Re: [PATCH] powerpc: allow cross-compilation of ppc64 kernel

On Fri, 2015-11-06 at 23:22 +0100, Laurent Vivier wrote:

Le 06/11/2015 22:09, Scott Wood a écrit :

quoted

On Thu, 2015-11-05 at 12:47 +0100, Laurent Vivier wrote:

quoted

When I try to cross compile a ppc64 kernel, it generally
fails on the VDSO stage. This is true for powerpc64 cross-
compiler, but also when I try to build a ppc64le kernel
on a ppc64 host.

VDSO64L fails:

  VDSO64L arch/powerpc/kernel/vdso64/vdso64.so.dbg
/usr/bin/powerpc64-linux-gnu-ld: arch/powerpc/kernel/vdso64/sigtramp.o:
file class ELFCLASS64 incompatible with ELFCLASS32
/usr/bin/powerpc64-linux-gnu-ld: final link failed: File in wrong format

This fails because gcc calls "collect2" with
"--oformat elf32-powerpcle" with ppc64 objects, without the
"--oformat" ld works well because it use the format of the
first object as output format.

As this case is correctly managed to build the other kernel
objects, this patch replaces $(GCC) by $(LD) to generate the
VDSO objects.

I cross-compile ppc64 kernels and have not seen this problem.  I do need 
to 
pass in -m64 as part of $(CC) if it's not the toolchain default, which is 
not 
nice, but the proper fix for that is to add -m64 in the makefiles -- and 
if I 
don't it fails way before VDSO.

Why is GCC building ppc64 object files but telling the linker --oformat 
elf32-
powerpcle?  Are different options somehow being passed to GCC in one case 
versus the other?

In fact, for all the other parts of the kernel, gcc is called with
"-mlittle-endian -m64", ld with "-EL -m elf64lppc", and thus generates
the good objects and calls ld with the good options ("elf64lppc"). I
think gcc is never used to link, only to compile.
This, I think, comes from:

arch/powerpc/Makefile:

ifeq ($(CONFIG_CPU_LITTLE_ENDIAN),y)
override CC     += -mlittle-endian
override LD     += -EL
...
ifeq ($(HAS_BIARCH),y)
override CC     += -m$(CONFIG_WORD_SIZE)
override LD     += -m elf$(CONFIG_WORD_SIZE)$(LDEMULATION)

OK, that works if I completely remove the CC environment setting rather than 
just remove the -m64 from it.  It looks like that's been there longer than 
I've been building 64-bit kernels, so I wonder what originally prompted me to 
add the environment setting.

Yes, the kernel itself is linked with ld rather than gcc, but that's unusual. 
 There's nothing wrong with using gcc to link.

So at this point, I can:

1- either fix my compiler,
2- or fix the vdso64 linker command.

Do #1, as that's where the bug is, and if you work around it you'll have the 
same problem building other projects if they happen to specify the endianness 
explicitly.

-Scott