Requiring /sys/fs/bpf path to be a mount point at the moment of cgroup
controllers realization does more harm than good, because:
* Realization happens early on boot, the mount point may not be ready at
the time. That happens if mounts are made by a .mount unit (the issue we
encountered).
* BPF filesystem may be mounted on another point.
Remove the check. Instead verify that path provided by BPFProgram= is
within BPF fs when unit properties are parsed.
Split in two commits for simple backport.
- Introduce support of cgroup-bpf programs managed (i.e. compiled,
loaded to and unloaded from kernel) externally. Systemd is only
responsible for attaching programs to unit cgroup hence the name
'foreign'.
Foreign BPF programs are identified by bpf program ID and attach type.
systemd:
- Gets kernel FD of BPF program;
- Makes a unique identifier of BPF program from BPF attach type and
program ID. Same program IDs mean the same program, i.e the same
chunk of kernel memory. Even if the same program is passed multiple
times, identical (program_id, attach_type) instances are collapsed
into one;
- Attaches programs to unit cgroup.
Daan De Meyer
Yu Watanabe
Julia Kartseva
Luca Boccassi