linux-kernel - eh_frame confusion

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Message-Id: <1583169883.zo43kx69lm.naveen@linux.ibm.com>
Date:   Mon, 02 Mar 2020 23:02:42 +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

Naveen N. Rao wrote:
> 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.

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