binder: implement binderfs

As discussed at Linux Plumbers Conference 2018 in Vancouver [1] this is the
implementation of binderfs.

/* Abstract */
binderfs is a backwards-compatible filesystem for Android's binder ipc
mechanism. Each ipc namespace will mount a new binderfs instance. Mounting
binderfs multiple times at different locations in the same ipc namespace
will not cause a new super block to be allocated and hence it will be the
same filesystem instance.
Each new binderfs mount will have its own set of binder devices only
visible in the ipc namespace it has been mounted in. All devices in a new
binderfs mount will follow the scheme binder%d and numbering will always
start at 0.

/* Backwards compatibility */
Devices requested in the Kconfig via CONFIG_ANDROID_BINDER_DEVICES for the
initial ipc namespace will work as before. They will be registered via
misc_register() and appear in the devtmpfs mount. Specifically, the
standard devices binder, hwbinder, and vndbinder will all appear in their
standard locations in /dev. Mounting or unmounting the binderfs mount in
the initial ipc namespace will have no effect on these devices, i.e. they
will neither show up in the binderfs mount nor will they disappear when the
binderfs mount is gone.

/* binder-control */
Each new binderfs instance comes with a binder-control device. No other
devices will be present at first. The binder-control device can be used to
dynamically allocate binder devices. All requests operate on the binderfs
mount the binder-control device resides in.
Assuming a new instance of binderfs has been mounted at /dev/binderfs
via mount -t binderfs binderfs /dev/binderfs. Then a request to create a
new binder device can be made as illustrated in [2].
Binderfs devices can simply be removed via unlink().

/* Implementation details */
- dynamic major number allocation:
  When binderfs is registered as a new filesystem it will dynamically
  allocate a new major number. The allocated major number will be returned
  in struct binderfs_device when a new binder device is allocated.
- global minor number tracking:
  Minor are tracked in a global idr struct that is capped at
  BINDERFS_MAX_MINOR. The minor number tracker is protected by a global
  mutex. This is the only point of contention between binderfs mounts.
- struct binderfs_info:
  Each binderfs super block has its own struct binderfs_info that tracks
  specific details about a binderfs instance:
  - ipc namespace
  - dentry of the binder-control device
  - root uid and root gid of the user namespace the binderfs instance
    was mounted in
- mountable by user namespace root:
  binderfs can be mounted by user namespace root in a non-initial user
  namespace. The devices will be owned by user namespace root.
- binderfs binder devices without misc infrastructure:
  New binder devices associated with a binderfs mount do not use the
  full misc_register() infrastructure.
  The misc_register() infrastructure can only create new devices in the
  host's devtmpfs mount. binderfs does however only make devices appear
  under its own mountpoint and thus allocates new character device nodes
  from the inode of the root dentry of the super block. This will have
  the side-effect that binderfs specific device nodes do not appear in
  sysfs. This behavior is similar to devpts allocated pts devices and
  has no effect on the functionality of the ipc mechanism itself.

[1]: https://goo.gl/JL2tfX
[2]: program to allocate a new binderfs binder device:

     #define _GNU_SOURCE
     #include <errno.h>
     #include <fcntl.h>
     #include <stdio.h>
     #include <stdlib.h>
     #include <string.h>
     #include <sys/ioctl.h>
     #include <sys/stat.h>
     #include <sys/types.h>
     #include <unistd.h>
     #include <linux/android/binder_ctl.h>

     int main(int argc, char *argv[])
     {
             int fd, ret, saved_errno;
             size_t len;
             struct binderfs_device device = { 0 };

             if (argc < 2)
                     exit(EXIT_FAILURE);

             len = strlen(argv[1]);
             if (len > BINDERFS_MAX_NAME)
                     exit(EXIT_FAILURE);

             memcpy(device.name, argv[1], len);

             fd = open("/dev/binderfs/binder-control", O_RDONLY | O_CLOEXEC);
             if (fd < 0) {
                     printf("%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) {
                     printf("%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);

             exit(EXIT_SUCCESS);
     }

Cc: Martijn Coenen <maco@android.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Acked-by: Todd Kjos <tkjos@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/android/Kconfig

@@ -20,6 +20,18 @@ config ANDROID_BINDER_IPC
 	  Android process, using Binder to identify, invoke and pass arguments
 	  between said processes.
 
