pkg/netns: add NewNSFrom()

The new API should replace the logic from Podman's setupNetNS() which
did several things incorrect. First and foremost it never bind mounted
the netns dir causing problems when the netns dir was later bind mounted
duplicating the netns mounts on diffeent parents which then caused all
unmount attempts to fail[1].

Also the Podman code did never check for conlicts so we way end up
overwriting an existing netns (super unlikely as we use 16 random bytes
but still). At the very least it duplicates the name logic for no reason
there so having this all here makes much more sense.

Of course we still have to port podman over to this new API.

[1] https://issues.redhat.com/browse/RHEL-59620
Backport for https://issues.redhat.com/browse/RHEL-59703

Signed-off-by: Paul Holzinger <[email protected]>

pkg/netns/netns_linux.go

@@ -40,6 +40,8 @@ import (
 // threadNsPath is the /proc path to the current netns handle for the current thread
 const threadNsPath = "/proc/thread-self/ns/net"
 
+var errNoFreeName = errors.New("failed to find free netns path name")
+
 // GetNSRunDir returns the dir of where to create the netNS. When running
 // rootless, it needs to be at a location writable by user.
 func GetNSRunDir() (string, error) {
@@ -61,12 +63,10 @@ func NewNSAtPath(nsPath string) (ns.NetNS, error) {
 // an object representing that namespace, without switching to it.
 func NewNS() (ns.NetNS, error) {
 	for i := 0; i < 10000; i++ {
-		b := make([]byte, 16)
-		_, err := rand.Reader.Read(b)
+		nsName, err := getRandomNetnsName()
 		if err != nil {
-			return nil, fmt.Errorf("failed to generate random netns name: %v", err)
+			return nil, err
 		}
-		nsName := fmt.Sprintf("netns-%x-%x-%x-%x-%x", b[0:4], b[4:6], b[6:8], b[8:10], b[10:])
 		ns, err := NewNSWithName(nsName)
 		if err == nil {
 			return ns, nil
@@ -77,7 +77,7 @@ func NewNS() (ns.NetNS, error) {
 		}
 		return nil, err
 	}
-	return nil, errors.New("failed to find free netns path name")
+	return nil, errNoFreeName
 }
 
 // NewNSWithName creates a new persistent (bind-mounted) network namespace and returns
@@ -100,6 +100,58 @@ func NewNSWithName(name string) (ns.NetNS, error) {
 	return newNSPath(nsPath)
 }
 
+// NewNSFrom creates a persistent (bind-mounted) network namespace from the
+// given netns path, i.e. /proc/<pid>/ns/net, and returns the new full path to
+// the bind mounted file in the netns run dir.
+func NewNSFrom(fromNetns string) (string, error) {
+	nsRunDir, err := GetNSRunDir()
+	if err != nil {
+		return "", err
+	}
+
+	err = makeNetnsDir(nsRunDir)
+	if err != nil {
+		return "", err
+	}
+
+	for i := 0; i < 10000; i++ {
+		nsName, err := getRandomNetnsName()
+		if err != nil {
+			return "", err
+		}
+		nsPath := filepath.Join(nsRunDir, nsName)
+
+		// create an empty file to use as at the mount point
+		err = createNetnsFile(nsPath)
+		if err != nil {
+			// retry when the name already exists
+			if errors.Is(err, os.ErrExist) {
+				continue
+			}
+			return "", err
+		}
+
+		err = unix.Mount(fromNetns, nsPath, "none", unix.MS_BIND|unix.MS_SHARED|unix.MS_REC, "")
+		if err != nil {
+			// Do not leak the ns on errors
+			_ = os.RemoveAll(nsPath)
+			return "", fmt.Errorf("failed to bind mount ns at %s: %v", nsPath, err)
+		}
+		return nsPath, nil
+	}
+
+	return "", errNoFreeName
+}
+
+func getRandomNetnsName() (string, error) {
+	b := make([]byte, 16)
+	_, err := rand.Reader.Read(b)
+	if err != nil {
+		return "", fmt.Errorf("failed to generate random netns name: %v", err)
+	}
+	return fmt.Sprintf("netns-%x-%x-%x-%x-%x", b[0:4], b[4:6], b[6:8], b[8:10], b[10:]), nil
+}
+
 func makeNetnsDir(nsRunDir string) error {
 	err := os.MkdirAll(nsRunDir, 0o755)
 	if err != nil {
@@ -151,16 +203,22 @@ func makeNetnsDir(nsRunDir string) error {
 	return nil
 }
 
-func newNSPath(nsPath string) (ns.NetNS, error) {
-	// create an empty file at the mount point
-	mountPointFd, err := os.OpenFile(nsPath, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0o600)
+// createNetnsFile created the file with O_EXCL to ensure there are no conflicts with others
+// Callers should check for ErrExist and loop over it to find a free file.
+func createNetnsFile(path string) error {
+	mountPointFd, err := os.OpenFile(path, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0o600)
 	if err != nil {
-		return nil, err
+		return err
 	}
-	if err := mountPointFd.Close(); err != nil {
+	return mountPointFd.Close()
+}
+
+func newNSPath(nsPath string) (ns.NetNS, error) {
+	// create an empty file to use as at the mount point
+	err := createNetnsFile(nsPath)
+	if err != nil {
 		return nil, err
 	}
-
 	// Ensure the mount point is cleaned up on errors; if the namespace
 	// was successfully mounted this will have no effect because the file
 	// is in-use