By separating parsing BTF from using all the information it provides, we allow BTF to encode new features even if they cannot be used. This is helpful in particular for cases where newer tools for BTF generation run on an older kernel; BTF kinds may be present that the kernel cannot yet use, but at least it can parse the BTF provided. Meanwhile userspace tools with newer libbpf may be able to use the newer information. The intent is to support encoding of kind layouts optionally so that tools like pahole can add this information. So for each kind we record - kind-related flags - length of singular element following struct btf_type - length of each of the btf_vlen() elements following In addition we make space in the BTF header for CRC32s computed over the BTF along with a CRC for the base BTF; this allows split BTF to identify a mismatch explicitly. The ideas here were discussed at [1], with further discussion at [2]. Future work can take more advantage of these features such as - using base CRC to identify base/module BTF mismatch explicitly - using absence of a base BTF CRC as evidence that BTF is standalone ...and new BTF kind addition should present less trouble, provided the kinds are optional. BTF parsing _will_ still fail if a non-optional kind is encountered, as the assumption is that such kinds are needed. To take a few examples, the tag kinds are optional, however enum64 is required, so BTF containing an enum64 (that did not know about enum64) would be rejected. This makes sense as if for example a struct contained an enum64 we would not be able to fully represent that struct unless we knew about enum64s. Patch 1 is the UAPI changes, patches 2-3 provide libbpf support for handling and using kind layouts. Patch 4 adds kernel support for handling and using kind layouts. Patch 5 adds libbpf support to add kind layouts and CRCs based on options passed to btf__new_empty_opts(). Patch 6 updates pahole flags for kernel/module BTF generation to generate kind layout/CRCs if that support is available. Patch 7 adds bpftool support to dump BTF metadata (header + kind layout). Patch 8 documents this. Patch 9 is a set of selftests covering encoding/decoding of BTF kind layout information, and patch 10 is a dwarves patch to add kind layout encoding and CRC support via btf__new_empty_opts(). Changes since RFC - Terminology change from meta -> kind_layout (Alexei and Andrii) - Simplify representation, removing meta header and just having kind layout section (Alexei) - Fixed bpftool to have JSON support, support prefix match, documented changes (Quentin) - Separated metadata opts into add_kind_layout and add_crc - Added additional positive/negative tests to cover basic unknown kind, one with an info_sz object following it and one with N elem_sz elements following it. - Updated pahole-flags to use help output rather than version to see if features are present [1] https://lore.kernel.org/bpf/CAEf4BzYjWHRdNNw4B=eOXOs_ONrDwrgX4bn=Nuc1g8JPFC34MA@xxxxxxxxxxxxxx/ [2] https://lore.kernel.org/bpf/20230531201936.1992188-1-alan.maguire@xxxxxxxxxx/ Alan Maguire (9): btf: add kind layout encoding, crcs to UAPI libbpf: support handling of kind layout section in BTF libbpf: use kind layout to compute an unknown kind size btf: support kernel parsing of BTF with kind layout libbpf: add kind layout encoding, crc support btf: generate BTF kind layout for vmlinux/module BTF bpftool: add BTF dump "format meta" to dump header/metadata bpftool: update doc to describe bpftool btf dump .. format meta selftests/bpf: test kind encoding/decoding include/uapi/linux/btf.h | 24 ++ kernel/bpf/btf.c | 102 +++++-- scripts/pahole-flags.sh | 7 + .../bpf/bpftool/Documentation/bpftool-btf.rst | 16 +- tools/bpf/bpftool/bash-completion/bpftool | 2 +- tools/bpf/bpftool/btf.c | 93 +++++- tools/include/uapi/linux/btf.h | 24 ++ tools/lib/bpf/btf.c | 272 +++++++++++++++--- tools/lib/bpf/btf.h | 11 + tools/lib/bpf/libbpf.map | 1 + .../selftests/bpf/prog_tests/btf_kind.c | 187 ++++++++++++ 11 files changed, 670 insertions(+), 69 deletions(-) create mode 100644 tools/testing/selftests/bpf/prog_tests/btf_kind.c -- 2.39.3
