join_classid(9)

join_classid(9) 🎀

Kernel API Manual bestie, we are looking at join_classid(9), and it is lowkey giving “tiny helper, big cgroupv1 energy” ✨

🌸 NAME 🌸

join_classid — join a cgroupv1 net_cls classid 💅

✨ SYNOPSIS ✨

int join_classid (void );

🦋 ARGUMENTS 🦋

• void — no arguments, bestie. Literally none. Not a single one. 🫶

💖 DESCRIPTION 💖

Okay so this function expects the cgroup work dir to be already created, because we join it right here. No cap.

That’s what causes the process sockets to be tagged with the given net_cls classid. Iconic behavior, honestly 🔥

On success, it returns 0. Otherwise, on failure, it returns 1. Periodt.

🌈 SEE ALSO 🌈

Kernel file ./tools/testing/selftests/bpf/cgroup_helpers.c is right there in the mix, and then there’s this whole squad — go read them too, iykyk 👑

• enable_controllers(9)
• write_cgroup_file(9)
• write_cgroup_file_parent(9)
• setup_cgroup_environment(9)
• join_cgroup(9)
• join_root_cgroup(9)
• join_parent_cgroup(9)
• set_cgroup_xattr(9)
• __cleanup_cgroup_environment(9)
• cleanup_cgroup_environment(9)
• get_root_cgroup(9)
• create_and_get_cgroup(9)
• get_cgroup_id_from_path(9)
• setup_classid_environment(9)
• set_classid(9)
• cleanup_classid_environment(9)
• get_classid_cgroup_id(9)
• get_cgroup1_hierarchy_id(9)
• open_classid(9)

Kernel API Manual, source/version ./tools/testing/selftests/bpf, October 2026 — bye bestie, mwah 💖