Replace os.File with fd.FD in fsgofer

os.NewFile() accounts for 38% of CPU time in localFile.Walk().
This change switchs to use fd.FD which is much cheaper to create.
Now, fd.New() in localFile.Walk() accounts for only 4%.

PiperOrigin-RevId: 244944983
Change-Id: Ic892df96cf2633e78ad379227a213cb93ee0ca46

pkg/fd/fd.go

@@ -167,6 +167,24 @@ func NewFromFile(file *os.File) (*FD, error) {
 	return New(fd), nil
 }
 
+// Open is equivallent to open(2).
+func Open(path string, openmode int, perm uint32) (*FD, error) {
+	f, err := syscall.Open(path, openmode|syscall.O_LARGEFILE, perm)
+	if err != nil {
+		return nil, err
+	}
+	return New(f), nil
+}
+
+// OpenAt is equivallent to openat(2).
+func OpenAt(dir *FD, path string, flags int, mode uint32) (*FD, error) {
+	f, err := syscall.Openat(dir.FD(), path, flags, mode)
+	if err != nil {
+		return nil, err
+	}
+	return New(f), nil
+}
+
 // Close closes the file descriptor contained in the FD.
 //
 // Close is safe to call multiple times, but will return an error after the

runsc/sandbox/network.go

@@ -257,6 +257,8 @@ func createInterfacesAndRoutesFromNS(conn *urpc.Client, nsPath string, enableGSO
 					return fmt.Errorf("unable to enable the PACKET_VNET_HDR option: %v", err)
 				}
 				link.GSOMaxSize = ifaceLink.Attrs().GSOMaxSize
+			} else {
+				log.Infof("GSO not available in host.")
 			}
 		}