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@xxxxxxxxxxxxxxxxxxxxxx
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
