Support associating BPF programs with struct_ops

This patchset adds a new BPF command BPF_STRUCT_OPS_ASSOCIATE_PROG 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
struct (i.e., kdata). struct_ops kfuncs can then get the associated
struct_ops by adding 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. However, the struct_ops can be in an
uninitialized or unattached state. The struct_ops implementer will be
responsible to maintain and check the state of the associated
struct_ops before accessing it.

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/sc...
[1] https://github.com/sched-ext/scx/blob/main/scheds/rust/sc...

Amery Hung (4):
  bpf: Allow verifier to fixup kernel module kfuncs
  bpf: Support associating BPF program with struct_ops
  libbpf: Add bpf_struct_ops_associate_prog() API
  selftests/bpf: Test BPF_STRUCT_OPS_ASSOCIATE_PROG command

 include/linux/bpf.h                           |  11 ++
 include/uapi/linux/bpf.h                      |  16 +++
 kernel/bpf/bpf_struct_ops.c                   |  32 ++++++
 kernel/bpf/core.c                             |   6 +
 kernel/bpf/syscall.c                          |  38 +++++++
 kernel/bpf/verifier.c                         |   3 +-
 tools/include/uapi/linux/bpf.h                |  16 +++
 tools/lib/bpf/bpf.c                           |  18 +++
 tools/lib/bpf/bpf.h                           |  19 ++++
 tools/lib/bpf/libbpf.map                      |   1 +
 .../bpf/prog_tests/test_struct_ops_assoc.c    |  76 +++++++++++++
 .../selftests/bpf/progs/struct_ops_assoc.c    | 105 ++++++++++++++++++
 .../selftests/bpf/test_kmods/bpf_testmod.c    |  17 +++
 .../bpf/test_kmods/bpf_testmod_kfunc.h        |   1 +
 14 files changed, 357 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

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2.47.3