I am trying to start with BPF CO:RE Development. Using Ubuntu 20.04 LTS in a VM, I needed to recompile the kernel and install pahole (from apt install dwarves) so that BTF is enabled (I set CONFIG_DEBUG_FS=y and CONFIG_DEBUG_INFO_BTF=y).
So my setup is:
/sys/kernel/btf/vmlinux exists and can be read out with cat.
But bpftool shows the following error:
$ sudo bpftool btf dump file /sys/kernel/btf/vmlinux format c
libbpf: failed to get EHDR from /sys/kernel/btf/vmlinux
Error: failed to load BTF from /sys/kernel/btf/vmlinux: Unknown error -4001
From https://github.com/libbpf/libbpf/blob/master/src/libbpf.h it looks like it is LIBBPF_ERRNO__FORMAT, /* BPF object format invalid */ but I can not find out what's wrong.
Does anybody know where the mistake might be?
Thanks in advance!
EDIT: Added bpftool version
You need to update bpftool to support a fallback to reading BTF as raw data if the input file is not an object file. The minimum bpftool version required is v5.5 as that's the Linux release where the patch landed. In general, I would recommend to always use the latest bpftool version as there are no backports.