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References: <20181207232509.31771-1-christian@brauner.io>
In-Reply-To: <20181207232509.31771-1-christian@brauner.io>
From:   Todd Kjos <tkjos@google.com>
Date:   Mon, 10 Dec 2018 09:50:25 -0800
Message-ID: <CAHRSSEzxR2wRw9UxaKkXTZ7cu989+70-V07PxMm6o+BfLTQ=6w@mail.gmail.com>
Subject: Re: [PATCH v1] binder: implement binderfs
To:     Christian Brauner <christian@brauner.io>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Todd Kjos <tkjos@android.com>,
        Martijn Coenen <maco@android.com>,
        LKML <linux-kernel@vger.kernel.org>,
        =?UTF-8?B?QXJ2ZSBIasO4bm5ldsOlZw==?= <arve@android.com>,
        joel@joelfernandes.org, darrick.wong@oracle.com,
        david@fromorbit.com, kilobyte@angband.pl,
        "open list:ANDROID DRIVERS" <devel@driverdev.osuosl.org>,
        chouryzhou@tencent.com
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Sender: linux-kernel-owner@vger.kernel.org
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chouryzhou@, can you confirm that this implementation works for your
android-in-container use-case?

-Todd


On Fri, Dec 7, 2018 at 3:26 PM Christian Brauner <christian@brauner.io> wrote:
>
> As discussed at Linux Plumbers Conference 2018 in Vancouver [1] this is the
> implementation of binderfs.
>
> 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:
> - BINDER_CTL_ADD
>   Allocate a new binder device.
> 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 via:
>
> struct binderfs_device device = {0};
> snprintf(device.name, BINDERFS_MAX_NAME, "%s", "my-device");
> int fd = open("/dev/binderfs/binder-control", O_RDWR);
> ioctl(fd, BINDER_CTL_ADD, &device);
>
> binderfs will then allocate a new minor number and create the device
> "my-device".
> The struct binderfs_device will then be used to return the major and minor
> number, for the device.
> Binderfs devices can simply be removed via unlink().
>
> /* Implementation details */
> - 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.
>   Minor numbers that have been given out 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.
> - Each binderfs super block has its own struct binderfs_info that tracks
>   specific details about a binderfs instance: the ipc namespace, the dentry
>   of the binder-control device, the root uid and gid of the user namespace
>   the binderfs instance was mounted in.
> - binderfs can be mounted by user namespace root in a non-initial user
>   namespace. The devices will be owned by user namespace root.
> - 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 devices 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.
>
> /* Create a new binder device in a binderfs mount */
> sudo mkdir /dev/binderfs
> sudo mount -t binder binder /dev/binderfs
>
>  #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;
>          struct binderfs_device device = { 0 };
>
>          if (argc < 2)
>                  exit(EXIT_FAILURE);
>
>          strncpy(device.name, argv[1], sizeof(device.name));
>
>          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);
>  }
>
> /* Demo */
> A demo of how binderfs works can be found under [2].
>
> [1]: https://goo.gl/JL2tfX
>
> Cc: Martijn Coenen <maco@android.com>
> Cc: Todd Kjos <tkjos@google.com>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
> ---
> v1:
> - simplify init_binderfs()
>   Move the creation of binder-control into binderfs_fill_super() so that we can
>   cleanly and without any complex error handling deallocate the super block on
>   failure.
> - switch from __u32 to __u8 in struct binderfs_device
>   __u8 is the correct value to cross the kernel <-> userspace boundary.
> - introduce BINDERFS_MAX_NAME
>   This determines the maximum length of a binderfs binder device name.
> - add name member struct binderfs_device
>   This lets userspace specify a name for the binder device. The maximum length
>   is determined by BINDERFS_MAX_NAME.
> - handle naming collisions
>   Since userspace now gives us a name to use for the device we need to handle
>   the case where userspace passes the same device name twice. This is done by
>   using d_lookup() (takes rename lock too). If a matching dentry under the root
>   dentry of the superblock is found we test whether it is still active and if
>   so return -EEXIST.
> - remove per-super block idr tracking and locking
>   Since userspace now determines the name remove the idr tracking since it's
>   not needed anymore.
> - remove ctl_mutex from struct binders_info
>   It was only needed to protect the per-super block idr which has been removed.
>   If I'm not mistaken we currently do not need to lock the ioctl() itself since
>   removing is handled by a simple unlink(). So remove the mutex.
> - ensure that binderfs_evict_inode() doesn't cause double-frees on iput()
>   When setting up the inode fails at a step where a new inode already has been
>   allocated we need to set inode->i_private to NULL to ensure that
>   binderfs_evict_inode() doesn't try to free up stuff that we already freed
>   while handling the error.
