Description

We are manually building docker / moby runtime in a project and after updating to 19.03.7 we remarked that the Backing Filesystem for overlay2 is now shown as "<unknown>" while it was "extfs" in 19.03.6.

Steps to reproduce the issue:

1. Install docker 19.03.7
2. Execute "docker info"

Describe the results you received:

Server:
 Server Version: 19.03.7
 Storage Driver: overlay2
  Backing Filesystem: <unknown>
  Supports d_type: true
  Native Overlay Diff: true

Describe the results you expected:

Server:
 Server Version: 19.03.7
 Storage Driver: overlay2
  Backing Filesystem: extfs
  Supports d_type: true
  Native Overlay Diff: true

Additional information:

This seems have been introduced with commit 5b6f2e1 which removes the filesystem check and now ommits setting backingFs variable, making it default to "<unknown>"

I am not a Go programmer, but probably something like the following can be readded in overlay.go in both overlay drivers:

fsMagic, err := graphdriver.GetFSMagic(testdir)
if err == nil {
    if fsName, ok := graphdriver.FsNames[fsMagic]; ok {
        backingFs = fsName
    }
}