Merge branch 'bpf-implement-mprog-api-on-top-of-existing-cgroup-progs'

Yonghong Song says:

====================
bpf: Implement mprog API on top of existing cgroup progs

Current cgroup prog ordering is appending at attachment time. This is not
ideal. In some cases, users want specific ordering at a particular cgroup
level. For example, in Meta, we have a case where three different
applications all have cgroup/setsockopt progs and they require specific
ordering. Current approach is to use a bpfchainer where one bpf prog
contains multiple global functions and each global function can be
freplaced by a prog for a specific application. The ordering of global
functions decides the ordering of those application specific bpf progs.
Using bpfchainer is a centralized approach and is not desirable as
one of applications acts as a daemon. The decentralized attachment
approach is more favorable for those applications.

To address this, the existing mprog API ([2]) seems an ideal solution with
supporting BPF_F_BEFORE and BPF_F_AFTER flags on top of existing cgroup
bpf implementation. More specifically, the support is added for prog/link
attachment with BPF_F_BEFORE and BPF_F_AFTER. The kernel mprog
interface ([2]) is not used and the implementation is directly done in
cgroup bpf code base. The mprog 'revision' is also implemented in
attach/detach/replace, so users can query revision number to check the
change of cgroup prog list.

The patch set contains 5 patches. Patch 1 adds revision support for
cgroup bpf progs. Patch 2 implements mprog API implementation for
prog/link attach and revision update. Patch 3 adds a new libbpf
API to do cgroup link attach with flags like BPF_F_BEFORE/BPF_F_AFTER.
Patches 4 and 5 add two tests to validate the implementation.

  [1] https://lore.kernel.org/r/20250224230116.283071-1-yonghong.song@linux.dev
  [2] https://lore.kernel.org/r/20230719140858.13224-2-daniel@iogearbox.net

Changelogs:
  v4 -> v5:
    - v4: https://lore.kernel.org/bpf/20250530173812.1823479-1-yonghong.song@linux.dev/
    - Remove early prog/link checking based flags and id_or_fd as later code
      will do checking as well.
    - Do proper cgroup flag checking for bpf_prog_attach().
  v3 -> v4:
    - v3: https://lore.kernel.org/bpf/20250517162720.4077882-1-yonghong.song@linux.dev/
    - Refactor some to make BPF_F_BEFORE/BPF_F_AFTER handling easier to understand.
    - Perviously, I degraded 'link' to 'prog' for later mprog handling. This is
      not correct. Similar to mprog.c, we should be check 'link' instead link->prog
      since it is possible two different links may have the same underlying prog and
      we do not want to miss supporting such use case.
  v2 -> v3:
    - v2: https://lore.kernel.org/bpf/20250508223524.487875-1-yonghong.song@linux.dev/
    - Big change to replace get_anchor_prog() to get_prog_list() so the
      'struct bpf_prog_list *' is returned directly.
    - Support 'BPF_F_BEFORE | BPF_F_AFTER' attachment if the prog list is empty
      and flags do not have 'BPF_F_LINK | BPF_F_ID' and id_or_fd is 0.
    - Add BPF_F_LINK support.
    - Patch 4 is added to reuse id_from_prog_fd() and id_from_link_fd().
  v1 -> v2:
    - v1: https://lore.kernel.org/bpf/20250411011523.1838771-1-yonghong.song@linux.dev/
    - Change cgroup_bpf.revisions from atomic64_t to u64.
    - Added missing bpf_prog_put in various places.
    - Rename get_cmp_prog() to get_anchor_prog(). The implementation tries to
      find the anchor prog regardless of whether id_or_fd is non-NULL or not.
    - Rename bpf_cgroup_prog_attached() to is_cgroup_prog_type() and handle
      BPF_PROG_TYPE_LSM properly (with BPF_LSM_CGROUP attach type).
    - I kept 'id || id_or_fd' condition as the condition 'id' is also used
      in mprog.c so I assume it is okay in cgroup.c as well.
====================

Link: https://patch.msgid.link/20250606163131.2428225-1-yonghong.song@linux.dev
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>