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authorChristian Brauner <christian@brauner.io>2019-01-11 16:40:59 +0300
committerJonathan Corbet <corbet@lwn.net>2019-01-15 03:21:37 +0300
commit35283f56626cbc6d5b2dcb42f6d2189ce34fee3d (patch)
tree3d38f282b162ecff6db0c70f8f94fff26527a683 /Documentation/filesystems
parent5591a3075e956af501430bdf2b3516a69f4fe65d (diff)
downloadlinux-35283f56626cbc6d5b2dcb42f6d2189ce34fee3d.tar.xz
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 <christian.brauner@ubuntu.com> [jc: tweaked markup and added to filesystems/index.rst] Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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-rw-r--r--Documentation/filesystems/index.rst7
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diff --git a/Documentation/filesystems/binderfs.rst b/Documentation/filesystems/binderfs.rst
new file mode 100644
index 000000000000..c009671f8434
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+++ b/Documentation/filesystems/binderfs.rst
@@ -0,0 +1,68 @@
+.. 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.
+
+Options
+-------
+max
+ 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: http://man7.org/linux/man-pages/man2/ioctl.2.html
+
+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: http://man7.org/linux/man-pages/man2/unlink.2.html
+.. _rm: http://man7.org/linux/man-pages/man1/rm.1.html
+
+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.
diff --git a/Documentation/filesystems/index.rst b/Documentation/filesystems/index.rst
index 605befab300b..61d2441b25d5 100644
--- a/Documentation/filesystems/index.rst
+++ b/Documentation/filesystems/index.rst
@@ -380,3 +380,10 @@ including:
:maxdepth: 2
path-lookup.rst
+
+binderfs
+========
+
+.. toctree::
+
+ binderfs.rst