CO-RE requires to have BTF information describing the kernel types in order to perform the relocations. This is usually provided by the kernel itself when it's configured with CONFIG_DEBUG_INFO_BTF. However, this configuration is not enabled in all the distributions and it's not available on kernels before 5.12. It's possible to use CO-RE in kernels without CONFIG_DEBUG_INFO_BTF support by providing the BTF information from an external source. BTFHub[0] contains BTF files to each released kernel not supporting BTF, for the most popular distributions. Providing this BTF file for a given kernel has some challenges: 1. Each BTF file is a few MBs big, then it's not possible to ship the eBPF program with all the BTF files needed to run in different kernels. (The BTF files will be in the order of GBs if you want to support a high number of kernels) 2. Downloading the BTF file for the current kernel at runtime delays the start of the program and it's not always possible to reach an external host to download such a file. Providing the BTF file with the information about all the data types of the kernel for running an eBPF program is an overkill in many of the cases. Usually the eBPF programs access only some kernel fields. This series implements BTFGen support in bpftool. This idea was discussed during the "Towards truly portable eBPF"[1] presentation at Linux Plumbers 2021. There is a good example[2] on how to use BTFGen and BTFHub together to generate multiple BTF files, to each existing/supported kernel, tailored to one application. For example: a complex bpf object might support nearly 400 kernels by having BTF files summing only 1.5 MB. [0]: https://github.com/aquasecurity/btfhub/ [1]: https://www.youtube.com/watch?v=igJLKyP1lFk&t=2418s [2]: https://github.com/aquasecurity/btfhub/tree/main/tools Changelog: v3 > v4: - parse BTF and BTF.ext sections in bpftool and use bpf_core_calc_relo_insn() directly - expose less internal details from libbpf to bpftool - implement support for enum-based relocations - split commits in a more granular way v2 > v3: - expose internal libbpf APIs to bpftool instead - implement btfgen in bpftool - drop btf__raw_data() from libbpf v1 > v2: - introduce bpf_object__prepare() and ‘record_core_relos’ to expose CO-RE relocations instead of bpf_object__reloc_info_gen() - rename btf__save_to_file() to btf__raw_data() v1: https://lore.kernel.org/bpf/20211027203727.208847-1-mauricio@xxxxxxxxxx/ v2: https://lore.kernel.org/bpf/20211116164208.164245-1-mauricio@xxxxxxxxxx/ v3: https://lore.kernel.org/bpf/20211217185654.311609-1-mauricio@xxxxxxxxxx/ Mauricio Vásquez (8): libbpf: split bpf_core_apply_relo() libbpf: Implement changes needed for BTFGen in bpftool bpftool: Add gen btf command bpftool: Implement btf_save_raw() bpftool: Add struct definitions and helpers for BTFGen bpftool: Implement btfgen() bpftool: Implement relocations recording for BTFGen bpftool: Implement btfgen_get_btf() kernel/bpf/btf.c | 13 +- tools/bpf/bpftool/gen.c | 849 ++++++++++++++++++++++++++++++++ tools/lib/bpf/Makefile | 2 +- tools/lib/bpf/libbpf.c | 127 +++-- tools/lib/bpf/libbpf_internal.h | 12 + tools/lib/bpf/relo_core.c | 79 +-- tools/lib/bpf/relo_core.h | 42 +- 7 files changed, 1012 insertions(+), 112 deletions(-) -- 2.25.1
