[v2,3/5] powerpc/boot: Ensure _zimage_start is a weak symbol

When building with clang crt0's _zimage_start is not marked weak, which
breaks the build when linking the kernel image:

 $ objdump -t arch/powerpc/boot/crt0.o |grep _zimage_start$
 0000000000000058 g       .text  0000000000000000 _zimage_start

 ld: arch/powerpc/boot/wrapper.a(crt0.o): in function '_zimage_start':
 (.text+0x58): multiple definition of '_zimage_start';
 arch/powerpc/boot/pseries-head.o:(.text+0x0): first defined here

Clang requires the .weak directive to appear after the symbol is
declared. The binutils manual says:

 This directive sets the weak attribute on the comma separated list of
 symbol names. If the symbols do not already exist, they will be
 created.

So it appears this is different with clang. The only reference I could
see for this was an OpenBSD mailing list post[1].

Changing it to be after the declaration fixes building with Clang, and
still works with GCC.

 $ objdump -t arch/powerpc/boot/crt0.o |grep _zimage_start$
 0000000000000058  w      .text	0000000000000000 _zimage_start

Reported to clang as https://bugs.llvm.org/show_bug.cgi?id=38921

[1] https://groups.google.com/forum/#!topic/fa.openbsd.tech/PAgKKen2YCY

Signed-off-by: Joel Stanley <joel@jms.id.au>
---
v2: Add comment explaining this is a workaround for clang. I have also
opened a clang bug to see if they plan on changing behaviour
---
 arch/powerpc/boot/crt0.S | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

On Thu, Sep 13, 2018 at 9:07 PM Joel Stanley <joel@jms.id.au> wrote:
>
> When building with clang crt0's _zimage_start is not marked weak, which
> breaks the build when linking the kernel image:
>
>  $ objdump -t arch/powerpc/boot/crt0.o |grep _zimage_start$
>  0000000000000058 g       .text  0000000000000000 _zimage_start
>
>  ld: arch/powerpc/boot/wrapper.a(crt0.o): in function '_zimage_start':
>  (.text+0x58): multiple definition of '_zimage_start';
>  arch/powerpc/boot/pseries-head.o:(.text+0x0): first defined here
>
> Clang requires the .weak directive to appear after the symbol is
> declared. The binutils manual says:
>
>  This directive sets the weak attribute on the comma separated list of
>  symbol names. If the symbols do not already exist, they will be
>  created.
>
> So it appears this is different with clang. The only reference I could
> see for this was an OpenBSD mailing list post[1].
>
> Changing it to be after the declaration fixes building with Clang, and
> still works with GCC.
>
>  $ objdump -t arch/powerpc/boot/crt0.o |grep _zimage_start$
>  0000000000000058  w      .text 0000000000000000 _zimage_start
>
> Reported to clang as https://bugs.llvm.org/show_bug.cgi?id=38921
>
> [1] https://groups.google.com/forum/#!topic/fa.openbsd.tech/PAgKKen2YCY
>
> Signed-off-by: Joel Stanley <joel@jms.id.au>
> ---
> v2: Add comment explaining this is a workaround for clang. I have also
> opened a clang bug to see if they plan on changing behaviour
> ---
>  arch/powerpc/boot/crt0.S | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/boot/crt0.S b/arch/powerpc/boot/crt0.S
> index e453e011d7e7..4e32bd251b99 100644
> --- a/arch/powerpc/boot/crt0.S
> +++ b/arch/powerpc/boot/crt0.S
> @@ -47,8 +47,10 @@ p_end:               .long   _end
>  p_pstack:      .long   _platform_stack_top
>  #endif
>
> -       .weak   _zimage_start
>         .globl  _zimage_start
> +       /* Clang appears to require the .weak directive to be after the symbol
> +        * is defined. See https://bugs.llvm.org/show_bug.cgi?id=38921  */
> +       .weak   _zimage_start
>  _zimage_start:
>         .globl  _zimage_start_lib
>  _zimage_start_lib:
> --
> 2.17.1
>

Sure, definitely a long tail of binutils/GAS functionality to match
from Clang's integrated assembler, but this is a small fix that
results in no functional change for GCC/binutils and enables
Clang/bfd.
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

On Fri, 2018-09-14 at 04:06:47 UTC, Joel Stanley wrote:
> When building with clang crt0's _zimage_start is not marked weak, which
> breaks the build when linking the kernel image:
> 
>  $ objdump -t arch/powerpc/boot/crt0.o |grep _zimage_start$
>  0000000000000058 g       .text  0000000000000000 _zimage_start
> 
>  ld: arch/powerpc/boot/wrapper.a(crt0.o): in function '_zimage_start':
>  (.text+0x58): multiple definition of '_zimage_start';
>  arch/powerpc/boot/pseries-head.o:(.text+0x0): first defined here
> 
> Clang requires the .weak directive to appear after the symbol is
> declared. The binutils manual says:
> 
>  This directive sets the weak attribute on the comma separated list of
>  symbol names. If the symbols do not already exist, they will be
>  created.
> 
> So it appears this is different with clang. The only reference I could
> see for this was an OpenBSD mailing list post[1].
> 
> Changing it to be after the declaration fixes building with Clang, and
> still works with GCC.
> 
>  $ objdump -t arch/powerpc/boot/crt0.o |grep _zimage_start$
>  0000000000000058  w      .text	0000000000000000 _zimage_start
> 
> Reported to clang as https://bugs.llvm.org/show_bug.cgi?id=38921
> 
> [1] https://groups.google.com/forum/#!topic/fa.openbsd.tech/PAgKKen2YCY
> 
> Signed-off-by: Joel Stanley <joel@jms.id.au>
> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

Applied to powerpc next, thanks.

https://git.kernel.org/powerpc/c/ee9d21b3b3583712029a0db65a4b7c

cheers