+config ANDROID_BINDERFS
+	bool "Android Binderfs filesystem"
+	depends on ANDROID_BINDER_IPC
+	default n
+	---help---
+	  Binderfs is a pseudo-filesystem for the Android Binder IPC driver
+	  which can be mounted per-ipc namespace allowing to run multiple
+	  instances of Android.
+	  Each binderfs mount initially only contains a binder-control device.
+	  It can be used to dynamically allocate new binder IPC devices via
+	  ioctls.
+
 config ANDROID_BINDER_DEVICES
 	string "Android Binder devices"
 	depends on ANDROID_BINDER_IPC

drivers/android/Makefile

@@ -1,4 +1,5 @@
 ccflags-y += -I$(src)			# needed for trace events
 
+obj-$(CONFIG_ANDROID_BINDERFS)		+= binderfs.o
 obj-$(CONFIG_ANDROID_BINDER_IPC)	+= binder.o binder_alloc.o
 obj-$(CONFIG_ANDROID_BINDER_IPC_SELFTEST) += binder_alloc_selftest.o

drivers/android/binder.c

@@ -79,6 +79,7 @@
 #include <asm/cacheflush.h>
 
 #include "binder_alloc.h"
+#include "binder_internal.h"
 #include "binder_trace.h"
 
 static HLIST_HEAD(binder_deferred_list);
@@ -249,20 +250,6 @@ static struct binder_transaction_log_entry *binder_transaction_log_add(
 	return e;
 }
 
-struct binder_context {
-	struct binder_node *binder_context_mgr_node;
-	struct mutex context_mgr_node_lock;
-
-	kuid_t binder_context_mgr_uid;
-	const char *name;
-};
-
-struct binder_device {
-	struct hlist_node hlist;
-	struct miscdevice miscdev;
-	struct binder_context context;
-};
-
 /**
  * struct binder_work - work enqueued on a worklist
  * @entry:             node enqueued on list
@@ -5024,8 +5011,12 @@ static int binder_open(struct inode *nodp, struct file *filp)
 	proc->tsk = current->group_leader;
 	INIT_LIST_HEAD(&proc->todo);
 	proc->default_priority = task_nice(current);
-	binder_dev = container_of(filp->private_data, struct binder_device,
-				  miscdev);
+	/* binderfs stashes devices in i_private */
+	if (is_binderfs_device(nodp))
+		binder_dev = nodp->i_private;
+	else
+		binder_dev = container_of(filp->private_data,
+					  struct binder_device, miscdev);
 	proc->context = &binder_dev->context;
 	binder_alloc_init(&proc->alloc);
 
@@ -5816,7 +5807,7 @@ static int transaction_log_show(struct seq_file *m, void *unused)
 	return 0;
 }
 
-static const struct file_operations binder_fops = {
+const struct file_operations binder_fops = {
 	.owner = THIS_MODULE,
 	.poll = binder_poll,
 	.unlocked_ioctl = binder_ioctl,

drivers/android/binder_internal.h

@@ -0,0 +1,49 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#ifndef _LINUX_BINDER_INTERNAL_H
+#define _LINUX_BINDER_INTERNAL_H
+
+#include <linux/export.h>
+#include <linux/fs.h>
+#include <linux/list.h>
+#include <linux/miscdevice.h>
+#include <linux/mutex.h>
+#include <linux/stddef.h>
+#include <linux/types.h>
+#include <linux/uidgid.h>
+
+struct binder_context {
+	struct binder_node *binder_context_mgr_node;
+	struct mutex context_mgr_node_lock;
+	kuid_t binder_context_mgr_uid;
+	const char *name;
+};
+
+/**
+ * struct binder_device - information about a binder device node
+ * @hlist:          list of binder devices (only used for devices requested via
+ *                  CONFIG_ANDROID_BINDER_DEVICES)
+ * @miscdev:        information about a binder character device node
+ * @context:        binder context information
+ * @binderfs_inode: This is the inode of the root dentry of the super block
+ *                  belonging to a binderfs mount.
+ */
+struct binder_device {
+	struct hlist_node hlist;
+	struct miscdevice miscdev;
+	struct binder_context context;
+	struct inode *binderfs_inode;
+};
+
+extern const struct file_operations binder_fops;
+
+#ifdef CONFIG_ANDROID_BINDERFS
+extern bool is_binderfs_device(const struct inode *inode);
+#else
+static inline bool is_binderfs_device(const struct inode *inode)
+{
+	return false;
+}
+#endif
+
+#endif /* _LINUX_BINDER_INTERNAL_H */

