samples: binderfs: really compile this sample and fix build issues

Even after commit c624adc9 ("samples: fix binderfs sample"), this
sample is never compiled.

'hostprogs' teaches Kbuild that this is a host program, but not enough
to order to compile it. You must add it to 'always-y' to really compile
it.

Since this sample has never been compiled in upstream, various issues
are left unnoticed.

[1] compilers without <linux/android/binderfs.h> are still widely used

<linux/android/binderfs.h> is only available since commit c13295ad
("binderfs: rename header to binderfs.h"), i.e., Linux 5.0

If your compiler is based on UAPI headers older than Linux 5.0, you
will see the following error:

  samples/binderfs/binderfs_example.c:16:10: fatal error: linux/android/binderfs.h: No such file or directory
   #include <linux/android/binderfs.h>
            ^~~~~~~~~~~~~~~~~~~~~~~~~~
  compilation terminated.

You cannot rely on compilers having such a new header.

The common approach is to install UAPI headers of this kernel into
usr/include, and then add it to the header search path.

I added 'depends on HEADERS_INSTALL' in Kconfig, and '-I usr/include'
compiler flag in Makefile.

[2] compile the sample for target architecture

Because headers_install works for the target architecture, only the
native compiler was able to build sample code that requires
'-I usr/include'.

Commit 7f3a59db ("kbuild: add infrastructure to build userspace
programs") added the new syntax 'userprogs' to compile user-space
programs for the target architecture.

Use it, and then 'ifndef CROSS_COMPILE' will go away.

I added 'depends on CC_CAN_LINK' because $(CC) is not necessarily
capable of linking user-space programs.

[3] use subdir-y to descend into samples/binderfs

Since this directory does not contain any kernel-space code, it has no
point in generating built-in.a or modules.order.

Replace obj-$(CONFIG_...) with subdir-$(CONFIG_...).

[4] -Wunused-variable warning

If I compile this, I see the following warning.

  samples/binderfs/binderfs_example.c: In function 'main':
  samples/binderfs/binderfs_example.c:21:9: warning: unused variable 'len' [-Wunused-variable]
     21 |  size_t len;
        |         ^~~

I removed the unused 'len'.

[5] CONFIG_ANDROID_BINDERFS is not required

Since this is a user-space standalone program, it is independent of
the kernel configuration.

Remove 'depends on ANDROID_BINDERFS'.

Fixes: 9762dc14 ("samples: add binderfs sample program")
Fixes: c624adc9 ("samples: fix binderfs sample")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>

samples/Kconfig

@@ -185,7 +185,7 @@ config SAMPLE_VFIO_MDEV_MBOCHS
 
 config SAMPLE_ANDROID_BINDERFS
 	bool "Build Android binderfs example"
-	depends on ANDROID_BINDERFS
+	depends on CC_CAN_LINK && HEADERS_INSTALL
 	help
 	  Builds a sample program to illustrate the use of the Android binderfs
 	  filesystem.

samples/Makefile

@@ -2,7 +2,7 @@
 # Makefile for Linux samples code
 
 subdir-$(CONFIG_SAMPLE_AUXDISPLAY)	+= auxdisplay
-obj-$(CONFIG_SAMPLE_ANDROID_BINDERFS)	+= binderfs/
+subdir-$(CONFIG_SAMPLE_ANDROID_BINDERFS) += binderfs
 obj-$(CONFIG_SAMPLE_CONFIGFS)		+= configfs/
 obj-$(CONFIG_SAMPLE_CONNECTOR)		+= connector/
 subdir-$(CONFIG_SAMPLE_HIDRAW)		+= hidraw

samples/binderfs/Makefile

 # SPDX-License-Identifier: GPL-2.0-only
-ifndef CROSS_COMPILE
-ifdef CONFIG_SAMPLE_ANDROID_BINDERFS
-hostprogs := binderfs_example
-endif
-endif
+userprogs := binderfs_example
+always-y := $(userprogs)
+
+userccflags += -I usr/include

samples/binderfs/binderfs_example.c

@@ -18,7 +18,6 @@
 int main(int argc, char *argv[])
 {
 	int fd, ret, saved_errno;
-	size_t len;
 	struct binderfs_device device = { 0 };
 
 	ret = unshare(CLONE_NEWNS);