[PATCH bpf-next v4 0/6] Support associating BPF programs with struct_ops

[Amery Hung <ameryhung@gmail.com>]

v4 -> v3
   - Fix potential dangling pointer in timer callback. Protect
     st_ops_assoc with RCU. The get helper now needs to be paired with
     bpf_struct_ops_put()
   - The command should only increase refcount once for a program
     (Andrii)
   - Test a struct_ops program reused in two struct_ops maps
   - Test getting associated struct_ops in timer callback
   Link: https://lore.kernel.org/bpf/20251017215627.722338-1-ameryhung@gmail.com/

v2 -> v3
   - Change the type of st_ops_assoc from void* (i.e., kdata) to bpf_map
     (Andrii)
   - Fix a bug that clears BPF_PTR_POISON when a struct_ops map is freed
     (Andrii)
   - Return NULL if the map is not fully initialized (Martin)
   - Move struct_ops map refcount inc/dec into internal helpers (Martin)
   - Add libbpf API, bpf_program__assoc_struct_ops (Andrii)
   Link: https://lore.kernel.org/bpf/20251016204503.3203690-1-ameryhung@gmail.com/

v1 -> v2
   - Poison st_ops_assoc when reusing the program in more than one
     struct_ops maps and add a helper to access the pointer (Andrii)
   - Minor style and naming changes (Andrii)
   Link: https://lore.kernel.org/bpf/20251010174953.2884682-1-ameryhung@gmail.com/

---

Hi,

This patchset adds a new BPF command BPF_PROG_ASSOC_STRUCT_OPS to
the bpf() syscall to allow associating a BPF program with a struct_ops.
The command is introduced to address a emerging need from struct_ops
users. As the number of subsystems adopting struct_ops grows, more
users are building their struct_ops-based solution with some help from
other BPF programs. For exmample, scx_layer uses a syscall program as
a user space trigger to refresh layers [0]. It also uses tracing program
to infer whether a task is using GPU and needs to be prioritized [1]. In
these use cases, when there are multiple struct_ops instances, the
struct_ops kfuncs called from different BPF programs, whether struct_ops
or not needs to be able to refer to a specific one, which currently is
not possible.

The new BPF command will allow users to explicitly associate a BPF
program with a struct_ops map. The libbpf wrapper can be called after
loading programs and before attaching programs and struct_ops.

Internally, it will set prog->aux->st_ops_assoc to the struct_ops
map. struct_ops kfuncs can then get the associated struct_ops struct
by calling bpf_prog_get_assoc_struct_ops() with prog->aux, which can
be acquired from a "__prog" argument. The value of the speical
argument will be fixed up by the verifier during verification.

The command conceptually associates the implementation of BPF programs
with struct_ops map, not the attachment. A program associated with the
map will take a refcount of it so that st_ops_assoc always points to a
valid struct_ops struct. struct_ops implementers can use the helper,
bpf_prog_get_assoc_struct_ops to get the pointer. The returned
struct_ops if not NULL is guaranteed to be valid and initialized.
However, it is not guarantted that the struct_ops is attached. The
struct_ops implementer still need to take stepis to track and check the
state of the struct_ops in kdata, if the use case demand the struct_ops
to be attached.

We can also consider support associating struct_ops link with BPF
programs, which on one hand make struct_ops implementer's job easier,
but might complicate libbpf workflow and does not apply to legacy
struct_ops attachment.

[0] https://github.com/sched-ext/scx/blob/main/scheds/rust/scx_layered/src/bpf/main.bpf.c#L557
[1] https://github.com/sched-ext/scx/blob/main/scheds/rust/scx_layered/src/bpf/main.bpf.c#L754

---

Amery Hung (6):
  bpf: Allow verifier to fixup kernel module kfuncs
  bpf: Support associating BPF program with struct_ops
  libbpf: Add support for associating BPF program with struct_ops
  selftests/bpf: Test BPF_PROG_ASSOC_STRUCT_OPS command
  selftests/bpf: Test ambiguous associated struct_ops
  selftests/bpf: Test getting associated struct_ops in timer callback

 include/linux/bpf.h                           |  16 ++
 include/uapi/linux/bpf.h                      |  17 ++
 kernel/bpf/bpf_struct_ops.c                   |  98 +++++++++
 kernel/bpf/core.c                             |   3 +
 kernel/bpf/syscall.c                          |  46 +++++
 kernel/bpf/verifier.c                         |   3 +-
 tools/include/uapi/linux/bpf.h                |  17 ++
 tools/lib/bpf/bpf.c                           |  19 ++
 tools/lib/bpf/bpf.h                           |  21 ++
 tools/lib/bpf/libbpf.c                        |  30 +++
 tools/lib/bpf/libbpf.h                        |  16 ++
 tools/lib/bpf/libbpf.map                      |   2 +
 .../bpf/prog_tests/test_struct_ops_assoc.c    | 190 ++++++++++++++++++
 .../selftests/bpf/progs/struct_ops_assoc.c    | 105 ++++++++++
 .../bpf/progs/struct_ops_assoc_in_timer.c     |  77 +++++++
 .../bpf/progs/struct_ops_assoc_reuse.c        |  75 +++++++
 .../selftests/bpf/test_kmods/bpf_testmod.c    |  19 ++
 .../bpf/test_kmods/bpf_testmod_kfunc.h        |   1 +
 18 files changed, 753 insertions(+), 2 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/test_struct_ops_assoc.c
 create mode 100644 tools/testing/selftests/bpf/progs/struct_ops_assoc.c
 create mode 100644 tools/testing/selftests/bpf/progs/struct_ops_assoc_in_timer.c
 create mode 100644 tools/testing/selftests/bpf/progs/struct_ops_assoc_reuse.c

-- 
2.47.3