[PATCH bpf-next v3 0/4] bpf, netns: Prepare for multi-prog attachment

[Jakub Sitnicki <jakub@cloudflare.com>]

This patch set prepares ground for link-based multi-prog attachment for
future netns attach types, with BPF_SK_LOOKUP attach type in mind [0].

Two changes are needed in order to attach and run a series of BPF programs:

  1) an bpf_prog_array of programs to run (patch #2), and
  2) a list of attached links to keep track of attachments (patch #3).

Nothing changes for BPF flow_dissector. Just as before only one program can
be attached to netns.


In v3 I've simplified patch #2 that introduces bpf_prog_array to take
advantage of the fact that it will hold at most one program for now.

In particular, I'm no longer using bpf_prog_array_copy. It turned out to be
less suitable for link operations than I thought as it fails to append the
same BPF program.

bpf_prog_array_replace_item is also gone, because we know we always want to
replace the first element in prog_array.

Naturally the code that handles bpf_prog_array will need change once
more when there is a program type that allows multi-prog attachment. But I
feel it will be better to do it gradually and present it together with
tests that actually exercise multi-prog code paths.

Thanks,
-jkbs

[0] https://lore.kernel.org/bpf/20200511185218.1422406-1-jakub@cloudflare.com/

Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: Stanislav Fomichev <sdf@google.com>

v2 -> v3:
- Don't check if run_array is null in link update callback. (Martin)
- Allow updating the link with the same BPF program. (Andrii)
- Add patch #4 with a test for the above case.
- Kill bpf_prog_array_replace_item. Access the run_array directly.
- Switch from bpf_prog_array_copy() to bpf_prog_array_alloc(1, ...).
- Replace rcu_deref_protected & RCU_INIT_POINTER with rcu_replace_pointer.
- Drop Andrii's Ack from patch #2. Code changed.

v1 -> v2:

- Show with a (void) cast that bpf_prog_array_replace_item() return value
  is ignored on purpose. (Andrii)
- Explain why bpf-cgroup cannot replace programs in bpf_prog_array based
  on bpf_prog pointer comparison in patch #2 description. (Andrii)

Jakub Sitnicki (4):
  flow_dissector: Pull BPF program assignment up to bpf-netns
  bpf, netns: Keep attached programs in bpf_prog_array
  bpf, netns: Keep a list of attached bpf_link's
  selftests/bpf: Test updating flow_dissector link with same program

 include/net/flow_dissector.h                  |   3 +-
 include/net/netns/bpf.h                       |   7 +-
 kernel/bpf/net_namespace.c                    | 162 ++++++++++++------
 net/core/flow_dissector.c                     |  32 ++--
 .../bpf/prog_tests/flow_dissector_reattach.c  |  32 +++-
 5 files changed, 160 insertions(+), 76 deletions(-)

-- 
2.25.4