drivers/android/binderfs.c

@@ -0,0 +1,544 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#include <linux/compiler_types.h>
+#include <linux/errno.h>
+#include <linux/fs.h>
+#include <linux/fsnotify.h>
+#include <linux/gfp.h>
+#include <linux/idr.h>
+#include <linux/init.h>
+#include <linux/ipc_namespace.h>
+#include <linux/kdev_t.h>
+#include <linux/kernel.h>
+#include <linux/list.h>
+#include <linux/magic.h>
+#include <linux/major.h>
+#include <linux/miscdevice.h>
+#include <linux/module.h>
+#include <linux/mutex.h>
+#include <linux/mount.h>
+#include <linux/parser.h>
+#include <linux/radix-tree.h>
+#include <linux/sched.h>
+#include <linux/slab.h>
+#include <linux/spinlock_types.h>
+#include <linux/stddef.h>
+#include <linux/string.h>
+#include <linux/types.h>
+#include <linux/uaccess.h>
+#include <linux/user_namespace.h>
+#include <linux/xarray.h>
+#include <uapi/asm-generic/errno-base.h>
+#include <uapi/linux/android/binder.h>
+#include <uapi/linux/android/binder_ctl.h>
+
+#include "binder_internal.h"
+
+#define FIRST_INODE 1
+#define SECOND_INODE 2
+#define INODE_OFFSET 3
+#define INTSTRLEN 21
+#define BINDERFS_MAX_MINOR (1U << MINORBITS)
+
+static struct vfsmount *binderfs_mnt;
+
+static dev_t binderfs_dev;
+static DEFINE_MUTEX(binderfs_minors_mutex);
+static DEFINE_IDA(binderfs_minors);
+
+/**
+ * binderfs_info - information about a binderfs mount
+ * @ipc_ns:         The ipc namespace the binderfs mount belongs to.
+ * @control_dentry: This records the dentry of this binderfs mount
+ *                  binder-control device.
+ * @root_uid:       uid that needs to be used when a new binder device is
+ *                  created.
+ * @root_gid:       gid that needs to be used when a new binder device is
+ *                  created.
+ */
+struct binderfs_info {
+	struct ipc_namespace *ipc_ns;
+	struct dentry *control_dentry;
+	kuid_t root_uid;
+	kgid_t root_gid;
+
+};
+
+static inline struct binderfs_info *BINDERFS_I(const struct inode *inode)
+{
+	return inode->i_sb->s_fs_info;
+}
+
+bool is_binderfs_device(const struct inode *inode)
+{
+	if (inode->i_sb->s_magic == BINDERFS_SUPER_MAGIC)
+		return true;
+
+	return false;
+}
+
+/**
+ * binderfs_binder_device_create - allocate inode from super block of a
+ *                                 binderfs mount
+ * @ref_inode: inode from wich the super block will be taken
+ * @userp:     buffer to copy information about new device for userspace to
+ * @req:       struct binderfs_device as copied from userspace
+ *
+ * This function allocated a new binder_device and reserves a new minor
+ * number for it.
+ * Minor numbers are limited and tracked globally in binderfs_minors. The
+ * function will stash a struct binder_device for the specific binder
+ * device in i_private of the inode.
+ * It will go on to allocate a new inode from the super block of the
+ * filesystem mount, stash a struct binder_device in its i_private field
+ * and attach a dentry to that inode.
+ *
+ * Return: 0 on success, negative errno on failure
+ */
+static int binderfs_binder_device_create(struct inode *ref_inode,
+					 struct binderfs_device __user *userp,
+					 struct binderfs_device *req)
+{
+	int minor, ret;
+	struct dentry *dentry, *dup, *root;
+	struct binder_device *device;
+	size_t name_len = BINDERFS_MAX_NAME + 1;
+	char *name = NULL;
+	struct inode *inode = NULL;
+	struct super_block *sb = ref_inode->i_sb;
+	struct binderfs_info *info = sb->s_fs_info;
+
+	/* Reserve new minor number for the new device. */
