1.. SPDX-License-Identifier: GPL-2.0 2 3The Android binderfs Filesystem 4=============================== 5 6Android binderfs is a filesystem for the Android binder IPC mechanism. It 7allows to dynamically add and remove binder devices at runtime. Binder devices 8located in a new binderfs instance are independent of binder devices located in 9other binderfs instances. Mounting a new binderfs instance makes it possible 10to get a set of private binder devices. 11 12Mounting binderfs 13----------------- 14 15Android binderfs can be mounted with:: 16 17 mkdir /dev/binderfs 18 mount -t binder binder /dev/binderfs 19 20at which point a new instance of binderfs will show up at ``/dev/binderfs``. 21In a fresh instance of binderfs no binder devices will be present. There will 22only be a ``binder-control`` device which serves as the request handler for 23binderfs. Mounting another binderfs instance at a different location will 24create a new and separate instance from all other binderfs mounts. This is 25identical to the behavior of e.g. ``devpts`` and ``tmpfs``. The Android 26binderfs filesystem can be mounted in user namespaces. 27 28Options 29------- 30max 31 binderfs instances can be mounted with a limit on the number of binder 32 devices that can be allocated. The ``max=<count>`` mount option serves as 33 a per-instance limit. If ``max=<count>`` is set then only ``<count>`` number 34 of binder devices can be allocated in this binderfs instance. 35 36stats 37 Using ``stats=global`` enables global binder statistics. 38 ``stats=global`` is only available for a binderfs instance mounted in the 39 initial user namespace. An attempt to use the option to mount a binderfs 40 instance in another user namespace will return a permission error. 41 42Allocating binder Devices 43------------------------- 44 45.. _ioctl: http://man7.org/linux/man-pages/man2/ioctl.2.html 46 47To allocate a new binder device in a binderfs instance a request needs to be 48sent through the ``binder-control`` device node. A request is sent in the form 49of an `ioctl() <ioctl_>`_. 50 51What a program needs to do is to open the ``binder-control`` device node and 52send a ``BINDER_CTL_ADD`` request to the kernel. Users of binderfs need to 53tell the kernel which name the new binder device should get. By default a name 54can only contain up to ``BINDERFS_MAX_NAME`` chars including the terminating 55zero byte. 56 57Once the request is made via an `ioctl() <ioctl_>`_ passing a ``struct 58binder_device`` with the name to the kernel it will allocate a new binder 59device and return the major and minor number of the new device in the struct 60(This is necessary because binderfs allocates a major device number 61dynamically.). After the `ioctl() <ioctl_>`_ returns there will be a new 62binder device located under /dev/binderfs with the chosen name. 63 64Deleting binder Devices 65----------------------- 66 67.. _unlink: http://man7.org/linux/man-pages/man2/unlink.2.html 68.. _rm: http://man7.org/linux/man-pages/man1/rm.1.html 69 70Binderfs binder devices can be deleted via `unlink() <unlink_>`_. This means 71that the `rm() <rm_>`_ tool can be used to delete them. Note that the 72``binder-control`` device cannot be deleted since this would make the binderfs 73instance unuseable. The ``binder-control`` device will be deleted when the 74binderfs instance is unmounted and all references to it have been dropped. 75