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.
Android binderfs can be mounted with:
mount -t binder binder /dev/binderfs
at which point a new instance of binderfs will show up at
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.
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.
stats=globalenables global binder statistics.
stats=globalis 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¶
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().
What a program needs to do is to open the
binder-control device node and
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
Once the request is made via an ioctl() passing a
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() returns there will be a new
binder device located under /dev/binderfs with the chosen name.
Deleting binder Devices¶
Binderfs binder devices can be deleted via unlink(). This means
that the 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.