Even after commit c624adc9cb ("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 c13295ad21
("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 7f3a59db27 ("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: 9762dc1432 ("samples: add binderfs sample program")
Fixes: c624adc9cb ("samples: fix binderfs sample")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
This adds a simple sample program mounting binderfs and adding, then
removing a binder device. Hopefully, it will be helpful to users who want
to know how binderfs is supposed to be used.
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Masahiro Yamada