+	mutex_lock(&binderfs_minors_mutex);
+	minor = ida_alloc_max(&binderfs_minors, BINDERFS_MAX_MINOR, GFP_KERNEL);
+	mutex_unlock(&binderfs_minors_mutex);
+	if (minor < 0)
+		return minor;
+
+	ret = -ENOMEM;
+	device = kzalloc(sizeof(*device), GFP_KERNEL);
+	if (!device)
+		goto err;
+
+	inode = new_inode(sb);
+	if (!inode)
+		goto err;
+
+	inode->i_ino = minor + INODE_OFFSET;
+	inode->i_mtime = inode->i_atime = inode->i_ctime = current_time(inode);
+	init_special_inode(inode, S_IFCHR | 0600,
+			   MKDEV(MAJOR(binderfs_dev), minor));
+	inode->i_fop = &binder_fops;
+	inode->i_uid = info->root_uid;
+	inode->i_gid = info->root_gid;
+
+	name = kmalloc(name_len, GFP_KERNEL);
+	if (!name)
+		goto err;
+
+	strscpy(name, req->name, name_len);
+
+	device->binderfs_inode = inode;
+	device->context.binder_context_mgr_uid = INVALID_UID;
+	device->context.name = name;
+	device->miscdev.name = name;
+	device->miscdev.minor = minor;
+	mutex_init(&device->context.context_mgr_node_lock);
+
+	req->major = MAJOR(binderfs_dev);
+	req->minor = minor;
+
+	ret = copy_to_user(userp, req, sizeof(*req));
+	if (ret) {
+		ret = -EFAULT;
+		goto err;
+	}
+
+	root = sb->s_root;
+	inode_lock(d_inode(root));
+	dentry = d_alloc_name(root, name);
+	if (!dentry) {
+		inode_unlock(d_inode(root));
+		ret = -ENOMEM;
+		goto err;
+	}
+
+	/* Verify that the name userspace gave us is not already in use. */
+	dup = d_lookup(root, &dentry->d_name);
+	if (dup) {
+		if (d_really_is_positive(dup)) {
+			dput(dup);
+			dput(dentry);
+			inode_unlock(d_inode(root));
+			ret = -EEXIST;
+			goto err;
+		}
+		dput(dup);
+	}
+
+	inode->i_private = device;
+	d_add(dentry, inode);
+	fsnotify_create(root->d_inode, dentry);
+	inode_unlock(d_inode(root));
+
+	return 0;
+
+err:
+	kfree(name);
+	kfree(device);
+	mutex_lock(&binderfs_minors_mutex);
+	ida_free(&binderfs_minors, minor);
+	mutex_unlock(&binderfs_minors_mutex);
+	iput(inode);
+
+	return ret;
+}
+
+/**
+ * binderfs_ctl_ioctl - handle binder device node allocation requests
+ *
+ * The request handler for the binder-control device. All requests operate on
+ * the binderfs mount the binder-control device resides in:
+ * - BINDER_CTL_ADD
+ *   Allocate a new binder device.
+ *
+ * Return: 0 on success, negative errno on failure
+ */
+static long binder_ctl_ioctl(struct file *file, unsigned int cmd,
+			     unsigned long arg)
+{
+	int ret = -EINVAL;
+	struct inode *inode = file_inode(file);
+	struct binderfs_device __user *device = (struct binderfs_device __user *)arg;
+	struct binderfs_device device_req;
+
+	switch (cmd) {
+	case BINDER_CTL_ADD:
+		ret = copy_from_user(&device_req, device, sizeof(device_req));
+		if (ret) {
+			ret = -EFAULT;
+			break;
+		}
+
+		ret = binderfs_binder_device_create(inode, device, &device_req);
+		break;
+	default:
+		break;
+	}
+
+	return ret;
+}
+
+static void binderfs_evict_inode(struct inode *inode)
+{
+	struct binder_device *device = inode->i_private;
+
+	clear_inode(inode);
+
+	if (!device)
+		return;
+
+	mutex_lock(&binderfs_minors_mutex);
+	ida_free(&binderfs_minors, device->miscdev.minor);
+	mutex_unlock(&binderfs_minors_mutex);
+
+	kfree(device->context.name);
+	kfree(device);
+}
+
+static const struct super_operations binderfs_super_ops = {
+	.statfs = simple_statfs,
+	.evict_inode = binderfs_evict_inode,
+};
+
+static int binderfs_rename(struct inode *old_dir, struct dentry *old_dentry,
+			   struct inode *new_dir, struct dentry *new_dentry,
+			   unsigned int flags)
+{
+	struct inode *inode = d_inode(old_dentry);
+
