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Subject[PATCH v1 1/2] Documentation/filesystems: add binderfs
This documents the Android binderfs filesystem used to dynamically add and
remove binder devices that are private to each instance.

Signed-off-by: Christian Brauner <>
/* Changelog */
- switch from *.txt to *.rst format
Documentation/filesystems/binderfs.rst | 70 ++++++++++++++++++++++++++
1 file changed, 70 insertions(+)
create mode 100644 Documentation/filesystems/binderfs.rst

diff --git a/Documentation/filesystems/binderfs.rst b/Documentation/filesystems/binderfs.rst
new file mode 100644
index 000000000000..74a744b42db7
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+++ b/Documentation/filesystems/binderfs.rst
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+.. SPDX-License-Identifier: GPL-2.0
+The Android binderfs Filesystem
+Android binderfs is a filesystem for the Android binder IPC mechanism. It
+allows to dynamically add and remove binder devices at runtime. Binder devices
+located in a new binderfs instance are independent of binder devices located in
+other binderfs instances. Mounting a new binderfs instance makes it possible
+to get a set of private binder devices.
+Mounting binderfs
+Android binderfs can be mounted with:
+ mkdir /dev/binderfs
+ mount -t binder binder /dev/binderfs
+at which point a new instance of binderfs will show up at ``/dev/binderfs``.
+In a fresh instance of binderfs no binder devices will be present. There will
+only be a ``binder-control`` device which serves as the request handler for
+binderfs. Mounting another binderfs instance at a different location will
+create a new and separate instance from all other binderfs mounts. This is
+identical to the behavior of e.g. ``devpts`` and ``tmpfs``. The Android
+binderfs filesystem can be mounted in user namespaces.
+ binderfs instances can be mounted with a limit on the number of binder
+ devices that can be allocated. The ``max=<count>`` mount option serves as
+ a per-instance limit. If ``max=<count>`` is set then only ``<count>`` number
+ of binder devices can be allocated in this binderfs instance.
+Allocating binder Devices
+.. _ioctl:
+To allocate a new binder device in a binderfs instance a request needs to be
+sent through the ``binder-control`` device node. A request is sent in the form
+of an `ioctl() <ioctl_>`_.
+What a program needs to do is to open the ``binder-control`` device node and
+send a ``BINDER_CTL_ADD`` request to the kernel. Users of binderfs need to
+tell the kernel which name the new binder device should get. By default a name
+can only contain up to ``BINDERFS_MAX_NAME`` chars including the terminating
+zero byte.
+Once the request is made via an `ioctl() <ioctl_>`_ passing a ``struct
+binder_device`` with the name to the kernel it will allocate a new binder
+device and return the major and minor number of the new device in the struct
+(This is necessary because binderfs allocates a major device number
+dynamically.). After the `ioctl() <ioctl_>`_ returns there will be a new
+binder device located under /dev/binderfs with the chosen name.
+Deleting binder Devices
+.. _unlink:
+.. _rm:
+Binderfs binder devices can be deleted via `unlink() <unlink_>`_. This means
+that the `rm() <rm_>`_ tool can be used to delete them. Note that the
+``binder-control`` device cannot be deleted since this would make the binderfs
+instance unuseable. The ``binder-control`` device will be deleted when the
+binderfs instance is unmounted and all references to it have been dropped.
 \ /
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