libcontainer: Add mountFds struct

We will need to pass more slices of fds to these functions in future
patches. Let's add a struct that just contains them all, instead of
adding lot of parameters to these functions.

Signed-off-by: Rodrigo Campos <rodrigoca@microsoft.com>

libcontainer/init_linux.go

@@ -47,6 +47,15 @@ type network struct {
 	TempVethPeerName string `json:"temp_veth_peer_name"`
 }
 
+type mountFds struct {
+	// Fds to use as source when mounting
+	// Size should be the same as container mounts, as it will be paired.
+	// The value -1 is used when no fd is needed for the mount.
+	// Can't have a valid fd in the same position that other slices in this struct.
+	// We need to use only one of these fds on any single mount.
+	sourceFds []int
+}
+
 // initConfig is used for transferring parameters from Exec() to Init()
 type initConfig struct {
 	Args             []string              `json:"args"`
@@ -128,7 +137,7 @@ func StartInitialization() (retErr error) {
 	}
 
 	// Get mount files (O_PATH).
-	mountFds, err := parseMountFds()
+	mountSrcFds, err := parseFdsFromEnv("_LIBCONTAINER_MOUNT_FDS")
 	if err != nil {
 		return err
 	}
@@ -148,10 +157,10 @@ func StartInitialization() (retErr error) {
 	}()
 
 	// If init succeeds, it will not return, hence none of the defers will be called.
-	return containerInit(it, pipe, consoleSocket, fifofd, logPipeFd, mountFds)
+	return containerInit(it, pipe, consoleSocket, fifofd, logPipeFd, mountFds{sourceFds: mountSrcFds})
 }
 
-func containerInit(t initType, pipe *os.File, consoleSocket *os.File, fifoFd, logFd int, mountFds []int) error {
+func containerInit(t initType, pipe *os.File, consoleSocket *os.File, fifoFd, logFd int, mountFds mountFds) error {
 	var config *initConfig
 	if err := json.NewDecoder(pipe).Decode(&config); err != nil {
 		return err
@@ -162,8 +171,8 @@ func containerInit(t initType, pipe *os.File, consoleSocket *os.File, fifoFd, lo
 	switch t {
 	case initSetns:
 		// mountFds must be nil in this case. We don't mount while doing runc exec.
-		if mountFds != nil {
-			return errors.New("mountFds must be nil; can't mount from exec")
+		if mountFds.sourceFds != nil {
+			return errors.New("mount source fds must be nil; can't mount from exec")
 		}
 
 		i := &linuxSetnsInit{

libcontainer/rootfs_linux.go

@@ -63,14 +63,14 @@ func needsSetupDev(config *configs.Config) bool {
 // prepareRootfs sets up the devices, mount points, and filesystems for use
 // inside a new mount namespace. It doesn't set anything as ro. You must call
 // finalizeRootfs after this function to finish setting up the rootfs.
-func prepareRootfs(pipe io.ReadWriter, iConfig *initConfig, mountFds []int) (err error) {
+func prepareRootfs(pipe io.ReadWriter, iConfig *initConfig, mountFds mountFds) (err error) {
 	config := iConfig.Config
 	if err := prepareRoot(config); err != nil {
 		return fmt.Errorf("error preparing rootfs: %w", err)
 	}
 
-	if mountFds != nil && len(mountFds) != len(config.Mounts) {
-		return fmt.Errorf("malformed mountFds slice. Expected size: %v, got: %v. Slice: %v", len(config.Mounts), len(mountFds), mountFds)
+	if mountFds.sourceFds != nil && len(mountFds.sourceFds) != len(config.Mounts) {
+		return fmt.Errorf("malformed mountFds slice. Expected size: %v, got: %v. Slice: %v", len(config.Mounts), len(mountFds.sourceFds), mountFds.sourceFds)
 	}
 
 	mountConfig := &mountConfig{
@@ -84,8 +84,8 @@ func prepareRootfs(pipe io.ReadWriter, iConfig *initConfig, mountFds []int) (err
 		entry := mountEntry{Mount: m}
 		// Just before the loop we checked that if not empty, len(mountFds) == len(config.Mounts).
 		// Therefore, we can access mountFds[i] without any concerns.
-		if mountFds != nil && mountFds[i] != -1 {
-			entry.srcFD = "/proc/self/fd/" + strconv.Itoa(mountFds[i])
+		if mountFds.sourceFds != nil && mountFds.sourceFds[i] != -1 {
+			entry.srcFD = "/proc/self/fd/" + strconv.Itoa(mountFds.sourceFds[i])
 		}
 
 		if err := mountToRootfs(mountConfig, entry); err != nil {

libcontainer/standard_init_linux.go

@@ -25,7 +25,7 @@ type linuxStandardInit struct {
 	parentPid     int
 	fifoFd        int
 	logFd         int
-	mountFds      []int
+	mountFds      mountFds
 	config        *initConfig
 }
 
@@ -86,15 +86,15 @@ func (l *linuxStandardInit) Init() error {
 	// initialises the labeling system
 	selinux.GetEnabled()
 
-	// We don't need the mountFds after prepareRootfs() nor if it fails.
+	// We don't need the mountFds.SourceFds after prepareRootfs() nor if it fails.
 	err := prepareRootfs(l.pipe, l.config, l.mountFds)
-	for _, m := range l.mountFds {
+	for _, m := range l.mountFds.sourceFds {
 		if m == -1 {
 			continue
 		}
 
 		if err := unix.Close(m); err != nil {
-			return fmt.Errorf("Unable to close mountFds fds: %w", err)
+			return fmt.Errorf("unable to close mount sourceFds: %w", err)
 		}
 	}