Allow customizing the base path and don't mount the BPF fs if it is already mounted

[dashpole]

Fixes #732

This enables beyla to be run without the SYS_ADMIN capability. See dashpole#1 for more details.

The code to determine if a mount is a bpf mount is based on https://github.com/cilium/cilium/blob/5130d33a835638c78dda2572d7dc92091ffb3dc1/pkg/mountinfo/mountinfo_linux.go#L27. Importing that would add a dependency on github.com/cilium/cilium, which the project doesn't currently have.

I could:

• Add a dependency on cilium, or
• Put the code in its own file with the cilium copyright header, and also copy the unit tests.

Let me know which you would prefer, and if there is anything else needed (docs updates, etc) for the change.

[mariomac]

Nice! It looks good to me. What do you think would be the correct approach to provide unit/integration tests for this feature?

[mariomac]

With respect to what to do with the Cilium imported code, I think that copying it and give proper attribution in the file headers is OK. Maybe also update this sentence: https://github.com/grafana/beyla/blob/main/NOTICE#L14-L16

[dashpole]

I've updated the copyright headers and the NOTICE file.

For unit/integration testing, i've followed the model used by cilium, which is to have some unit tests that are run as privileged. I have unit tests for IsMountFS, as well as mountBpfPinPath and unmountBpfPinPath that are only run when the PRIVILEGED_TESTS env var is set. I've added a test-privileged case to the Makefile, which can be run with sudo make test-privileged.

But it isn't run as a presubmit right now, as that setup looks complex: https://github.com/cilium/cilium/blob/e3ea2ed7c1250f6f3410c3fe4d6b1f51ea759d65/.github/workflows/conformance-runtime.yaml#L374

[grcevski]

We have an intermittent test issue, I've kicked off a re-run on the integration tests. I think it should pass now...

[grcevski]

LGTM! Looks great, thanks for this contribution @dashpole !!!