From: Masahiro Yamada <masahiroy@kernel.org>
Date: Wed, 29 Apr 2020 12:45:14 +0900
Subject: kbuild: add infrastructure to build userspace programs
Patch-mainline: v5.8-rc1
Git-commit: 7f3a59db274c3e3d884c785e363a054110f1c266
References: bsc#1177028

Kbuild supports the infrastructure to build host programs, but there
was no support to build userspace programs for the target architecture
(i.e. the same architecture as the kernel).

Sam Ravnborg worked on this in 2014 (https://lkml.org/lkml/2014/7/13/154),
but it was not merged. One problem at that time was, there was no good way
to know whether $(CC) can link standalone programs. In fact, pre-built
kernel.org toolchains [1] are often used for building the kernel, but they
do not provide libc.

Now, we can handle this cleanly because the compiler capability is
evaluated at the Kconfig time. If $(CC) cannot link standalone programs,
the relevant options are hidden by 'depends on CC_CAN_LINK'.

The implementation just mimics scripts/Makefile.host

The userspace programs are compiled with the same flags as the host
programs. In addition, it uses -m32 or -m64 if it is found in
$(KBUILD_CFLAGS).

This new syntax has two usecases.

- Sample programs

  Several userspace programs under samples/ include UAPI headers
  installed in usr/include. Most of them were previously built for
  the host architecture just to use the 'hostprogs' syntax.

  However, 'make headers' always works for the target architecture.
  This caused the arch mismatch in cross-compiling. To fix this
  distortion, sample code should be built for the target architecture.

- Bpfilter

  net/bpfilter/Makefile compiles bpfilter_umh as the user mode helper,
  and embeds it into the kernel. Currently, it overrides HOSTCC with
  CC to use the 'hostprogs' syntax. This hack should go away.

[1]: https://mirrors.edge.kernel.org/pub/tools/crosstool/
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Gary Lin <glin@suse.com>

NOTE from Gary:
  Fix the conflicts in Makefile and scripts/Makefile.clean

---
 Makefile                   |   13 ++++++++++---
 scripts/Makefile.build     |    6 ++++++
 scripts/Makefile.clean     |    2 +-
 scripts/Makefile.userprogs |   45 +++++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 62 insertions(+), 4 deletions(-)
 create mode 100644 scripts/Makefile.userprogs

--- a/Makefile
+++ b/Makefile
@@ -400,9 +400,12 @@ HOST_LFS_LIBS := $(shell getconf LFS_LIB
 
 HOSTCC       = gcc
 HOSTCXX      = g++
-KBUILD_HOSTCFLAGS   := -Wall -Wmissing-prototypes -Wstrict-prototypes -O2 \
-		-fomit-frame-pointer -std=gnu89 $(HOST_LFS_CFLAGS) \
-		$(HOSTCFLAGS)
+
+export KBUILD_USERCFLAGS := -Wall -Wmissing-prototypes -Wstrict-prototypes \
+			      -O2 -fomit-frame-pointer -std=gnu89
+export KBUILD_USERLDFLAGS :=
+
+KBUILD_HOSTCFLAGS   := $(KBUILD_USERCFLAGS) $(HOST_LFS_CFLAGS) $(HOSTCFLAGS)
 KBUILD_HOSTCXXFLAGS := -O2 $(HOST_LFS_CFLAGS) $(HOSTCXXFLAGS)
 KBUILD_HOSTLDFLAGS  := $(HOST_LFS_LDFLAGS) $(HOSTLDFLAGS)
 KBUILD_HOSTLDLIBS   := $(HOST_LFS_LIBS) $(HOSTLDLIBS)
@@ -912,6 +915,10 @@ ifeq ($(CONFIG_STRIP_ASM_SYMS),y)
 LDFLAGS_vmlinux	+= $(call ld-option, -X,)
 endif
 
