kbuild: add CONFIG_HEADERS_INSTALL and loosen the dependency of samples

Commit 5318321d ("samples: disable CONFIG_SAMPLES for UML") used
a big hammer to fix the build errors under the samples/ directory.
Only some samples actually include uapi headers from usr/include.

Introduce CONFIG_HEADERS_INSTALL since 'depends on HEADERS_INSTALL' is
clearer than 'depends on !UML'. If this option is enabled, uapi headers
are installed before starting directory descending.

I added 'depends on HEADERS_INSTALL' to per-sample CONFIG options.
This allows UML to compile some samples.

$ make ARCH=um allmodconfig samples/
  [ snip ]
  CC [M]  samples/configfs/configfs_sample.o
  CC [M]  samples/kfifo/bytestream-example.o
  CC [M]  samples/kfifo/dma-example.o
  CC [M]  samples/kfifo/inttype-example.o
  CC [M]  samples/kfifo/record-example.o
  CC [M]  samples/kobject/kobject-example.o
  CC [M]  samples/kobject/kset-example.o
  CC [M]  samples/trace_events/trace-events-sample.o
  CC [M]  samples/trace_printk/trace-printk.o
  AR      samples/vfio-mdev/built-in.a
  AR      samples/built-in.a
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Makefile

@@ -1053,9 +1053,6 @@ vmlinux: scripts/link-vmlinux.sh autoksyms_recursive $(vmlinux-deps) FORCE
 
 targets := vmlinux
 
-# Some samples need headers_install.
-samples: headers_install
-
 # The actual objects are generated when descending,
 # make sure no implicit rule kicks in
 $(sort $(vmlinux-deps)): $(vmlinux-dirs) ;
@@ -1199,6 +1196,10 @@ headers_check: headers_install
 	$(Q)$(MAKE) $(hdr-inst)=include/uapi dst=include HDRCHECK=1
 	$(Q)$(MAKE) $(hdr-inst)=arch/$(SRCARCH)/include/uapi dst=include HDRCHECK=1
 
+ifdef CONFIG_HEADERS_INSTALL
+prepare: headers_install
+endif
+
 ifdef CONFIG_HEADERS_CHECK
 all: headers_check
 endif
@@ -1744,7 +1745,6 @@ build-dir = $(patsubst %/,%,$(dir $(build-target)))
 PHONY += /
 /: ./
 
-samples/: headers_install
 %/: prepare FORCE
 	$(Q)$(MAKE) KBUILD_MODULES=1 $(build)=$(build-dir)
 

arch/arc/configs/tb10x_defconfig

@@ -94,6 +94,7 @@ CONFIG_CONFIGFS_FS=y
 CONFIG_DEBUG_INFO=y
 CONFIG_STRIP_ASM_SYMS=y
 CONFIG_DEBUG_FS=y
+CONFIG_HEADERS_INSTALL=y
 CONFIG_HEADERS_CHECK=y
 CONFIG_DEBUG_SECTION_MISMATCH=y
 CONFIG_MAGIC_SYSRQ=y

arch/nds32/configs/defconfig

@@ -92,6 +92,7 @@ CONFIG_DEBUG_INFO=y
 CONFIG_DEBUG_INFO_DWARF4=y
 CONFIG_GDB_SCRIPTS=y
 CONFIG_READABLE_ASM=y
+CONFIG_HEADERS_INSTALL=y
 CONFIG_HEADERS_CHECK=y
 CONFIG_DEBUG_SECTION_MISMATCH=y
 CONFIG_MAGIC_SYSRQ=y

arch/parisc/configs/a500_defconfig

@@ -166,6 +166,7 @@ CONFIG_NLS_ISO8859_1=m
 CONFIG_NLS_ISO8859_15=m
 CONFIG_NLS_UTF8=m
 CONFIG_DEBUG_FS=y
+CONFIG_HEADERS_INSTALL=y
 CONFIG_HEADERS_CHECK=y
 CONFIG_MAGIC_SYSRQ=y
 # CONFIG_DEBUG_BUGVERBOSE is not set

arch/parisc/configs/b180_defconfig

@@ -90,6 +90,7 @@ CONFIG_NLS_ASCII=m
 CONFIG_NLS_ISO8859_1=m
 CONFIG_NLS_ISO8859_15=m
 CONFIG_NLS_UTF8=m
+CONFIG_HEADERS_INSTALL=y
 CONFIG_HEADERS_CHECK=y
 CONFIG_MAGIC_SYSRQ=y
 CONFIG_DEBUG_KERNEL=y

arch/parisc/configs/c3000_defconfig

@@ -139,6 +139,7 @@ CONFIG_NLS_ISO8859_1=m
 CONFIG_NLS_ISO8859_15=m
 CONFIG_NLS_UTF8=m
 CONFIG_DEBUG_FS=y
+CONFIG_HEADERS_INSTALL=y
 CONFIG_HEADERS_CHECK=y
 CONFIG_MAGIC_SYSRQ=y
 CONFIG_DEBUG_MUTEXES=y

arch/parisc/configs/default_defconfig

@@ -183,6 +183,7 @@ CONFIG_NLS_KOI8_R=m
 CONFIG_NLS_KOI8_U=m
 CONFIG_NLS_UTF8=y
 CONFIG_DEBUG_FS=y
+CONFIG_HEADERS_INSTALL=y
 CONFIG_HEADERS_CHECK=y
 CONFIG_MAGIC_SYSRQ=y
 CONFIG_DEBUG_KERNEL=y

