• Masahiro Yamada's avatar
    samples: binderfs: really compile this sample and fix build issues · fca5e949
    Masahiro Yamada authored
    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: default avatarMasahiro Yamada <masahiroy@kernel.org>
    Acked-by: default avatarChristian Brauner <christian.brauner@ubuntu.com>
    fca5e949
Makefile 1.16 KB