+	/* binderfs doesn't support directories. */
+	if (d_is_dir(old_dentry))
+		return -EPERM;
+
+	if (flags & ~RENAME_NOREPLACE)
+		return -EINVAL;
+
+	if (!simple_empty(new_dentry))
+		return -ENOTEMPTY;
+
+	if (d_really_is_positive(new_dentry))
+		simple_unlink(new_dir, new_dentry);
+
+	old_dir->i_ctime = old_dir->i_mtime = new_dir->i_ctime =
+		new_dir->i_mtime = inode->i_ctime = current_time(old_dir);
+
+	return 0;
+}
+
+static int binderfs_unlink(struct inode *dir, struct dentry *dentry)
+{
+	/*
+	 * The control dentry is only ever touched during mount so checking it
+	 * here should not require us to take lock.
+	 */
+	if (BINDERFS_I(dir)->control_dentry == dentry)
+		return -EPERM;
+
+	return simple_unlink(dir, dentry);
+}
+
+static const struct file_operations binder_ctl_fops = {
+	.owner		= THIS_MODULE,
+	.open		= nonseekable_open,
+	.unlocked_ioctl	= binder_ctl_ioctl,
+	.compat_ioctl	= binder_ctl_ioctl,
+	.llseek		= noop_llseek,
+};
+
+/**
+ * binderfs_binder_ctl_create - create a new binder-control device
+ * @sb: super block of the binderfs mount
+ *
+ * This function creates a new binder-control device node in the binderfs mount
+ * referred to by @sb.
+ *
+ * Return: 0 on success, negative errno on failure
+ */
+static int binderfs_binder_ctl_create(struct super_block *sb)
+{
+	int minor, ret;
+	struct dentry *dentry;
+	struct binder_device *device;
+	struct inode *inode = NULL;
+	struct dentry *root = sb->s_root;
+	struct binderfs_info *info = sb->s_fs_info;
+
+	device = kzalloc(sizeof(*device), GFP_KERNEL);
+	if (!device)
+		return -ENOMEM;
+
+	inode_lock(d_inode(root));
+
+	/* If we have already created a binder-control node, return. */
+	if (info->control_dentry) {
+		ret = 0;
+		goto out;
+	}
+
+	ret = -ENOMEM;
+	inode = new_inode(sb);
+	if (!inode)
+		goto out;
+
+	/* Reserve a new minor number for the new device. */
+	mutex_lock(&binderfs_minors_mutex);
+	minor = ida_alloc_max(&binderfs_minors, BINDERFS_MAX_MINOR, GFP_KERNEL);
+	mutex_unlock(&binderfs_minors_mutex);
+	if (minor < 0) {
+		ret = minor;
+		goto out;
+	}
+
+	inode->i_ino = SECOND_INODE;
+	inode->i_mtime = inode->i_atime = inode->i_ctime = current_time(inode);
+	init_special_inode(inode, S_IFCHR | 0600,
+			   MKDEV(MAJOR(binderfs_dev), minor));
+	inode->i_fop = &binder_ctl_fops;
+	inode->i_uid = info->root_uid;
+	inode->i_gid = info->root_gid;
+
+	device->binderfs_inode = inode;
+	device->miscdev.minor = minor;
+
+	dentry = d_alloc_name(root, "binder-control");
+	if (!dentry)
+		goto out;
+
+	inode->i_private = device;
+	info->control_dentry = dentry;
+	d_add(dentry, inode);
+	inode_unlock(d_inode(root));
+
+	return 0;
+
+out:
+	inode_unlock(d_inode(root));
+	kfree(device);
+	iput(inode);
+
+	return ret;
+}
+
+static const struct inode_operations binderfs_dir_inode_operations = {
+	.lookup = simple_lookup,
+	.rename = binderfs_rename,
+	.unlink = binderfs_unlink,
+};
+
+static int binderfs_fill_super(struct super_block *sb, void *data, int silent)
+{
+	struct binderfs_info *info;
+	int ret = -ENOMEM;
+	struct inode *inode = NULL;
+	struct ipc_namespace *ipc_ns = sb->s_fs_info;
+
+	get_ipc_ns(ipc_ns);
+
+	sb->s_blocksize = PAGE_SIZE;
+	sb->s_blocksize_bits = PAGE_SHIFT;
+
+	/*
+	 * The binderfs filesystem can be mounted by userns root in a
+	 * non-initial userns. By default such mounts have the SB_I_NODEV flag
+	 * set in s_iflags to prevent security issues where userns root can
+	 * just create random device nodes via mknod() since it owns the
+	 * filesystem mount. But binderfs does not allow to create any files
