So far we didn't provide any concept to enumerate local Varlink
services. Let's change that.
Let's define very light-weight scheme for this: provide a well-known dir
/run/varlink/registry/ where services that implement public interfaces
can link their sockets into. When enumerating services it's thus
sufficient to enumerate inodes in that directory.
The usecase for this is twofold:
1. It's simply very useful to be able to see which public services are
bound on the local system, for debugging/admin/development purposes.
2. At Amutable we'd like to optionally provide a HTTP-to-Varlink bridge
on individual nodes, that allows remote peers (after authentication)
to access local Varlink services. For that it's essential we know the
list of services and their entrypoints to expose, it would be
security-wise highly problematic for clients to provide AF_UNIX
entrypoint paths when connecting. hence: let's instead just have a
dir with the public stuff, and let's ensure the HTTP-to-Varlink
bridge simply exposes that stuff, and nothing else.
Non-public interfaces (such as the oomd interfaces between PID 1 and
oomd), and interfaces with multiple implementors (such as the resolved
hook interface, or the metrics collection stuff) should not be linked
in.
This is inspired by the Varlink.org "registry" concept, briefly
explained here:
https://varlink.org/FAQ#how-do-i-find-the-service-which-implements-a-local-interface
Note however that the described Varlink interface is not actually
implemented here, the directory is introduced however in a fashion that
conceptually matches the registry defined there, and would allow us to
implement the registry interface on top of it. (One of the reason the
registry Varlink API is not implemented right now is that the URI format
it relies on is entirely unspecified in the Varlink docs right now. Some
research needs to be done to extract what's implemented in the reference
implementation and to determine how it maps to the Varlink entrypoint
address format systemd's own tooling currently uses)
This primarily installs the symlinks via Symlinks= in unit files and via
a new tmpfiles.d/ drop-in. But since we touch all .socket units relating
to Varlink this also sets the FileDescriptorName= to varlink for each,
just to minimize diffrences and make things work more alike (the
services in questin don't care about the name, so this doesn't change).
In one case we replace a pair of separate sockets for two closely
related varlink services by a socket and a symlink, so that we can
safely use Symlinks= to also install the registry symlinks.
This adds a small, socket-activated Varlink daemon that can delegate UID
ranges for user namespaces to clients asking for it.
The primary call is AllocateUserRange() where the user passes in an
uninitialized userns fd, which is then set up.
There are other calls that allow assigning a mount fd to a userns
allocated that way, to set up permissions for a cgroup subtree, and to
allocate a veth for such a user namespace.
Since the UID assignments are supposed to be transitive, i.e. not
permanent, care is taken to ensure that users cannot create inodes owned
by these UIDs, so that persistancy cannot be acquired. This is
implemented via a BPF-LSM module that ensures that any member of a
userns allocated that way cannot create files unless the mount it
operates on is owned by the userns itself, or is explicitly
allowelisted.
BPF LSM program with contributions from Alexei Starovoitov.
Lennart Poettering
Lennart Poettering