[RFC bpf-next v3 00/11] Registrating struct_ops types from modules

[thinker.li@gmail.com]

From: Kui-Feng Lee <thinker.li@gmail.com>

Given the current constraints of the current implementation,
struct_ops cannot be registered dynamically. This presents a
significant limitation for modules like coming fuse-bpf, which seeks
to implement a new struct_ops type. To address this issue, a new API
is introduced that allows the registration of new struct_ops types
from modules.

Previously, struct_ops types were defined in bpf_struct_ops_types.h
and collected as a static array. The new API lets callers add new
struct_ops types dynamically. The static array has been removed and
replaced by the per-btf struct_ops_tab.

The struct_ops subsystem relies on BTF to determine the layout of
values in a struct_ops map and identify the subsystem that the
struct_ops map registers to. However, the kernel BTF does not include
the type information of struct_ops types defined by a module. The
struct_ops subsystem requires knowledge of the corresponding module
for a given struct_ops map and the utilization of BTF information from
that module. We empower libbpf to determine the correct module for
accessing the BTF information and pass an identity (FD) of the module
btf to the kernel. The kernel looks up type information and registered
struct_ops types directly from the given btf.

If a module exits while one or more struct_ops maps still refer to a
struct_ops type defined by the module, it can lead to unforeseen
complications. Therefore, it is crucial to ensure that a module
remains intact as long as any struct_ops map is still linked to a
struct_ops type defined by the module. To achieve this, every
struct_ops map holds a reference to the module for each struct_ops
map.

Changes from v2:

 - Remove struct_ops array, and add a per-btf (module) struct_ops_tab
   to collect registered struct_ops types.

 - Validate value_type by checking member names and types.

---
v2: https://lore.kernel.org/all/20230913061449.1918219-1-thinker.li@gmail.com/

Kui-Feng Lee (11):
  bpf: refactory struct_ops type initialization to a function.
  bpf: add struct_ops_tab to btf.
  bpf: add register and unregister functions for struct_ops.
  bpf: attach a module BTF to a bpf_struct_ops
  bpf: hold module for bpf_struct_ops_map.
  bpf: validate value_type
  bpf, net: switch to storing struct_ops in btf
  bpf: pass attached BTF to find correct type info of struct_ops progs.
  libbpf: Find correct module BTFs for struct_ops maps and progs.
  bpf: export btf_ctx_access to modules.
  selftests/bpf: test case for register_bpf_struct_ops().

 include/linux/bpf.h                           |  15 +-
 include/linux/btf.h                           |  36 ++
 include/uapi/linux/bpf.h                      |   4 +
 kernel/bpf/bpf_struct_ops.c                   | 378 ++++++++++++------
 kernel/bpf/bpf_struct_ops_types.h             |  12 -
 kernel/bpf/btf.c                              |  87 +++-
 kernel/bpf/syscall.c                          |   2 +-
 kernel/bpf/verifier.c                         |   4 +-
 net/bpf/bpf_dummy_struct_ops.c                |  12 +-
 net/ipv4/bpf_tcp_ca.c                         |  20 +-
 tools/include/uapi/linux/bpf.h                |   4 +
 tools/lib/bpf/bpf.c                           |   3 +-
 tools/lib/bpf/bpf.h                           |   4 +-
 tools/lib/bpf/libbpf.c                        | 121 +++---
 .../selftests/bpf/bpf_testmod/bpf_testmod.c   |  63 +++
 .../selftests/bpf/bpf_testmod/bpf_testmod.h   |   5 +
 .../bpf/prog_tests/test_struct_ops_module.c   |  43 ++
 .../selftests/bpf/progs/struct_ops_module.c   |  30 ++
 18 files changed, 638 insertions(+), 205 deletions(-)
 delete mode 100644 kernel/bpf/bpf_struct_ops_types.h
 create mode 100644 tools/testing/selftests/bpf/prog_tests/test_struct_ops_module.c
 create mode 100644 tools/testing/selftests/bpf/progs/struct_ops_module.c

-- 
2.34.1