linux-stable/samples/binderfs/binderfs_example.c
Masahiro Yamada fca5e94921 samples: binderfs: really compile this sample and fix build issues
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>
2020-06-11 20:14:41 +09:00

82 lines
1.9 KiB
C

// SPDX-License-Identifier: GPL-2.0
#define _GNU_SOURCE
#include <errno.h>
#include <fcntl.h>
#include <sched.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ioctl.h>
#include <sys/mount.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
#include <linux/android/binder.h>
#include <linux/android/binderfs.h>
int main(int argc, char *argv[])
{
int fd, ret, saved_errno;
struct binderfs_device device = { 0 };
ret = unshare(CLONE_NEWNS);
if (ret < 0) {
fprintf(stderr, "%s - Failed to unshare mount namespace\n",
strerror(errno));
exit(EXIT_FAILURE);
}
ret = mount(NULL, "/", NULL, MS_REC | MS_PRIVATE, 0);
if (ret < 0) {
fprintf(stderr, "%s - Failed to mount / as private\n",
strerror(errno));
exit(EXIT_FAILURE);
}
ret = mkdir("/dev/binderfs", 0755);
if (ret < 0 && errno != EEXIST) {
fprintf(stderr, "%s - Failed to create binderfs mountpoint\n",
strerror(errno));
exit(EXIT_FAILURE);
}
ret = mount(NULL, "/dev/binderfs", "binder", 0, 0);
if (ret < 0) {
fprintf(stderr, "%s - Failed to mount binderfs\n",
strerror(errno));
exit(EXIT_FAILURE);
}
memcpy(device.name, "my-binder", strlen("my-binder"));
fd = open("/dev/binderfs/binder-control", O_RDONLY | O_CLOEXEC);
if (fd < 0) {
fprintf(stderr, "%s - Failed to open binder-control device\n",
strerror(errno));
exit(EXIT_FAILURE);
}
ret = ioctl(fd, BINDER_CTL_ADD, &device);
saved_errno = errno;
close(fd);
errno = saved_errno;
if (ret < 0) {
fprintf(stderr, "%s - Failed to allocate new binder device\n",
strerror(errno));
exit(EXIT_FAILURE);
}
printf("Allocated new binder device with major %d, minor %d, and name %s\n",
device.major, device.minor, device.name);
ret = unlink("/dev/binderfs/my-binder");
if (ret < 0) {
fprintf(stderr, "%s - Failed to delete binder device\n",
strerror(errno));
exit(EXIT_FAILURE);
}
/* Cleanup happens when the mount namespace dies. */
exit(EXIT_SUCCESS);
}