[RFC PATCH bpf-next 0/9] Add support for type-based and enum value-based CO-RE relocations

[Andrii Nakryiko <andriin@fb.com>]

N.B. Posting this patch set as an RFC to raise awareness and let people think
about other ways to apply these in practical applications.

This patch set adds libbpf support to two new classes of CO-RE relocations:
type-based (TYPE_EXISTS/TYPE_SIZE/TYPE_ID_LOCAL/TYPE_ID_TARGET) and enum
value-vased (ENUMVAL_EXISTS/ENUMVAL_VALUE):
  - TYPE_EXISTS allows to detect presence in kernel BTF of a locally-recorded
    BTF type. Useful for feature detection (new functionality often comes with
    new internal kernel types), as well as handling type renames and bigger
    refactorings.
  - TYPE_SIZE allows to get the real size (in bytes) of a specified kernel
    type. Useful for dumping internal structure as-is through perfbuf or
    ringbuf.
  - TYPE_ID_LOCAL/TYPE_ID_TARGET allow to capture BTF type ID of a BTF type in
    program's BTF or kernel BTF, respectively. These could be used for
    high-performance and space-efficient generic data dumping/logging by
    relying on small and cheap BTF type ID as a data layout descriptor, for
    post-processing on user-space side.
  - ENUMVAL_EXISTS can be used for detecting the presence of enumerator value
    in kernel's enum type. Most direct application is to detect BPF helper
    support in kernel.
  - ENUMVAL_VALUE allows to relocate real integer value of kernel enumerator
    value, which is subject to change (e.g., always a potential issue for
    internal, non-UAPI, kernel enums).

I've indicated potential applications for these relocations, but relocations
themselves are generic and unassuming and are designed to work correctly even
in unintended applications. Furthermore, relocated values become constants,
known to the verifier and could and would be used for dead branch code
detection and elimination. This makes them ideal to do all sorts of feature
detection and guarding functionality that's not available on some older (but
still supported by BPF program) kernels, while having to compile and maintain
one unified source code.

As part of this patch set, one potential issue with ambiguous CO-RE
relocations was solved (see patch #3). There are also some improvements to the
way debug relocation logs are emitted, helping to get a high-level idea of
what's going on for users that are willing to dive deeper into the internals
of libbpf (or libbpf contributors, of course).

Selftests are added for all the new features and relocation ambiguity issue is
excercised as well.

LLVM patches adding these relocation in Clang:
  - __builtin_btf_type_id() ([0], [1], [2]);
  - __builtin_preserve_type_info(), __builtin_preserve_enum_value() ([3], [4]).

  [0] https://reviews.llvm.org/D74572
  [1] https://reviews.llvm.org/D74668
  [2] https://reviews.llvm.org/D85174
  [3] https://reviews.llvm.org/D83878
  [4] https://reviews.llvm.org/D83242

Andrii Nakryiko (9):
  libbpf: improve error logging for mismatched BTF kind cases
  libbpf: clean up and improve CO-RE reloc logging
  libbpf: improve relocation ambiguity detection
  selftests/bpf: add test validating failure on ambiguous relocation
    value
  libbpf: implement type-based CO-RE relocations support
  selftests/bpf: test TYPE_EXISTS and TYPE_SIZE CO-RE relocations
  selftests/bpf: add CO-RE relo test for TYPE_ID_LOCAL/TYPE_ID_TARGET
  libbpf: implement enum value-based CO-RE relocations
  selftests/bpf: add tests for ENUMVAL_EXISTS/ENUMVAL_VALUE relocations

 tools/lib/bpf/Makefile                        |   2 +-
 tools/lib/bpf/bpf_core_read.h                 |  80 +-
 tools/lib/bpf/btf.c                           |  17 +-
 tools/lib/bpf/btf.h                           |  38 -
 tools/lib/bpf/libbpf.c                        | 754 ++++++++++++++----
 tools/lib/bpf/libbpf_internal.h               |  84 +-
 .../selftests/bpf/prog_tests/core_reloc.c     | 328 +++++++-
 .../bpf/progs/btf__core_reloc_enumval.c       |   3 +
 .../progs/btf__core_reloc_enumval___diff.c    |   3 +
 .../btf__core_reloc_enumval___err_missing.c   |   3 +
 .../btf__core_reloc_enumval___val3_missing.c  |   3 +
 .../btf__core_reloc_size___err_ambiguous.c    |   4 +
 .../bpf/progs/btf__core_reloc_type_based.c    |   3 +
 ...btf__core_reloc_type_based___all_missing.c |   3 +
 .../btf__core_reloc_type_based___diff_sz.c    |   3 +
 ...f__core_reloc_type_based___fn_wrong_args.c |   3 +
 .../btf__core_reloc_type_based___incompat.c   |   3 +
 .../bpf/progs/btf__core_reloc_type_id.c       |   3 +
 ...tf__core_reloc_type_id___missing_targets.c |   3 +
 .../selftests/bpf/progs/core_reloc_types.h    | 352 +++++++-
 .../bpf/progs/test_core_reloc_enumval.c       |  69 ++
 .../bpf/progs/test_core_reloc_type_based.c    | 107 +++
 .../bpf/progs/test_core_reloc_type_id.c       |  94 +++
 23 files changed, 1728 insertions(+), 234 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/progs/btf__core_reloc_enumval.c
 create mode 100644 tools/testing/selftests/bpf/progs/btf__core_reloc_enumval___diff.c
 create mode 100644 tools/testing/selftests/bpf/progs/btf__core_reloc_enumval___err_missing.c
 create mode 100644 tools/testing/selftests/bpf/progs/btf__core_reloc_enumval___val3_missing.c
 create mode 100644 tools/testing/selftests/bpf/progs/btf__core_reloc_size___err_ambiguous.c
 create mode 100644 tools/testing/selftests/bpf/progs/btf__core_reloc_type_based.c
 create mode 100644 tools/testing/selftests/bpf/progs/btf__core_reloc_type_based___all_missing.c
 create mode 100644 tools/testing/selftests/bpf/progs/btf__core_reloc_type_based___diff_sz.c
 create mode 100644 tools/testing/selftests/bpf/progs/btf__core_reloc_type_based___fn_wrong_args.c
 create mode 100644 tools/testing/selftests/bpf/progs/btf__core_reloc_type_based___incompat.c
 create mode 100644 tools/testing/selftests/bpf/progs/btf__core_reloc_type_id.c
 create mode 100644 tools/testing/selftests/bpf/progs/btf__core_reloc_type_id___missing_targets.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_core_reloc_enumval.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_core_reloc_type_based.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_core_reloc_type_id.c

-- 
2.24.1