> ---
>  drivers/android/Kconfig                 |  12 +
>  drivers/android/Makefile                |   1 +
>  drivers/android/binder.c                |  25 +-
>  drivers/android/binder_internal.h       |  49 ++
>  drivers/android/binderfs.c              | 565 ++++++++++++++++++++++++
>  include/uapi/linux/android/binder_ctl.h |  35 ++
>  include/uapi/linux/magic.h              |   1 +
>  7 files changed, 671 insertions(+), 17 deletions(-)
>  create mode 100644 drivers/android/binder_internal.h
>  create mode 100644 drivers/android/binderfs.c
>  create mode 100644 include/uapi/linux/android/binder_ctl.h
>
> diff --git a/drivers/android/Kconfig b/drivers/android/Kconfig
> index 51e8250d113f..4c190f8d1f4c 100644
> --- a/drivers/android/Kconfig
> +++ b/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
> diff --git a/drivers/android/Makefile b/drivers/android/Makefile
> index a01254c43ee3..c7856e3200da 100644
> --- a/drivers/android/Makefile
> +++ b/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
> diff --git a/drivers/android/binder.c b/drivers/android/binder.c
> index cb30a524d16d..3ed8bc4b7451 100644
> --- a/drivers/android/binder.c
> +++ b/drivers/android/binder.c
> @@ -78,6 +78,7 @@
>  #include <asm/cacheflush.h>
>
>  #include "binder_alloc.h"
> +#include "binder_internal.h"
>  #include "binder_trace.h"
>
>  static HLIST_HEAD(binder_deferred_list);
> @@ -262,20 +263,6 @@ 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
> @@ -4935,8 +4922,12 @@ 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);
>
> @@ -5724,7 +5715,7 @@ static int binder_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,
> diff --git a/drivers/android/binder_internal.h b/drivers/android/binder_internal.h
> new file mode 100644
> index 000000000000..7fb97f503ef2
> --- /dev/null
> +++ b/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 */
> diff --git a/drivers/android/binderfs.c b/drivers/android/binderfs.c
> new file mode 100644
> index 000000000000..ac435210eb53
> --- /dev/null
> +++ b/drivers/android/binderfs.c
> @@ -0,0 +1,565 @@
> +/* 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(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_new_inode - 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
> + * @device:    binder device for which the new inode will be allocated
> + * @req:       struct binderfs_device as copied from userspace
> + *
> + * This function will allocate a new inode from the super block of the
> + * filesystem mount and attach a dentry to that inode.
> + * 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.
> + *
> + * Return: 0 on success, negative errno on failure
> + */
> +static int binderfs_new_inode(struct inode *ref_inode,
> +                             struct binder_device *device,
> +                             struct binderfs_device __user *userp,
> +                             struct binderfs_device *req)
> +{
> +       int minor, ret;
> +       struct dentry *dentry, *dup, *root;
> +       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;
> +       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;
> +       inode->i_private = device;
> +
> +       name = kmalloc(name_len, GFP_KERNEL);
> +       if (!name)
> +               goto err;
> +
> +       ret = snprintf(name, name_len, "%s", req->name);
> +       if (ret < 0 || (size_t)ret >= name_len) {
> +               ret = -EINVAL;
> +               goto err;
> +       }
> +
> +       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)
> +               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));
> +                       /*
> +                        * Prevent double free since iput() calls
> +                        * binderfs_evict_inode().
> +                        */
> +                       inode->i_private = NULL;
> +                       ret = -EEXIST;
> +                       goto err;
> +               }
> +               dput(dup);
> +       }
> +
> +       d_add(dentry, inode);
> +       fsnotify_create(root->d_inode, dentry);
> +       inode_unlock(d_inode(root));
> +
> +       return 0;
> +
> +err:
> +       kfree(name);
> +       mutex_lock(&binderfs_minors_mutex);
> +       ida_free(&binderfs_minors, minor);
> +       mutex_unlock(&binderfs_minors_mutex);
> +       iput(inode);
> +
> +       return ret;
> +}
> +
> +static int binderfs_binder_device_create(struct inode *inode,
> +                                        struct binderfs_device __user *userp,
> +                                        struct binderfs_device *req)
> +{
> +       struct binder_device *device;
> +       int ret;
> +
> +       device = kzalloc(sizeof(*device), GFP_KERNEL);
> +       if (!device)
> +               return -ENOMEM;
> +
> +       ret = binderfs_new_inode(inode, device, userp, req);
> +       if (ret < 0) {
> +               kfree(device);
> +               return ret;
> +       }
> +
> +       return 0;
> +}
> +
> +/**
> + * 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)
> +{
> +       struct binderfs_info *info;
> +       int ret = -EINVAL;
> +       struct inode *inode = file_inode(file);
> +       struct binderfs_device *device = (struct binderfs_device __user *)arg;
> +       struct binderfs_device device_req;
> +
> +       info = BINDERFS_I(inode);
> +       switch (cmd) {
> +       case BINDER_CTL_ADD:
> +               ret = copy_from_user(&device_req, device, sizeof(device_req));
> +               if (ret)
> +                       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;
> +       struct dentry *dentry;
> +       struct binder_device *device;
> +       int ret = 0;
> +       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 (info->control_dentry)
> +               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;
> +       inode->i_private = device;
> +
> +       device->binderfs_inode = inode;
> +       device->miscdev.minor = minor;
> +
> +       dentry = d_alloc_name(root, "binder-control");
> +       if (!dentry)
> +               goto out;
> +
> +       info->control_dentry = dentry;
> +       d_add(dentry, inode);
> +       inode_unlock(d_inode(root));
> +
> +       return 0;
> +
> +out:
> +       inode_unlock(d_inode(root));
> +       kfree(device);
> +       if (inode) {
> +               inode->i_private = NULL;
> +               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)
> +               return ret;
> +
> +       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:
> +       if (inode)
> +               iput(inode);
> +       kfree(info);
> +       put_ipc_ns(ipc_ns);
> +
> +       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 < 0)
> +               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);
> diff --git a/include/uapi/linux/android/binder_ctl.h b/include/uapi/linux/android/binder_ctl.h
> new file mode 100644
> index 000000000000..65b2efd1a0a5
> --- /dev/null
> +++ b/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 */
> +
> diff --git a/include/uapi/linux/magic.h b/include/uapi/linux/magic.h
> index 96c24478d8ce..f8c00045d537 100644
> --- a/include/uapi/linux/magic.h
> +++ b/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
> --
> 2.19.1
>