I was attempting to build an SPM project on an Ubuntu 16.04 system that has the release package of Swift 4.2 installed from https://swift.org/download/#releases.
When building, I'm getting an permission denied error accessing swift-4.2-RELEASE-ubuntu16.04/usr/lib/swift/CoreFoundation/module.modulemap. Looking at the CoreFoundation directory, it seems like all the files are set with permissions 640 and, with my install, the user and group were root:root.
For comparison, the files in the other usr/lib/swift subdirectories had 644 as the permission. In the Ubuntu 18.04 download, the files in the CoreFoundation directory also had 644 as their permissions.
I checked this against a clean download of both the 16.04 and 18.04 packages. I'm not sure if there are other problematic-seeming permissions in there.