arch/powerpc/configs/ppc6xx_defconfig

@@ -1124,6 +1124,7 @@ CONFIG_NLS_KOI8_R=m
 CONFIG_NLS_KOI8_U=m
 CONFIG_DEBUG_INFO=y
 CONFIG_UNUSED_SYMBOLS=y
+CONFIG_HEADERS_INSTALL=y
 CONFIG_HEADERS_CHECK=y
 CONFIG_MAGIC_SYSRQ=y
 CONFIG_DEBUG_KERNEL=y

arch/s390/configs/debug_defconfig

@@ -586,6 +586,7 @@ CONFIG_GDB_SCRIPTS=y
 CONFIG_FRAME_WARN=1024
 CONFIG_READABLE_ASM=y
 CONFIG_UNUSED_SYMBOLS=y
+CONFIG_HEADERS_INSTALL=y
 CONFIG_HEADERS_CHECK=y
 CONFIG_DEBUG_SECTION_MISMATCH=y
 CONFIG_MAGIC_SYSRQ=y

lib/Kconfig.debug

@@ -305,14 +305,23 @@ config DEBUG_FS
 
 	  If unsure, say N.
 
+config HEADERS_INSTALL
+	bool "Install uapi headers to usr/include"
+	depends on !UML
+	help
+	  This option will install uapi headers (headers exported to user-space)
+	  into the usr/include directory for use during the kernel build.
+	  This is unneeded for building the kernel itself, but needed for some
+	  user-space program samples. It is also needed by some features such
+	  as uapi header sanity checks.
+
 config HEADERS_CHECK
 	bool "Run sanity checks on uapi headers when building 'all'"
-	depends on !UML
+	depends on HEADERS_INSTALL
 	help
-	  This option will extract the user-visible kernel headers when
-	  building the 'all' target, and will run basic sanity checks on them to
-	  ensure that exported files do not attempt to include files which
-	  were not exported, etc.
+	  This option will run basic sanity checks on uapi headers when
+	  building the 'all' target, for example, ensure that they do not
+	  attempt to include files which were not exported, etc.
 
 	  If you're making modifications to header files which are
 	  relevant for userspace, say 'Y', and check the headers

samples/Kconfig

 # SPDX-License-Identifier: GPL-2.0-only
 menuconfig SAMPLES
 	bool "Sample kernel code"
-	depends on !UML
 	help
 	  You can build and test sample kernel code here.
 
@@ -95,16 +94,24 @@ config SAMPLE_CONFIGFS
 
 config SAMPLE_CONNECTOR
 	tristate "Build connector sample -- loadable modules only"
-	depends on CONNECTOR && m
+	depends on CONNECTOR && HEADERS_INSTALL && m
 	help
 	  When enabled, this builds both a sample kernel module for
 	  the connector interface and a user space tool to communicate
 	  with it.
 	  See also Documentation/connector/connector.txt
 
+config SAMPLE_HIDRAW
+	bool "hidraw sample"
+	depends on HEADERS_INSTALL
+
+config SAMPLE_PIDFD
+	bool "pidfd sample"
+	depends on HEADERS_INSTALL
+
 config SAMPLE_SECCOMP
 	bool "Build seccomp sample code"
-	depends on SECCOMP_FILTER
+	depends on SECCOMP_FILTER && HEADERS_INSTALL
 	help
 	  Build samples of seccomp filters using various methods of
 	  BPF filter construction.
@@ -156,6 +163,7 @@ config SAMPLE_ANDROID_BINDERFS
 
 config SAMPLE_VFS
 	bool "Build example programs that use new VFS system calls"
+	depends on HEADERS_INSTALL
 	help
 	  Build example userspace programs that use new VFS system calls such
 	  as mount API and statx().  Note that this is restricted to the x86

samples/Makefile

@@ -4,14 +4,14 @@
 obj-$(CONFIG_SAMPLE_ANDROID_BINDERFS)	+= binderfs/
 obj-$(CONFIG_SAMPLE_CONFIGFS)		+= configfs/
 obj-$(CONFIG_SAMPLE_CONNECTOR)		+= connector/
-subdir-y				+= hidraw
+subdir-$(CONFIG_SAMPLE_HIDRAW)		+= hidraw
 obj-$(CONFIG_SAMPLE_HW_BREAKPOINT)	+= hw_breakpoint/
 obj-$(CONFIG_SAMPLE_KDB)		+= kdb/
 obj-$(CONFIG_SAMPLE_KFIFO)		+= kfifo/
 obj-$(CONFIG_SAMPLE_KOBJECT)		+= kobject/
 obj-$(CONFIG_SAMPLE_KPROBES)		+= kprobes/
 obj-$(CONFIG_SAMPLE_LIVEPATCH)		+= livepatch/
-subdir-y				+= pidfd
+subdir-$(CONFIG_SAMPLE_PIDFD)		+= pidfd
 obj-$(CONFIG_SAMPLE_QMI_CLIENT)		+= qmi/
 obj-$(CONFIG_SAMPLE_RPMSG_CLIENT)	+= rpmsg/
 subdir-$(CONFIG_SAMPLE_SECCOMP)		+= seccomp