Resolve mount paths while setting up root fs mount

It's hard to resolve symlinks inside the sandbox because rootfs and mounts
may be read-only, forcing us to create mount points inside lower layer of an
overlay, **before** the volumes are mounted.

Since the destination must already be resolved outside the sandbox when creating
mounts, take this opportunity to rewrite the spec with paths resolved.
"runsc boot" will use the "resolved" spec to load mounts. In addition, symlink
traversals were disabled while mounting containers inside the sandbox.

It haven't been able to write a good test for it. So I'm relying on manual tests
for now.

PiperOrigin-RevId: 217749904
Change-Id: I7ac434d5befd230db1488446cda03300cc0751a9

2 files changed, 50 insertions, 96 deletions

diff --git a/runsc/boot/config.go b/runsc/boot/config.go
index 41af084b9..51d20d06d 100644
--- a/runsc/boot/config.go
+++ b/runsc/boot/config.go
@@ -192,9 +192,6 @@ type Config struct {
 	// disabled. Pardon the double negation, but default to enabled is important.
 	DisableSeccomp bool
 
-	// SpecFile is the file containing the OCI spec.
-	SpecFile string
-
 	// WatchdogAction sets what action the watchdog takes when triggered.
 	WatchdogAction watchdog.Action
 
diff --git a/runsc/boot/fs.go b/runsc/boot/fs.go
index 42e011beb..ea825e571 100644
--- a/runsc/boot/fs.go
+++ b/runsc/boot/fs.go
@@ -103,9 +103,14 @@ func createMountNamespace(userCtx context.Context, rootCtx context.Context, spec
 		return nil, fmt.Errorf("failed to create root mount namespace: %v", err)
 	}
 
-	if err := setMounts(rootCtx, conf, mns, fds, mounts); err != nil {
-		return nil, fmt.Errorf("failed to configure mounts: %v", err)
+	root := mns.Root()
+	defer root.DecRef()
+	for _, m := range mounts {
+		if err := mountSubmount(rootCtx, conf, mns, root, fds, m, mounts); err != nil {
+			return nil, fmt.Errorf("mount submount: %v", err)
+		}
 	}
+
 	if !fds.empty() {
 		return nil, fmt.Errorf("not all mount points were consumed, remaining: %v", fds)
 	}
@@ -184,17 +189,6 @@ func compileMounts(spec *specs.Spec) []specs.Mount {
 	return mounts
 }
 
-// setMounts iterates over mounts and mounts them in the specified
-// mount namespace.
-func setMounts(ctx context.Context, conf *Config, mns *fs.MountNamespace, fds *fdDispenser, mounts []specs.Mount) error {
-	for _, m := range mounts {
-		if err := mountSubmount(ctx, conf, mns, fds, m, mounts, m.Destination); err != nil {
-			return err
-		}
-	}
-	return nil
-}
-
 // createRootMount creates the root filesystem.
 func createRootMount(ctx context.Context, spec *specs.Spec, conf *Config, fds *fdDispenser, mounts []specs.Mount) (*fs.Inode, error) {
 	// First construct the filesystem from the spec.Root.
@@ -207,9 +201,9 @@ func createRootMount(ctx context.Context, spec *specs.Spec, conf *Config, fds *f
 
 	fd := fds.remove()
 	log.Infof("Mounting root over 9P, ioFD: %d", fd)
-	hostFS := mustFindFilesystem("9p")
+	p9FS := mustFindFilesystem("9p")
 	opts := p9MountOptions(fd, conf.FileAccess)
-	rootInode, err = hostFS.Mount(ctx, rootDevice, mf, strings.Join(opts, ","))
+	rootInode, err = p9FS.Mount(ctx, rootDevice, mf, strings.Join(opts, ","))
 	if err != nil {
 		return nil, fmt.Errorf("failed to generate root mount point: %v", err)
 	}
@@ -294,7 +288,11 @@ func getMountNameAndOptions(conf *Config, m specs.Mount, fds *fdDispenser) (stri
 	return fsName, opts, useOverlay, err
 }
 
-func mountSubmount(ctx context.Context, conf *Config, mns *fs.MountNamespace, fds *fdDispenser, m specs.Mount, mounts []specs.Mount, dest string) error {
+// mountSubmount mounts volumes inside the container's root. Because mounts may
+// be readonly, a lower ramfs overlay is added to create the mount point dir.
+// Another overlay is added with tmpfs on top if Config.Overlay is true.
+// 'm.Destination' must be an absolute path with '..' and symlinks resolved.
+func mountSubmount(ctx context.Context, conf *Config, mns *fs.MountNamespace, root *fs.Dirent, fds *fdDispenser, m specs.Mount, mounts []specs.Mount) error {
 	// Map mount type to filesystem name, and parse out the options that we are
 	// capable of dealing with.
 	fsName, opts, useOverlay, err := getMountNameAndOptions(conf, m, fds)
@@ -340,60 +338,16 @@ func mountSubmount(ctx context.Context, conf *Config, mns *fs.MountNamespace, fd
 		}
 	}
 
