[PATCH 03/16] kbuild: add infrastructure to build userspace programs

[Masahiro Yamada]

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 a long time ago.

  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] 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 at least 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 '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 code. 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@xxxxxxxxxx>
---

 Makefile                   | 11 +++++++---
 scripts/Makefile.build     |  5 +++++
 scripts/Makefile.clean     |  2 +-
 scripts/Makefile.userprogs | 44 ++++++++++++++++++++++++++++++++++++++
 4 files changed, 58 insertions(+), 4 deletions(-)
 create mode 100644 scripts/Makefile.userprogs

diff --git a/Makefile b/Makefile
index 49b2709ff44e..f20597820131 100644
--- a/Makefile
+++ b/Makefile
@@ -406,9 +406,11 @@ else
 HOSTCC	= gcc
 HOSTCXX	= g++
 endif
-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
+
+KBUILD_HOSTCFLAGS   := $(KBUILD_USERCFLAGS) $(HOST_LFS_CFLAGS) $(HOSTCFLAGS)
 KBUILD_HOSTCXXFLAGS := -Wall -O2 $(HOST_LFS_CFLAGS) $(HOSTCXXFLAGS)
 KBUILD_HOSTLDFLAGS  := $(HOST_LFS_LDFLAGS) $(HOSTLDFLAGS)
 KBUILD_HOSTLDLIBS   := $(HOST_LFS_LIBS) $(HOSTLDLIBS)
@@ -937,6 +939,9 @@ ifeq ($(CONFIG_RELR),y)
 LDFLAGS_vmlinux	+= --pack-dyn-relocs=relr
 endif
 
+# Align the bit size of userspace programs with the kernel
+KBUILD_USERCFLAGS += $(filter -m32 -m64, $(KBUILD_CFLAGS))
+
 # make the checker run with the right architecture
 CHECKFLAGS += --arch=$(ARCH)
 
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 9fcbfac15d1d..94f2f7016172 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -50,6 +50,11 @@ ifneq ($(hostprogs)$(hostcxxlibs-y)$(hostcxxlibs-m),)
 include scripts/Makefile.host
 endif
 
+# Do not include userprogs rules unless needed
+ifneq ($(userprogs),)
+include scripts/Makefile.userprogs
+endif
+
 ifndef obj
 $(warning kbuild: Makefile.build is included improperly)
 endif
diff --git a/scripts/Makefile.clean b/scripts/Makefile.clean
index 075f0cc2d8d7..e2c76122319d 100644
--- a/scripts/Makefile.clean
+++ b/scripts/Makefile.clean
@@ -29,7 +29,7 @@ subdir-ymn	:= $(addprefix $(obj)/,$(subdir-ymn))
 
 __clean-files	:= $(extra-y) $(extra-m) $(extra-)       \
 		   $(always) $(always-y) $(always-m) $(always-) $(targets) $(clean-files)   \
-		   $(hostprogs) $(hostprogs-y) $(hostprogs-m) $(hostprogs-) \
+		   $(hostprogs) $(hostprogs-y) $(hostprogs-m) $(hostprogs-) $(userprogs) \
 		   $(hostcxxlibs-y) $(hostcxxlibs-m)
 
 __clean-files   := $(filter-out $(no-clean-files), $(__clean-files))
diff --git a/scripts/Makefile.userprogs b/scripts/Makefile.userprogs
new file mode 100644
index 000000000000..0d987085819b
--- /dev/null
+++ b/scripts/Makefile.userprogs
@@ -0,0 +1,44 @@
+# SPDX-License-Identifier: GPL-2.0-only
+#
+# Build userspace programs for the target system
+#
+
+userprogs 	:= $(sort $(userprogs))
+
+# Executables compiled from a single .c file
+user-csingle	:= $(foreach m, $(userprogs), $(if $($(m)-objs),,$(m)))
+
+# C executables linked based on several .o files
+user-cmulti	:= $(foreach m, $(userprogs), $(if $($(m)-objs),$(m)))
+
+# Object (.o) files compiled from .c files
+user-cobjs	:= $(foreach m, $(userprogs), $($(m)-objs))
+
+user-csingle	:= $(addprefix $(obj)/, $(user-csingle))
+user-cmulti	:= $(addprefix $(obj)/, $(user-cmulti))
+user-cobjs	:= $(addprefix $(obj)/, $(user-cobjs))
+
+user_c_flags	= -Wp,-MMD,$(depfile) $(KBUILD_USERCFLAGS) $(user-ccflags) \
+			$($(target-stem)-ccflags)
+
+# Create an executable from a single .c file
+quiet_cmd_user_cc_c = CC [U]  $@
+      cmd_user_cc_c = $(CC) $(user_c_flags) -o $@ $<
+$(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) -o $@ $(addprefix $(obj)/, $($(target-stem)-objs)) \
+                      $($(target-stem)-ldlibs)
+$(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_c_flags) -c -o $@ $<
+$(user-cobjs): $(obj)/%.o: $(src)/%.c FORCE
+	$(call if_changed_dep,user_cc_o_c)
+
+targets += $(user-csingle) $(user-cmulti) $(user-cobjs)
-- 
2.25.1