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 the 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 set of commits extend libbpf to provide an API to generate a BTF file with only the types that are needed by an eBPF object. These generated files are very small compared to the ones that contain all the kernel types (for a program like execsnoop it's around 4kB). This allows to ship an eBPF program together with the BTF information that it needs to run for many different kernels. 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 Mauricio Vásquez (2): libbpf: Implement btf__save_to_file() libbpf: Implement API for generating BTF for ebpf objects tools/lib/bpf/Makefile | 2 +- tools/lib/bpf/btf.c | 22 ++ tools/lib/bpf/btf.h | 2 + tools/lib/bpf/libbpf.c | 28 ++- tools/lib/bpf/libbpf.h | 4 + tools/lib/bpf/libbpf.map | 6 + tools/lib/bpf/relo_core.c | 514 +++++++++++++++++++++++++++++++++++++- tools/lib/bpf/relo_core.h | 11 +- 8 files changed, 579 insertions(+), 10 deletions(-) -- 2.25.1
