CO-RE requires to have BTF information describing the types of the kernel 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 older kernels. 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 extends libbpf to provide an API that can be used for this purpose. The major proposed change is to introduce a new bpf_object__prepare() method that performs all the preparation steps without creating the maps nor loading the programs. This idea was discussed during the "Towards truly portable eBPF"[1] presentation at Linux Plumbers 2021. We prepared a BTFGen repository[2] with an example of how this API can be used. Our plan is to include this support in bpftool once it's merged in libbpf. There is also a good example[3] 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/kinvolk/btfgen [3]: https://github.com/aquasecurity/btfhub/tree/main/tools Changelog: 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/ Mauricio Vásquez (4): libbpf: Implement btf__save_raw() libbpf: Introduce 'btf_custom' to 'bpf_obj_open_opts' libbpf: Introduce 'bpf_object__prepare()' libbpf: Expose CO-RE relocation results tools/lib/bpf/btf.c | 30 ++++++ tools/lib/bpf/btf.h | 2 + tools/lib/bpf/libbpf.c | 213 +++++++++++++++++++++++++++++++------- tools/lib/bpf/libbpf.h | 58 ++++++++++- tools/lib/bpf/libbpf.map | 4 + tools/lib/bpf/relo_core.c | 28 ++++- tools/lib/bpf/relo_core.h | 21 +--- 7 files changed, 294 insertions(+), 62 deletions(-) -- 2.25.1
