[PATCH] uml: allow building as 32-bit binary on 64bit host

This patch makes the command:

make ARCH=um SUBARCH=i386

work on x86_64 hosts (with support for building 32-bit binaries). This is
especially needed since 64-bit UMLs don't support 32-bit emulation for guest
binaries, currently. This has been tested in all possible cases and works.

Only exception is that I've built but not tested a 64-bit binary, because I
hadn't a 64-bit filesystem available.

Signed-off-by: Paolo 'Blaisorblade' Giarrusso <[e-mail]>
Cc: Jeff Dike <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

arch/um/Makefile

···
 endif
 SYS_DIR		:= $(ARCH_DIR)/include/sysdep-$(SUBARCH)
 
-include $(srctree)/$(ARCH_DIR)/Makefile-$(SUBARCH)
-
-core-y += $(SUBARCH_CORE)
-libs-y += $(SUBARCH_LIBS)
-
 # -Dvmap=kernel_vmap affects everything, and prevents anything from
 # referencing the libpcap.o symbol so named.
 
···
 
 USER_CFLAGS := $(patsubst -I%,,$(CFLAGS))
 USER_CFLAGS := $(patsubst -D__KERNEL__,,$(USER_CFLAGS)) $(ARCH_INCLUDE) \
-	$(MODE_INCLUDE) $(ARCH_USER_CFLAGS)
+	$(MODE_INCLUDE)
 
 # -Derrno=kernel_errno - This turns all kernel references to errno into
 # kernel_errno to separate them from the libc errno.  This allows -fno-common
···
 
 CFLAGS += -Derrno=kernel_errno -Dsigprocmask=kernel_sigprocmask
 CFLAGS += $(call cc-option,-fno-unit-at-a-time,)
+
+include $(srctree)/$(ARCH_DIR)/Makefile-$(SUBARCH)
 
 #This will adjust *FLAGS accordingly to the platform.
 include $(srctree)/$(ARCH_DIR)/Makefile-os-$(OS)
···
 #The wrappers will select whether using "malloc" or the kernel allocator.
 LINK_WRAPS = -Wl,--wrap,malloc -Wl,--wrap,free -Wl,--wrap,calloc
 
-CFLAGS_vmlinux = $(LINK-y) $(LINK_WRAPS)
+CFLAGS_vmlinux := $(LINK-y) $(LINK_WRAPS)
 define cmd_vmlinux__
 	$(CC) $(CFLAGS_vmlinux) -o $@ \
 	-Wl,-T,$(vmlinux-lds) $(vmlinux-init) \

arch/um/Makefile-i386

···
-SUBARCH_CORE := arch/um/sys-i386/ arch/i386/crypto/
+core-y += arch/um/sys-i386/ arch/i386/crypto/
 
 TOP_ADDR := $(CONFIG_TOP_ADDR)
 
···
   endif
 endif
 
+LDFLAGS			+= -m elf_i386
+ELF_ARCH		:= $(SUBARCH)
+ELF_FORMAT 		:= elf32-$(SUBARCH)
+OBJCOPYFLAGS  		:= -O binary -R .note -R .comment -S
+
+ifeq ("$(origin SUBARCH)", "command line")
+ifneq ("$(shell uname -m | sed -e s/i.86/i386/)", "$(SUBARCH)")
+CFLAGS			+= $(call cc-option,-m32)
+USER_CFLAGS		+= $(call cc-option,-m32)
+HOSTCFLAGS		+= $(call cc-option,-m32)
+HOSTLDFLAGS		+= $(call cc-option,-m32)
+AFLAGS			+= $(call cc-option,-m32)
+LINK-y			+= $(call cc-option,-m32)
+UML_OBJCOPYFLAGS	+= -F $(ELF_FORMAT)
+
+export LDFLAGS HOSTCFLAGS HOSTLDFLAGS UML_OBJCOPYFLAGS
+endif
+endif
+
 CFLAGS += -U__$(SUBARCH)__ -U$(SUBARCH) $(STUB_CFLAGS)
-ARCH_USER_CFLAGS :=
 
 ifneq ($(CONFIG_GPROF),y)
 ARCH_CFLAGS += -DUM_FASTCALL
 endif
 
-ELF_ARCH := $(SUBARCH)
-ELF_FORMAT := elf32-$(SUBARCH)
-
-OBJCOPYFLAGS  := -O binary -R .note -R .comment -S
-
-SYS_UTIL_DIR	:= $(ARCH_DIR)/sys-i386/util
-
-SYS_HEADERS := $(SYS_DIR)/sc.h $(SYS_DIR)/thread.h
+SYS_HEADERS	:= $(SYS_DIR)/sc.h $(SYS_DIR)/thread.h
 
 prepare: $(SYS_HEADERS)
 

arch/um/Makefile-x86_64

···
 # Copyright 2003 - 2004 Pathscale, Inc
 # Released under the GPL
 
-SUBARCH_LIBS := arch/um/sys-x86_64/
+libs-y += arch/um/sys-x86_64/
 START := 0x60000000
 
+#We #undef __x86_64__ for kernelspace, not for userspace where
+#it's needed for headers to work!
 CFLAGS += -U__$(SUBARCH)__ -fno-builtin $(STUB_CFLAGS)
-ARCH_USER_CFLAGS := -D__x86_64__
+USER_CFLAGS += -fno-builtin
 
 ELF_ARCH := i386:x86-64
 ELF_FORMAT := elf64-x86-64

arch/um/scripts/Makefile.unmap

···
 
 quiet_cmd_wrapld = LD      $@
 define cmd_wrapld
-	$(LD) -r -o $(obj)/unmap_tmp.o $< $(shell $(CC) -print-file-name=libc.a); \
-	$(OBJCOPY) $(obj)/unmap_tmp.o $@ -G switcheroo
+	$(LD) $(LDFLAGS) -r -o $(obj)/unmap_tmp.o $< $(shell $(CC) $(CFLAGS) -print-file-name=libc.a); \
+	$(OBJCOPY) $(UML_OBJCOPYFLAGS) $(obj)/unmap_tmp.o $@ -G switcheroo
 endef
 
 $(obj)/unmap_fin.o : $(obj)/unmap.o FORCE