EBUSY when trying to attach a tracing program

[mmat11]

From Slack:

I am trying to attach a Fentry program in a VM built from https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf.git/.

The userspace part of the program is almost copypasted from the one in the examples folder
and it works well on my Fedora laptop (5.14/5.15).
I've compiled the kernel with all the bpf related config enabled but I get device or resource busy
when I try to attach it.

The error comes from this function https://github.com/cilium/ebpf/blob/master/link/tracing.go#L106,
any idea what it could be?

[mmat11]

Solution: set CONFIG_FUNCTION_TRACER=y in the kernel .config (thanks @fntlnz)