Made this change per suggestion in #2868 (comment)

Looking at the logic now, I'm wondering if return true is "correct" if we have an error; is it ok to assume that we are in userns if we receive an error, or should we assume the reverse?

I guess if this file is not parsable, it means things are non-functional anyway, so perhaps not an issue (but looking from the other perspective; if the file-mapping is not-parsable, we likely won't be able to be using user-namespaces, in which case false would make more sense)

@AkihiroSuda @kolyshkin wdyt?

Alternative would be:

func uidMapInUserNS(uidMap string) bool {
	var a, b, c int64
	_, err := fmt.Sscanf(uidMap, "%d %d %d", &a, &b, &c)
	if err != nil {
		return false
	}

	/*
	 * We assume we are in the initial user namespace if we have a full
	 * range - 4294967295 uids starting at uid 0.
	 */
	if a == 0 && b == 0 && c == 4294967295 {
		return false
	}
	return true
}

One other way would be to panic() if we can't parse this. That is too radical I think.

As to whether return true or false in case we're not sure, I'd say emit a warning and return false as e.g. cgroups/fs drivers use true to skip devices setting, and we don't want that.

So I like your last version, which can be further simplified by doing

initns := a == 0 && b == 0 && c == 4294967295
return !initns

I'd also replace the comment ("We assume we are ...") with a reference to user_namespaces(7):

// As per user_namespaces(7), /proc/self/uid_map of
// the initial user namespace shows 0 0 4294967295.

Yes, I think in lxc, they don't check uidMap == "", so they return true when got an EOF error.
But we checked this, so return false is correct, I think it will never happen.

This link is gone now. Now it's in here: https://github.com/lxc/incus/blob/main/shared/util.go#L678-L700

Thx; updated to use a permalink