1 files changed, 136 insertions, 0 deletions
diff --git a/pkg/sentry/fs/mount_overlay.go b/pkg/sentry/fs/mount_overlay.go
new file mode 100644
index 000000000..535f812c8
--- /dev/null
+++ b/pkg/sentry/fs/mount_overlay.go
@@ -0,0 +1,136 @@
+// Copyright 2018 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package fs
+
+import (
+	"gvisor.googlesource.com/gvisor/pkg/sentry/context"
+)
+
+// overlayMountSourceOperations implements MountSourceOperations for an overlay
+// mount point. The upper filesystem determines the caching behavior of the
+// overlay.
+//
+// +stateify savable
+type overlayMountSourceOperations struct {
+	upper *MountSource
+	lower *MountSource
+}
+
+func newOverlayMountSource(upper, lower *MountSource, flags MountSourceFlags) *MountSource {
+	upper.IncRef()
+	lower.IncRef()
+	msrc := NewMountSource(&overlayMountSourceOperations{
+		upper: upper,
+		lower: lower,
+	}, &overlayFilesystem{}, flags)
+
+	// Use the minimum number to keep resource usage under limits.
+	size := lower.fscache.maxSize
+	if size > upper.fscache.maxSize {
+		size = upper.fscache.maxSize
+	}
+	msrc.fscache.setMaxSize(size)
+
+	return msrc
+}
+
+// Revalidate implements MountSourceOperations.Revalidate for an overlay by
+// delegating to the upper filesystem's Revalidate method. We cannot reload
+// files from the lower filesystem, so we panic if the lower filesystem's
+// Revalidate method returns true.
+func (o *overlayMountSourceOperations) Revalidate(ctx context.Context, name string, parent, child *Inode) bool {
+	if child.overlay == nil {
+		panic("overlay cannot revalidate inode that is not an overlay")
+	}
+
+	// Revalidate is never called on a mount point, so parent and child
+	// must be from the same mount, and thus must both be overlay inodes.
+	if parent.overlay == nil {
+		panic("trying to revalidate an overlay inode but the parent is not an overlay")
+	}
+
+	// We can't revalidate from the lower filesystem.
+	if child.overlay.lower != nil && o.lower.Revalidate(ctx, name, parent.overlay.lower, child.overlay.lower) {
+		panic("an overlay cannot revalidate file objects from the lower fs")
+	}
+
+	// Do we have anything to revalidate?
+	if child.overlay.upper == nil {
+		return false
+	}
+
+	// Does the upper require revalidation?
+	return o.upper.Revalidate(ctx, name, parent.overlay.upper, child.overlay.upper)
+}
+
+// Keep implements MountSourceOperations by delegating to the upper
+// filesystem's Keep method.
+func (o *overlayMountSourceOperations) Keep(dirent *Dirent) bool {
+	return o.upper.Keep(dirent)
+}
+
+// ResetInodeMappings propagates the call to both upper and lower MountSource.
+func (o *overlayMountSourceOperations) ResetInodeMappings() {
+	o.upper.ResetInodeMappings()
+	o.lower.ResetInodeMappings()
+}
+
+// SaveInodeMapping propagates the call to both upper and lower MountSource.
+func (o *overlayMountSourceOperations) SaveInodeMapping(inode *Inode, path string) {
+	inode.overlay.copyMu.RLock()
+	defer inode.overlay.copyMu.RUnlock()
+	if inode.overlay.upper != nil {
+		o.upper.SaveInodeMapping(inode.overlay.upper, path)
+	}
+	if inode.overlay.lower != nil {
+		o.lower.SaveInodeMapping(inode.overlay.lower, path)
+	}
+}
+
+// Destroy drops references on the upper and lower MountSource.
+func (o *overlayMountSourceOperations) Destroy() {
+	o.upper.DecRef()
+	o.lower.DecRef()
+}
+
+// type overlayFilesystem is the filesystem for overlay mounts.
+//
+// +stateify savable
+type overlayFilesystem struct{}
+
+// Name implements Filesystem.Name.
+func (ofs *overlayFilesystem) Name() string {
+	return "overlayfs"
+}
+
+// Flags implements Filesystem.Flags.
+func (ofs *overlayFilesystem) Flags() FilesystemFlags {
+	return 0
+}
+
+// AllowUserMount implements Filesystem.AllowUserMount.
+func (ofs *overlayFilesystem) AllowUserMount() bool {
+	return false
+}
+
+// AllowUserList implements Filesystem.AllowUserList.
+func (*overlayFilesystem) AllowUserList() bool {
+	return true
+}
+
+// Mount implements Filesystem.Mount.
+func (ofs *overlayFilesystem) Mount(ctx context.Context, device string, flags MountSourceFlags, data string, _ interface{}) (*Inode, error) {
+	panic("overlayFilesystem.Mount should not be called!")
+}