Comment 8 for bug 1884810

/dev/binder is obsolete. Installing the binder_linux module creates a filesystem type now.

https://brauner.github.io/2019/01/09/android-binderfs.html

Summary (excerpts, see the blog entry link for more):
Problem 1: Using the misc major Device Number
Problem 2: Containers and IPC namespaces
To solve both problems we came up with a solution that I presented at the Linux Plumbers Conference in Vancouver this year.
Android binderfs is a tiny filesystem that allows users to dynamically allocate binder devices, i.e. it allows to add and remove binder devices at runtime.

mkdir /dev/binderfs
mount -t binder binder /dev/binderfs

See also
https://askubuntu.com/questions/1267990/ls-1-dev-ashmem-binder-yields-anticipated-result-for-dev-ashmem-but-an-err