On Wed, Jan 6, 2021 at 10:04 AM Vamsi Kodavanty <vamsi@xxxxxxxxxxxxxxxxxx> wrote: > > Had a few questions on CO-RE dependencies and usage. From what I read > CO-RE needs a supported kernel version and be compiled with > `CONFIG_DEBUG_INFO_BTF=y`. > > I also understand there are three pieces to enable CO-RE > functionality. (1) The BTF format. For efficient/compressed kernel > symbol table. (2) clang changes to emit the BTF relocations. (3) BTF is not really a symbol table, rather a type information. Like simpler and more compact DWARF. > `libbpf` changes to locate a BTF file and fix-up relocations. Once > these 3 steps are done the resulting byte code is no different from > non-CO-RE byte code. > > Given this I am hoping the knowledgeable folks on this mailer correct > and guide me if I am stating something incorrectly. > > (1) Is the kernel support requirement ONLY for the purposes of > generating and exposing the BTF file information on > `/sys/kernel/btf/vmlinux`? So that the eBPF CO-RE applications > `libbpf` can find the BTF information at a standard location?. /sys/kernel/btf/vmlinux is a standardized place, but libbpf will also try to search for vmlinux image (and BTF info within it) in a few standard locations, see [0]. Early versions of in-kernel BTF didn't even expose /sys/kernel/btf/vmlinux. [0] https://github.com/libbpf/libbpf/blob/master/src/btf.c#L4580 > > (2) If the answer to the above question is YES. Could the below > mechanism be used so that it works on all kernels whether they support > the `CONFIG_DEBUG_INFO_BTF` flag or not?. > (a) Extract BTF generation process outside of the kernel build. > Use this to generate the equivalent BTF file for it. Yes, CONFIG_DEBUG_INFO_BTF=y is the most convenient way to add BTF info, but it's also possible to just embed BTF manually with a direct invocation of pahole -J, see [1] on how it's done for CONFIG_DEBUG_INFO_BTF. You can do that for *any* kernel image, no matter the version, and it will work with CO-RE relocations. [1] https://github.com/torvalds/linux/blob/master/scripts/link-vmlinux.sh#L137-L170 > (b) Make changes to `libbpf` to look for BTF not only at the > standard locations but also at a user specified location. The BTF file > generated in (a) can be presented here. You can already do that, actually, though it's not very obvious. You can specify (or override) kernel BTF location by using bpf_object__load_xattr() and passing target_btf_path pointing to your BTF location (see [2]). I've been meaning to add it instead to a bpf_object_open_opts, btw, to make its use possible with a BPF skeleton. Also keep in mind that currently libbpf expects that custom BTF to be an ELF file with .BTF section, not just a raw BTF data. But we can improve that, of course. [2] https://github.com/libbpf/libbpf/blob/master/src/libbpf.h#L136-L141 > > This should provide us a way to enable CO-RE functionality on older > kernel versions as well. I tried to make the above changes and tried > against a 4.14 kernel and it did not work. Either I am not doing > something right or my assumptions are wrong. > > Thanks in advance for your time. And I hope someone here can guide me > in the right direction. > > Regards > Vamsi.
