[PATCH bpf-next v2 00/15] libbpf: type suffixes and autocreate flag for struct_ops maps

[Eduard Zingerman <eddyz87@gmail.com>]

Tweak struct_ops related APIs to allow the following features:
- specify version suffixes for stuct_ops map types;
- share same BPF program between several map definitions with
  different local BTF types, assuming only maps with same
  kernel BTF type would be selected for load;
- toggle autocreate flag for struct_ops maps;
- automatically toggle autoload for struct_ops programs referenced
  from struct_ops maps, depending on autocreate status of the
  corresponding map;
- use SEC("?.struct_ops") and SEC("?.struct_ops.link")
  to define struct_ops maps with autocreate == false after object open.

This would allow loading programs like below:

    SEC("struct_ops/foo") int BPF_PROG(foo) { ... }
    SEC("struct_ops/bar") int BPF_PROG(bar) { ... }

    struct bpf_testmod_ops___v1 {
        int (*foo)(void);
    };

    struct bpf_testmod_ops___v2 {
        int (*foo)(void);
        int (*bar)(void);
    };

    /* Assume kernel type name to be 'test_ops' */
    SEC(".struct_ops.link")
    struct test_ops___v1 map_v1 = {
        /* Program 'foo' shared by maps with
         * different local BTF type
         */
        .foo = (void *)foo
    };

    SEC(".struct_ops.link")
    struct test_ops___v2 map_v2 = {
        .foo = (void *)foo,
        .bar = (void *)bar
    };

Assuming the following tweaks are done before loading:

    /* to load v1 */
    bpf_map__set_autocreate(skel->maps.map_v1, true);
    bpf_map__set_autocreate(skel->maps.map_v2, false);

    /* to load v2 */
    bpf_map__set_autocreate(skel->maps.map_v1, false);
    bpf_map__set_autocreate(skel->maps.map_v2, true);

Patch #8 ties autocreate and autoload flags for struct_ops maps and
programs. While discussion for v1 concluded that such feature is
useful, it is an outlier compared to the rest of libbpf API that
treats autoload / autocreate flags in most simple way possible.
An alternative might be a dedicated API call, e.g.:

  int bpf_object__infer_struct_ops_progs_autoload(struct bpf_object *)
  
That would set programs autoload flags depending on user defined state
of autocreate flags of struct_ops map.

It might even be accompanied by an API call:

  int bpf_object__infer_struct_ops_maps_autocreate(struct bpf_object *)

That would check which struct_ops maps definitions could be loaded
with current kernel, or report error if there are ambiguities.

Changelog:
- v1 [1] -> v2:
  - fixed memory leak in patch #1 (Kui-Feng);
  - improved error messages in patch #2 (Martin, Andrii);
  - in bad_struct_ops selftest from patch #6 added .test_2
    map member setup (David);
  - added utility functions to capture libbpf log from selftests (David)
  - in selftests replaced usage of ...__open_and_load by separate
    calls to ..._open() and ..._load() (Andrii);
  - removed serial_... in selftest definitions (Andrii);
  - improved comments in selftest struct_ops_autocreate
    from patch #7 (David);
  - removed autoload toggling logic incompatible with shadow variables
    from bpf_map__set_autocreate(), instead struct_ops programs
    autoload property is computed at struct_ops maps load phase,
    see patch #8 (Kui-Feng, Martin, Andrii);
  - added support for SEC("?.struct_ops") and SEC("?.struct_ops.link")
    (Andrii).

[1] https://lore.kernel.org/bpf/20240227204556.17524-1-eddyz87@gmail.com/

Eduard Zingerman (15):
  libbpf: allow version suffixes (___smth) for struct_ops types
  libbpf: tie struct_ops programs to kernel BTF ids, not to local ids
  libbpf: honor autocreate flag for struct_ops maps
  selftests/bpf: test struct_ops map definition with type suffix
  selftests/bpf: utility functions to capture libbpf log in test_progs
  selftests/bpf: bad_struct_ops test
  selftests/bpf: test autocreate behavior for struct_ops maps
  libbpf: sync progs autoload with maps autocreate for struct_ops maps
  selftests/bpf: verify struct_ops autoload/autocreate sync
  libbpf: replace elf_state->st_ops_* fields with SEC_ST_OPS sec_type
  libbpf: struct_ops in SEC("?.struct_ops") and SEC("?.struct_ops.link")
  libbpf: rewrite btf datasec names starting from '?'
  selftests/bpf: test case for SEC("?.struct_ops")
  bpf: allow '?' at the beginning of DATASEC names
  selftests/bpf: test cases for '?' in BTF names

 kernel/bpf/btf.c                              |  17 +-
 tools/lib/bpf/features.c                      |  22 +++
 tools/lib/bpf/libbpf.c                        | 174 ++++++++++++------
 tools/lib/bpf/libbpf_internal.h               |   2 +
 .../selftests/bpf/bpf_testmod/bpf_testmod.c   |  25 +++
 .../selftests/bpf/bpf_testmod/bpf_testmod.h   |   4 +
 .../selftests/bpf/prog_tests/bad_struct_ops.c |  67 +++++++
 tools/testing/selftests/bpf/prog_tests/btf.c  |  46 +++++
 .../bpf/prog_tests/struct_ops_autocreate.c    | 124 +++++++++++++
 .../bpf/prog_tests/test_struct_ops_module.c   |  33 +++-
 .../selftests/bpf/progs/bad_struct_ops.c      |  25 +++
 .../selftests/bpf/progs/bad_struct_ops2.c     |  14 ++
 .../bpf/progs/struct_ops_autocreate.c         |  52 ++++++
 .../selftests/bpf/progs/struct_ops_module.c   |  21 ++-
 tools/testing/selftests/bpf/test_progs.c      |  57 ++++++
 tools/testing/selftests/bpf/test_progs.h      |   3 +
 16 files changed, 614 insertions(+), 72 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/bad_struct_ops.c
 create mode 100644 tools/testing/selftests/bpf/prog_tests/struct_ops_autocreate.c
 create mode 100644 tools/testing/selftests/bpf/progs/bad_struct_ops.c
 create mode 100644 tools/testing/selftests/bpf/progs/bad_struct_ops2.c
 create mode 100644 tools/testing/selftests/bpf/progs/struct_ops_autocreate.c

-- 
2.43.0