-	// Create destination in case it doesn't exist. This is required, in addition
-	// to 'addSubmountOverlay', in case there are symlinks to create directories
-	// in the right location, e.g.
-	//   mount: /var/run/secrets, may be created in '/run/secrets' if
-	//   '/var/run' => '/var'.
-	if err := mkdirAll(ctx, mns, dest); err != nil {
-		return err
-	}
-
-	root := mns.Root()
-	defer root.DecRef()
-	dirent, err := mns.FindInode(ctx, root, nil, dest, linux.MaxSymlinkTraversals)
+	dirent, err := mns.FindInode(ctx, root, root, m.Destination, 0 /* maxTraversals */)
 	if err != nil {
-		return fmt.Errorf("failed to find mount destination %q: %v", dest, err)
+		return fmt.Errorf("failed to find mount destination %q: %v", m.Destination, err)
 	}
 	defer dirent.DecRef()
 	if err := mns.Mount(ctx, dirent, inode); err != nil {
-		return fmt.Errorf("failed to mount at destination %q: %v", dest, err)
+		return fmt.Errorf("failed to mount at destination %q: %v", m.Destination, err)
 	}
 
-	log.Infof("Mounted %q to %q type %s", m.Source, dest, m.Type)
-	return nil
-}
-
-func mkdirAll(ctx context.Context, mns *fs.MountNamespace, path string) error {
-	log.Infof("mkdirAll called with path %s", path)
-	root := mns.Root()
-	defer root.DecRef()
-
-	// Starting at the root, walk the path.
-	parent := root
-	ps := strings.Split(filepath.Clean(path), string(filepath.Separator))
-	for _, pathElem := range ps {
-		if pathElem == "" {
-			// This will be case for the first and last element, if the path
-			// begins or ends with '/'. Note that we always treat the path as
-			// absolute, regardless of what the first character contains.
-			continue
-		}
-		d, err := mns.FindInode(ctx, root, parent, pathElem, fs.DefaultTraversalLimit)
-		if err == syserror.ENOENT {
-			// If we encounter a path that does not exist, then
-			// create it.
-			if err := parent.CreateDirectory(ctx, root, pathElem, fs.FilePermsFromMode(0755)); err != nil {
-				return fmt.Errorf("failed to create directory %q: %v", pathElem, err)
-			}
-			if d, err = parent.Walk(ctx, root, pathElem); err != nil {
-				return fmt.Errorf("walk to %q failed: %v", pathElem, err)
-			}
-		} else if err != nil {
-			return fmt.Errorf("failed to find inode %q: %v", pathElem, err)
-		}
-		parent = d
-	}
+	log.Infof("Mounted %q to %q type %s", m.Source, m.Destination, m.Type)
 	return nil
 }
 
@@ -437,14 +391,6 @@ func parseAndFilterOptions(opts []string, allowedKeys ...string) ([]string, erro
 	return out, nil
 }
 
-func destinations(mounts []specs.Mount, extra ...string) []string {
-	var ds []string
-	for _, m := range mounts {
-		ds = append(ds, m.Destination)
-	}
-	return append(ds, extra...)
-}
-
 // mountDevice returns a device string based on the fs type and target
 // of the mount.
 func mountDevice(m specs.Mount) string {
@@ -544,7 +490,8 @@ func mustFindFilesystem(name string) fs.Filesystem {
 func addSubmountOverlay(ctx context.Context, inode *fs.Inode, submounts []string) (*fs.Inode, error) {
 	// There is no real filesystem backing this ramfs tree, so we pass in
 	// "nil" here.
-	mountTree, err := ramfs.MakeDirectoryTree(ctx, fs.NewNonCachingMountSource(nil, fs.MountSourceFlags{}), submounts)
+	msrc := fs.NewNonCachingMountSource(nil, fs.MountSourceFlags{})
+	mountTree, err := ramfs.MakeDirectoryTree(ctx, msrc, submounts)
 	if err != nil {
 		return nil, fmt.Errorf("error creating mount tree: %v", err)
 	}
@@ -608,12 +555,16 @@ func setupContainerFS(procArgs *kernel.CreateProcessArgs, spec *specs.Spec, conf
 	// namespace.
 	mns := k.RootMountNamespace()
 	if mns == nil {
+		// Setup the root container.
+
 		// Create the virtual filesystem.
 		mns, err := createMountNamespace(ctx, rootCtx, spec, conf, goferFDs)
 		if err != nil {
 			return fmt.Errorf("error creating mounts: %v", err)
 		}
 		k.SetRootMountNamespace(mns)
+
+		// We're done with root container.
 		return nil
 	}
 