+# Align the bit size of userspace programs with the kernel
+KBUILD_USERCFLAGS  += $(filter -m32 -m64, $(KBUILD_CFLAGS))
+KBUILD_USERLDFLAGS += $(filter -m32 -m64, $(KBUILD_CFLAGS))
+
 # insure the checker run with the right endianness
 CHECKFLAGS += $(if $(CONFIG_CPU_BIG_ENDIAN),-mbig-endian,-mlittle-endian)
 
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -48,6 +48,12 @@ ifneq ($(hostprogs-y)$(hostprogs-m)$(hos
 include scripts/Makefile.host
 endif
 
+# Do not include userprogs rules unless needed.
+userprogs := $(sort $(userprogs))
+ifneq ($(userprogs),)
+include scripts/Makefile.userprogs
+endif
+
 ifndef obj
 $(warning kbuild: Makefile.build is included improperly)
 endif
--- a/scripts/Makefile.clean
+++ b/scripts/Makefile.clean
@@ -38,7 +38,7 @@ subdir-ymn	:= $(addprefix $(obj)/,$(subd
 
 __clean-files	:= $(extra-y) $(extra-m) $(extra-)       \
 		   $(always) $(targets) $(clean-files)   \
-		   $(hostprogs-y) $(hostprogs-m) $(hostprogs-) \
+		   $(hostprogs-y) $(hostprogs-m) $(hostprogs-) $(userprogs) \
 		   $(hostcxxlibs-y) $(hostcxxlibs-m)
 
 __clean-files   := $(filter-out $(no-clean-files), $(__clean-files))
--- /dev/null
+++ b/scripts/Makefile.userprogs
@@ -0,0 +1,45 @@
+# SPDX-License-Identifier: GPL-2.0-only
+#
+# Build userspace programs for the target system
+#
+
+# Executables compiled from a single .c file
+user-csingle	:= $(foreach m, $(userprogs), $(if $($(m)-objs),,$(m)))
+
+# Executables linked based on several .o files
+user-cmulti	:= $(foreach m, $(userprogs), $(if $($(m)-objs),$(m)))
+
+# Objects compiled from .c files
+user-cobjs	:= $(sort $(foreach m, $(userprogs), $($(m)-objs)))
+
+user-csingle	:= $(addprefix $(obj)/, $(user-csingle))
+user-cmulti	:= $(addprefix $(obj)/, $(user-cmulti))
+user-cobjs	:= $(addprefix $(obj)/, $(user-cobjs))
+
+user_ccflags	= -Wp,-MMD,$(depfile) $(KBUILD_USERCFLAGS) $(userccflags) \
+			$($(target-stem)-userccflags)
+user_ldflags	= $(KBUILD_USERLDFLAGS) $(userldflags) $($(target-stem)-userldflags)
+
+# Create an executable from a single .c file
+quiet_cmd_user_cc_c = CC [U]  $@
+      cmd_user_cc_c = $(CC) $(user_ccflags) $(user_ldflags) -o $@ $< \
+		      $($(target-stem)-userldlibs)
+$(user-csingle): $(obj)/%: $(src)/%.c FORCE
+	$(call if_changed_dep,user_cc_c)
+
+# Link an executable based on list of .o files
+quiet_cmd_user_ld = LD [U]  $@
+      cmd_user_ld = $(CC) $(user_ldflags) -o $@ \
+		    $(addprefix $(obj)/, $($(target-stem)-objs)) \
+		    $($(target-stem)-userldlibs)
+$(user-cmulti): FORCE
+	$(call if_changed,user_ld)
+$(call multi_depend, $(user-cmulti), , -objs)
+
+# Create .o file from a .c file
+quiet_cmd_user_cc_o_c = CC [U]  $@
+      cmd_user_cc_o_c = $(CC) $(user_ccflags) -c -o $@ $<
+$(user-cobjs): $(obj)/%.o: $(src)/%.c FORCE
+	$(call if_changed_dep,user_cc_o_c)
+
+targets += $(user-csingle) $(user-cmulti) $(user-cobjs)