -v "Offset " | cut -d' ' -f4 | sort | uniq

R_PPC64_ADDR32
R_PPC64_ADDR64
R_PPC64_ENTRY
R_PPC64_REL24
R_PPC64_REL32
R_PPC64_REL64
R_PPC64_TOC
R_PPC64_TOC16_HA
R_PPC64_TOC16_LO
R_PPC64_TOC16_LO_DS

All three environments show up similar set of relocations, all of which 
we handle in the module loader today.

If Rasmus/Christophe can confirm that this is true for ppc32 as well, 
then we should be fine.

- Naveen