C++ – Linux BTF : bpftool: Failed to get EHDR from/sys/kernel/btf/vmlinux

I’m trying to start with BPF CO:RE development.
With Ubuntu 20.04 LTS in the VM, I need to recompile the kernel and install pahole (from apt install dwarves) to enable BTF (I set CONFIG_DEBUG_FS=y and CONFIG_DEBUG_INFO_BTF=y).

So my setup is:

• Ubuntu 20.04
• Kernel 5.4.0-90-generic
• bpftool –version:/usr/lib/linux-tools/5.4.0-90-generic/bpftool v5.4.148

/

sys/kernel/btf/vmlinux exists and can be read out with cat.

But bpftool displays 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
Appears to be LIBBPF_ERRNO__FORMAT,/* BPF object format invalid */
But I couldn’t figure out what the problem was.

Does anyone know where the error 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. In general, I recommend always using the latest bpftool version as there is no backporting.