.. 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. stats Using ``stats=global`` enables global binder statistics. ``stats=global`` is only available for a binderfs instance mounted in the initial user namespace. An attempt to use the option to mount a binderfs instance in another user namespace will return a permission error. 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.