eh_frame confusion

Naveen N. Rao wrote:

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

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?

quoted

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.

Can you check if the below patch works? I am yet to test this in more 
detail, but would be good to know the implications for ppc32.

- Naveen


---

diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
index f35730548e42..5b5bf98b8217 100644
--- a/arch/powerpc/Makefile
+++ b/arch/powerpc/Makefile

@@ -239,10 +239,7 @@ KBUILD_CFLAGS += $(call cc-option,-mno-vsx)
 KBUILD_CFLAGS += $(call cc-option,-mno-spe)
 KBUILD_CFLAGS += $(call cc-option,-mspe=no)
 
-# FIXME: the module load should be taught about the additional relocs
-# generated by this.
-# revert to pre-gcc-4.4 behaviour of .eh_frame
-KBUILD_CFLAGS  += $(call cc-option,-fno-dwarf2-cfi-asm)
+KBUILD_CFLAGS  += $(call cc-option,-fno-asynchronous-unwind-tables)
 
 # Never use string load/store instructions as they are
 # often slow when they are implemented at all

diff --git a/arch/powerpc/kernel/vdso32/Makefile b/arch/powerpc/kernel/vdso32/Makefile
index e147bbdc12cd..d43b0b18137c 100644
--- a/arch/powerpc/kernel/vdso32/Makefile
+++ b/arch/powerpc/kernel/vdso32/Makefile

@@ -25,6 +25,7 @@ KCOV_INSTRUMENT := n
 UBSAN_SANITIZE := n
 
 ccflags-y := -shared -fno-common -fno-builtin -nostdlib \
+       -fasynchronous-unwind-tables \
        -Wl,-soname=linux-vdso32.so.1 -Wl,--hash-style=both
 asflags-y := -D__VDSO32__ -s
 

diff --git a/arch/powerpc/kernel/vdso64/Makefile b/arch/powerpc/kernel/vdso64/Makefile
index 32ebb3522ea1..b2cbb5c49bad 100644
--- a/arch/powerpc/kernel/vdso64/Makefile
+++ b/arch/powerpc/kernel/vdso64/Makefile

@@ -13,6 +13,7 @@ KCOV_INSTRUMENT := n
 UBSAN_SANITIZE := n
 
 ccflags-y := -shared -fno-common -fno-builtin -nostdlib \
+       -fasynchronous-unwind-tables \
        -Wl,-soname=linux-vdso64.so.1 -Wl,--hash-style=both
 asflags-y := -D__VDSO64__ -s