linux-kernel - eh_frame confusion

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Message-Id: <1583412126.fc8zskkv66.naveen@linux.ibm.com>
Date:   Thu, 05 Mar 2020 18:17:21 +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>
Cc:     Segher Boessenkool <segher@...nel.crashing.org>
Subject: eh_frame confusion

Naveen N. Rao wrote:
> Naveen N. Rao wrote:
>> Rasmus Villemoes wrote:
<snip>
> 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)

In terms of the CFI information, the primary difference I see with 
-fno-dwarf2-cfi-asm is that when dumping the debug frames, CIE indicates 
version 3, while otherwise (i.e., without -fno-dwarf2-cfi-asm and 
with/without -fasynchronous-unwind-tables), it is version 1, regardless 
of -gdwarf-2/-gdwarf-4. There are few more minor changes, but none of 
these looked significant to me.

>  
>  # 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

The above vdso hunks can be dropped since all our VDSO are assembly, so 
the above have no impact.


- Naveen