linux-kernel - Re: [PATCH] powerpc: vdso: fix building with wrong-endian toolchain

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Message-Id: <23ddf649-f70b-47b0-90df-af572fd50504@app.fastmail.com>
Date: Fri, 07 Jun 2024 16:11:25 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Michael Ellerman" <mpe@...erman.id.au>, "Arnd Bergmann" <arnd@...nel.org>
Cc: "Masahiro Yamada" <masahiroy@...nel.org>,
 "Nathan Chancellor" <nathan@...nel.org>,
 "Nicolas Schier" <nicolas@...sle.eu>, linux-kbuild@...r.kernel.org,
 "Nicholas Piggin" <npiggin@...il.com>,
 "Christophe Leroy" <christophe.leroy@...roup.eu>,
 "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>, linuxppc-dev@...ts.ozlabs.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc: vdso: fix building with wrong-endian toolchain

On Fri, Jun 7, 2024, at 14:42, Michael Ellerman wrote:
> Arnd Bergmann <arnd@...nel.org> writes:
>>
>> Signed-off-by: Arnd Bergmann <arnd@...db.de>
>> ---
>> I'm fairly sure this worked in the past, but I did not try to bisect the
>> issue.
>
> It still works for me.
>
> I use the korg toolchains every day, and kisskb uses them too.
>
> What commit / defconfig are you seeing the errors with?
>
> Is it just the 12.3.0 toolchain or all of them? I just tested 12.3.0
> here and it built OK.
>
> I guess you're building on x86 or arm64? I build on ppc64le, I wonder if
> that makes a difference.
>
> The patch is probably OK regardless, but I'd rather understand what the
> actual problem is.

I tested again and found that the problem is actually part of my
local build setup, which overrides the 'CPP' variable in the
top-level makefile that I use for building multiple kernels
concurrently.

This ends up clashing with this other line that only
powerpc sets:

arch/powerpc/Makefile:CPP               = $(CC) -E $(KBUILD_CFLAGS)

It's rare that someone overrides CPP, so quite possibly I'm
the only one that has seen this so far, but it also seems like
it should be possible to do that.

This patch seems to work as well for me, and is a little
more logical, but it's also more invasive and has a
higher regression risk:

8<---------
diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
index 65261cbe5bfd..9ad4ca318e34 100644
--- a/arch/powerpc/Makefile
+++ b/arch/powerpc/Makefile
@@ -62,14 +62,14 @@ KBUILD_LDFLAGS_MODULE += arch/powerpc/lib/crtsavres.o
 endif
 
 ifdef CONFIG_CPU_LITTLE_ENDIAN
-KBUILD_CFLAGS  += -mlittle-endian
+KBUILD_CPPFLAGS        += -mlittle-endian
 KBUILD_LDFLAGS += -EL
 LDEMULATION    := lppc
 GNUTARGET      := powerpcle
 MULTIPLEWORD   := -mno-multiple
 KBUILD_CFLAGS_MODULE += $(call cc-option,-mno-save-toc-indirect)
 else
-KBUILD_CFLAGS += $(call cc-option,-mbig-endian)
+KBUILD_CPPFLAGS += $(call cc-option,-mbig-endian)
 KBUILD_LDFLAGS += -EB
 LDEMULATION    := ppc
 GNUTARGET      := powerpc
@@ -95,7 +95,7 @@ aflags-$(CONFIG_CPU_BIG_ENDIAN)               += $(call cc-option,-mbig-endian)
 aflags-$(CONFIG_CPU_LITTLE_ENDIAN)     += -mlittle-endian
 
 ifeq ($(HAS_BIARCH),y)
-KBUILD_CFLAGS  += -m$(BITS)
+KBUILD_CPPFLAGS        += -m$(BITS)
 KBUILD_AFLAGS  += -m$(BITS)
 KBUILD_LDFLAGS += -m elf$(BITS)$(LDEMULATION)
 endif
@@ -176,7 +176,6 @@ KBUILD_CPPFLAGS     += -I $(srctree)/arch/powerpc $(asinstr)
 KBUILD_AFLAGS  += $(AFLAGS-y)
 KBUILD_CFLAGS  += $(call cc-option,-msoft-float)
 KBUILD_CFLAGS  += $(CFLAGS-y)
-CPP            = $(CC) -E $(KBUILD_CFLAGS)
 
 CHECKFLAGS     += -m$(BITS) -D__powerpc__ -D__powerpc$(BITS)__
 ifdef CONFIG_CPU_BIG_ENDIAN
diff --git a/arch/powerpc/kernel/vdso/Makefile b/arch/powerpc/kernel/vdso/Makefile
index 1b93655c2857..3516e71926e5 100644
--- a/arch/powerpc/kernel/vdso/Makefile
+++ b/arch/powerpc/kernel/vdso/Makefile
@@ -59,7 +59,7 @@ ldflags-$(CONFIG_LD_IS_LLD) += $(call cc-option,--ld-path=$(LD),-fuse-ld=lld)
 ldflags-$(CONFIG_LD_ORPHAN_WARN) += -Wl,--orphan-handling=$(CONFIG_LD_ORPHAN_WARN_LEVEL)
 
 # Filter flags that clang will warn are unused for linking
-ldflags-y += $(filter-out $(CC_AUTO_VAR_INIT_ZERO_ENABLER) $(CC_FLAGS_FTRACE) -Wa$(comma)%, $(KBUILD_CFLAGS))
+ldflags-y += $(filter-out $(CC_AUTO_VAR_INIT_ZERO_ENABLER) $(CC_FLAGS_FTRACE) -Wa$(comma)%, $(KBUILD_CPPFLAGS) $(KBUILD_CFLAGS))
 
 CC32FLAGS := -m32
 LD32FLAGS := -Wl,-soname=linux-vdso32.so.1
--------->8

     Arnd