@@ -627,42 +578,48 @@ func setupContainerFS(procArgs *kernel.CreateProcessArgs, spec *specs.Spec, conf
 		return fmt.Errorf("error creating filesystem for container: %v", err)
 	}
 
-	// Make directories for submounts within the container.
-	rootDir := mns.Root()
-	defer rootDir.DecRef()
-	containerRoot := filepath.Join(ChildContainersDir, cid)
-	mkdirAll(ctx, mns, containerRoot)
+	globalRoot := mns.Root()
+	defer globalRoot.DecRef()
 
-	// Mount the container's root filesystem to the newly created
-	// mount point.
-	containerRootDirent, err := mns.FindInode(ctx, rootDir, nil, containerRoot, linux.MaxSymlinkTraversals)
+	// Create mount point for the container's rootfs.
+	contDir, err := mns.FindInode(ctx, globalRoot, nil, ChildContainersDir, 0 /* TraversalLimit */)
 	if err != nil {
-		return fmt.Errorf("failed to find mount destination: %q: %v", containerRoot, err)
+		return fmt.Errorf("couldn't find child container dir %q: %v", ChildContainersDir, err)
 	}
-	if err := mns.Mount(ctx, containerRootDirent, rootInode); err != nil {
-		return fmt.Errorf("failed to mount at destination %q: %v", containerRoot, err)
+	if err := contDir.CreateDirectory(ctx, globalRoot, cid, fs.FilePermsFromMode(0755)); err != nil {
+		return fmt.Errorf("create directory %q: %v", cid, err)
+	}
+	containerRoot, err := contDir.Walk(ctx, globalRoot, cid)
+	if err != nil {
+		return fmt.Errorf("walk to %q failed: %v", cid, err)
+	}
+	defer containerRoot.DecRef()
+
+	// Mount the container's root filesystem to the newly created mount point.
+	if err := mns.Mount(ctx, containerRoot, rootInode); err != nil {
+		return fmt.Errorf("mount container root: %v", err)
 	}
-	containerRootDirent.DecRef()
 
 	// We have to re-walk to the dirent to find the mounted
 	// directory. The old dirent is invalid at this point.
-	containerRootDirent, err = mns.FindInode(ctx, rootDir, nil, containerRoot, linux.MaxSymlinkTraversals)
+	containerRoot, err = contDir.Walk(ctx, globalRoot, cid)
 	if err != nil {
-		return fmt.Errorf("failed to find mount destination2: %q: %v", containerRoot, err)
+		return fmt.Errorf("find container mount point %q: %v", cid, err)
 	}
-	log.Infof("Mounted child's root fs to %q", containerRoot)
+
+	log.Infof("Mounted child's root fs to %q", filepath.Join(ChildContainersDir, cid))
 
 	// Mount all submounts.
 	mounts := compileMounts(spec)
 	for _, m := range mounts {
-		dest := filepath.Join(containerRoot, m.Destination)
-		if err := mountSubmount(rootCtx, conf, k.RootMountNamespace(), fds, m, mounts, dest); err != nil {
+		if err := mountSubmount(rootCtx, conf, k.RootMountNamespace(), containerRoot, fds, m, mounts); err != nil {
+			containerRoot.DecRef()
 			return fmt.Errorf("error mounting filesystem for container: %v", err)
 		}
 	}
 
 	// Set the procArgs root directory.
-	procArgs.Root = containerRootDirent
+	procArgs.Root = containerRoot
 	return nil
 }
 
@@ -686,7 +643,7 @@ func destroyContainerFS(ctx context.Context, cid string, k *kernel.Kernel) error
 	mnsRoot := mns.Root()
 	defer mnsRoot.DecRef()
 	containerRoot := path.Join(ChildContainersDir, cid)
-	containerRootDirent, err := mns.FindInode(ctx, mnsRoot, nil, containerRoot, linux.MaxSymlinkTraversals)
+	containerRootDirent, err := mns.FindInode(ctx, mnsRoot, nil, containerRoot, 0 /* maxTraversals */)
 	if err == syserror.ENOENT {
 		// Container must have been destroyed already. That's fine.
 		return nil
@@ -720,7 +677,7 @@ func destroyContainerFS(ctx context.Context, cid string, k *kernel.Kernel) error
 
 	// Get a reference to the parent directory and remove the root
 	// container directory.
-	containersDirDirent, err := mns.FindInode(ctx, mnsRoot, nil, ChildContainersDir, linux.MaxSymlinkTraversals)
+	containersDirDirent, err := mns.FindInode(ctx, mnsRoot, nil, ChildContainersDir, 0 /* maxTraversals */)
 	if err != nil {
 		return fmt.Errorf("error finding containers directory %q: %v", ChildContainersDir, err)
 	}