+	 * including devices nodes. The only way to create binder devices nodes
+	 * is through the binder-control device which userns root is explicitly
+	 * allowed to do. So removing the SB_I_NODEV flag from s_iflags is both
+	 * necessary and safe.
+	 */
+	sb->s_iflags &= ~SB_I_NODEV;
+	sb->s_iflags |= SB_I_NOEXEC;
+	sb->s_magic = BINDERFS_SUPER_MAGIC;
+	sb->s_op = &binderfs_super_ops;
+	sb->s_time_gran = 1;
+
+	info = kzalloc(sizeof(struct binderfs_info), GFP_KERNEL);
+	if (!info)
+		goto err_without_dentry;
+
+	info->ipc_ns = ipc_ns;
+	info->root_gid = make_kgid(sb->s_user_ns, 0);
+	if (!gid_valid(info->root_gid))
+		info->root_gid = GLOBAL_ROOT_GID;
+	info->root_uid = make_kuid(sb->s_user_ns, 0);
+	if (!uid_valid(info->root_uid))
+		info->root_uid = GLOBAL_ROOT_UID;
+
+	sb->s_fs_info = info;
+
+	inode = new_inode(sb);
+	if (!inode)
+		goto err_without_dentry;
+
+	inode->i_ino = FIRST_INODE;
+	inode->i_fop = &simple_dir_operations;
+	inode->i_mode = S_IFDIR | 0755;
+	inode->i_mtime = inode->i_atime = inode->i_ctime = current_time(inode);
+	inode->i_op = &binderfs_dir_inode_operations;
+	set_nlink(inode, 2);
+
+	sb->s_root = d_make_root(inode);
+	if (!sb->s_root)
+		goto err_without_dentry;
+
+	ret = binderfs_binder_ctl_create(sb);
+	if (ret)
+		goto err_with_dentry;
+
+	return 0;
+
+err_with_dentry:
+	dput(sb->s_root);
+	sb->s_root = NULL;
+
+err_without_dentry:
+	put_ipc_ns(ipc_ns);
+	iput(inode);
+	kfree(info);
+
+	return ret;
+}
+
+static int binderfs_test_super(struct super_block *sb, void *data)
+{
+	struct binderfs_info *info = sb->s_fs_info;
+
+	if (info)
+		return info->ipc_ns == data;
+
+	return 0;
+}
+
+static int binderfs_set_super(struct super_block *sb, void *data)
+{
+	sb->s_fs_info = data;
+	return set_anon_super(sb, NULL);
+}
+
+static struct dentry *binderfs_mount(struct file_system_type *fs_type,
+				     int flags, const char *dev_name,
+				     void *data)
+{
+	struct super_block *sb;
+	struct ipc_namespace *ipc_ns = current->nsproxy->ipc_ns;
+
+	if (!ns_capable(ipc_ns->user_ns, CAP_SYS_ADMIN))
+		return ERR_PTR(-EPERM);
+
+	sb = sget_userns(fs_type, binderfs_test_super, binderfs_set_super,
+			 flags, ipc_ns->user_ns, ipc_ns);
+	if (IS_ERR(sb))
+		return ERR_CAST(sb);
+
+	if (!sb->s_root) {
+		int ret = binderfs_fill_super(sb, data, flags & SB_SILENT ? 1 : 0);
+		if (ret) {
+			deactivate_locked_super(sb);
+			return ERR_PTR(ret);
+		}
+
+		sb->s_flags |= SB_ACTIVE;
+	}
+
+	return dget(sb->s_root);
+}
+
+static void binderfs_kill_super(struct super_block *sb)
+{
+	struct binderfs_info *info = sb->s_fs_info;
+
+	if (info && info->ipc_ns)
+		put_ipc_ns(info->ipc_ns);
+
+	kfree(info);
+	kill_litter_super(sb);
+}
+
+static struct file_system_type binder_fs_type = {
+	.name		= "binder",
+	.mount		= binderfs_mount,
+	.kill_sb	= binderfs_kill_super,
+	.fs_flags	= FS_USERNS_MOUNT,
+};
+
+static int __init init_binderfs(void)
+{
+	int ret;
+
+	/* Allocate new major number for binderfs. */
+	ret = alloc_chrdev_region(&binderfs_dev, 0, BINDERFS_MAX_MINOR,
+				  "binder");
+	if (ret)
+		return ret;
+
+	ret = register_filesystem(&binder_fs_type);
+	if (ret) {
+		unregister_chrdev_region(binderfs_dev, BINDERFS_MAX_MINOR);
+		return ret;
+	}
+
+	binderfs_mnt = kern_mount(&binder_fs_type);
+	if (IS_ERR(binderfs_mnt)) {
+		ret = PTR_ERR(binderfs_mnt);
+		binderfs_mnt = NULL;
+		unregister_filesystem(&binder_fs_type);
+		unregister_chrdev_region(binderfs_dev, BINDERFS_MAX_MINOR);
+	}
+
+	return ret;
+}
+
+device_initcall(init_binderfs);

