From: "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>
To: Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
"linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>,
Rasmus Villemoes <linux@rasmusvillemoes.dk>,
Michael Ellerman <mpe@ellerman.id.au>
Subject: eh_frame confusion
Date: Mon, 02 Mar 2020 23:02:42 +0530 [thread overview]
Message-ID: <1583169883.zo43kx69lm.naveen@linux.ibm.com> (raw)
In-Reply-To: <1583168442.ovqnxu16tp.naveen@linux.ibm.com>
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
next prev parent reply other threads:[~2020-03-02 17:32 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-02 10:56 eh_frame confusion Rasmus Villemoes
2020-03-02 12:44 ` Segher Boessenkool
2020-03-02 17:17 ` Naveen N. Rao
2020-03-02 17:09 ` Naveen N. Rao
2020-03-02 17:32 ` Naveen N. Rao [this message]
2020-03-03 9:50 ` Rasmus Villemoes
2020-03-05 12:47 ` Naveen N. Rao
2020-03-03 10:28 ` Michael Ellerman
2020-03-05 12:41 ` Naveen N. Rao
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