map: detect map Name and PinPath separately

[hao-lee]

libbpf uses /sys/fs/bpf as the default pinpath if the user doesn't specify
one. We'd better keep consistent with it.

static int build_map_pin_path(struct bpf_map *map, const char *path)
{
...
	if (!path)
		path = "/sys/fs/bpf";
...
}

Signed-off-by: Hao Lee haolee@didiglobal.com

[lmb]

Hi @hao-lee,

I think this is not a safe default: /sys/fs/bpf is shared across the system, so using it as a default path will lead to conflicts if multiple projects using this library want to pin maps with the same name. For that reason, it's best to always use a subdirectory of /sys/fs/bpf.

If we want to tackle this in the library we need to come up with a safer default. Maybe it's possible to derive a subdirectory using os.Args[0]? This needs some thinking through of corner cases, etc. For example, what happens when using go run file.go like you can do in the examples?

[hao-lee]

Hello,

I think this is not a safe default: /sys/fs/bpf is shared across the system, so using it as a default path will lead to conflicts if multiple projects using this library want to pin maps with the same name. For that reason, it's best to always use a subdirectory of /sys/fs/bpf.

I think maybe it's the responsibility of the eBPF program developer to make sure the map's name is unique.

If we want to tackle this in the library we need to come up with a safer default. Maybe it's possible to derive a subdirectory using os.Args[0]?

This is indeed a solution if we want to avoid conflicts as soon as possible, although os.Args[0] is not foolproof.

---

If the developer doesn't configure the map's PinPath attribute in the Go code, the program will fail to load because of the missing PinPath. I think this behavior is too mandatory, especially for some beginners who only want to fire up a runnable eBPF program as simple as possible. 😅

[lmb]

I think maybe it's the responsibility of the eBPF program developer to make sure the map's name is unique.

From my experience this unfortunately doesn't happen.

I think this behavior is too mandatory, especially for some beginners who only want to fire up a runnable eBPF program as simple as possible.

Will beginners start by pinning maps?

Maybe we can improve the error message?

[hao-lee]

Maybe we can improve the error message?

Agree. The error message confused me.

[hao-lee]

@lmb Hello, I have updated the commit. The current implementation separates error detections. This should be more reasonable than setting default path.

[lmb]

Maybe MapOptions.PinPath?