include/uapi/linux/android/binder_ctl.h

@@ -0,0 +1,35 @@
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+/*
+ * Copyright (C) 2018 Canonical Ltd.
+ *
+ */
+
+#ifndef _UAPI_LINUX_BINDER_CTL_H
+#define _UAPI_LINUX_BINDER_CTL_H
+
+#include <linux/android/binder.h>
+#include <linux/types.h>
+#include <linux/ioctl.h>
+
+#define BINDERFS_MAX_NAME 255
+
+/**
+ * struct binderfs_device - retrieve information about a new binder device
+ * @name:   the name to use for the new binderfs binder device
+ * @major:  major number allocated for binderfs binder devices
+ * @minor:  minor number allocated for the new binderfs binder device
+ *
+ */
+struct binderfs_device {
+	char name[BINDERFS_MAX_NAME + 1];
+	__u8 major;
+	__u8 minor;
+};
+
+/**
+ * Allocate a new binder device.
+ */
+#define BINDER_CTL_ADD _IOWR('b', 1, struct binderfs_device)
+
+#endif /* _UAPI_LINUX_BINDER_CTL_H */
+

include/uapi/linux/magic.h

@@ -73,6 +73,7 @@
 #define DAXFS_MAGIC             0x64646178
 #define BINFMTFS_MAGIC          0x42494e4d
 #define DEVPTS_SUPER_MAGIC	0x1cd1
+#define BINDERFS_SUPER_MAGIC	0x6c6f6f70
 #define FUTEXFS_SUPER_MAGIC	0xBAD1DEA
 #define PIPEFS_MAGIC            0x50495045
 #define PROC_SUPER_MAGIC	0x9fa0