1 files changed, 345 insertions, 0 deletions
diff --git a/pkg/sentry/fs/file_overlay.go b/pkg/sentry/fs/file_overlay.go
new file mode 100644
index 000000000..0c6e622b9
--- /dev/null
+++ b/pkg/sentry/fs/file_overlay.go
@@ -0,0 +1,345 @@
+// Copyright 2018 Google Inc.
+//
+// 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 (
+	"sync"
+
+	"gvisor.googlesource.com/gvisor/pkg/log"
+	"gvisor.googlesource.com/gvisor/pkg/sentry/arch"
+	"gvisor.googlesource.com/gvisor/pkg/sentry/context"
+	"gvisor.googlesource.com/gvisor/pkg/sentry/memmap"
+	"gvisor.googlesource.com/gvisor/pkg/sentry/usermem"
+	"gvisor.googlesource.com/gvisor/pkg/syserror"
+	"gvisor.googlesource.com/gvisor/pkg/waiter"
+)
+
+// overlayFile gets a handle to a file from the upper or lower filesystem
+// in an overlay. The caller is responsible for calling File.DecRef on
+// the returned file.
+func overlayFile(ctx context.Context, inode *Inode, flags FileFlags) (*File, error) {
+	// Do a song and dance to eventually get to:
+	//
+	//   File -> single reference
+	//   Dirent -> single reference
+	//   Inode -> multiple references
+	//
+	// So that File.DecRef() -> File.destroy -> Dirent.DecRef -> Dirent.destroy,
+	// and both the transitory File and Dirent can be GC'ed but the Inode
+	// remains.
+
+	// Take another reference on the Inode.
+	inode.IncRef()
+
+	// Start with a single reference on the Dirent. It inherits the reference
+	// we just took on the Inode above.
+	dirent := NewTransientDirent(inode)
+
+	// Get a File. This will take another reference on the Dirent.
+	f, err := inode.GetFile(ctx, dirent, flags)
+
+	// Drop the extra reference on the Dirent. Now there's only one reference
+	// on the dirent, either owned by f (if non-nil), or the Dirent is about
+	// to be destroyed (if GetFile failed).
+	dirent.DecRef()
+
+	return f, err
+}
+
+// overlayFileOperations implements FileOperations for a file in an overlay.
+type overlayFileOperations struct {
+	// upperMu protects upper below. In contrast lower is stable.
+	upperMu sync.Mutex `state:"nosave"`
+
+	// We can't share Files in upper and lower filesystems between all Files
+	// in an overlay because some file systems expect to get distinct handles
+	// that are not consistent with each other on open(2).
+	//
+	// So we lazily acquire an upper File when the overlayEntry acquires an
+	// upper Inode (it might have one from the start). This synchronizes with
+	// copy up.
+	//
+	// If upper is non-nil and this is not a directory, then lower is ignored.
+	//
+	// For directories, upper and lower are ignored because it is always
+	// necessary to acquire new directory handles so that the directory cursors
+	// of the upper and lower Files are not exhausted.
+	upper *File
+	lower *File
+
+	// dirCursor is a directory cursor for a directory in an overlay.
+	dirCursor string
+
+	// dirCache is cache of DentAttrs from upper and lower Inodes.
+	dirCache *SortedDentryMap
+}
+
+// Release implements FileOperations.Release.
+func (f *overlayFileOperations) Release() {
+	if f.upper != nil {
+		f.upper.DecRef()
+	}
+	if f.lower != nil {
+		f.lower.DecRef()
+	}
+}
+
+// EventRegister implements FileOperations.EventRegister.
+func (f *overlayFileOperations) EventRegister(we *waiter.Entry, mask waiter.EventMask) {
+	f.upperMu.Lock()
+	defer f.upperMu.Unlock()
+	if f.upper != nil {
+		f.upper.EventRegister(we, mask)
+		return
+	}
+	f.lower.EventRegister(we, mask)
+}
+
+// EventUnregister implements FileOperations.Unregister.
+func (f *overlayFileOperations) EventUnregister(we *waiter.Entry) {
+	f.upperMu.Lock()
+	defer f.upperMu.Unlock()
+	if f.upper != nil {
+		f.upper.EventUnregister(we)
+		return
+	}
+	f.lower.EventUnregister(we)
+}
+
+// Readiness implements FileOperations.Readiness.
+func (f *overlayFileOperations) Readiness(mask waiter.EventMask) waiter.EventMask {
+	f.upperMu.Lock()
+	defer f.upperMu.Unlock()
+	if f.upper != nil {
+		return f.upper.Readiness(mask)
+	}
+	return f.lower.Readiness(mask)
+}
+
+// Seek implements FileOperations.Seek.
+func (f *overlayFileOperations) Seek(ctx context.Context, file *File, whence SeekWhence, offset int64) (int64, error) {
+	f.upperMu.Lock()
+	defer f.upperMu.Unlock()
+
+	var seekDir bool
+	var n int64
+	if f.upper != nil {
+		var err error
+		if n, err = f.upper.FileOperations.Seek(ctx, file, whence, offset); err != nil {
+			return n, err
+		}
+		seekDir = IsDir(f.upper.Dirent.Inode.StableAttr)
+	} else {
+		var err error
+		if n, err = f.lower.FileOperations.Seek(ctx, file, whence, offset); err != nil {
+			return n, err
+		}
+		seekDir = IsDir(f.lower.Dirent.Inode.StableAttr)
+	}
+
+	// If this was a seek on a directory, we must update the cursor.
+	if seekDir && whence == SeekSet && offset == 0 {
+		// Currenly only seeking to 0 on a directory is supported.
+		// FIXME: Lift directory seeking limitations.
+		f.dirCursor = ""
+	}
+	return n, nil
+}
+
+// Readdir implements FileOperations.Readdir.
+func (f *overlayFileOperations) Readdir(ctx context.Context, file *File, serializer DentrySerializer) (int64, error) {
+	o := file.Dirent.Inode.overlay
+
+	o.copyMu.RLock()
+	defer o.copyMu.RUnlock()
+
+	var err error
+	f.dirCache, err = readdirEntries(ctx, o)
+	if err != nil {
+		return file.Offset(), err
+	}
+
+	root := RootFromContext(ctx)
+	defer root.DecRef()
+
+	dirCtx := &DirCtx{
+		Serializer: serializer,
+		DirCursor:  &f.dirCursor,
+	}
+	return DirentReaddir(ctx, file.Dirent, f, root, dirCtx, file.Offset())
+}
+
+// IterateDir implements DirIterator.IterateDir.
+func (f *overlayFileOperations) IterateDir(ctx context.Context, dirCtx *DirCtx, offset int) (int, error) {
+	n, err := GenericReaddir(dirCtx, f.dirCache)
+	return offset + n, err
+}
+
+// Read implements FileOperations.Read.
+func (f *overlayFileOperations) Read(ctx context.Context, file *File, dst usermem.IOSequence, offset int64) (int64, error) {
+	o := file.Dirent.Inode.overlay
+
+	o.copyMu.RLock()
+	defer o.copyMu.RUnlock()
+
+	if o.upper != nil {
+		// We may need to acquire an open file handle to read from if
+		// copy up has occurred. Otherwise we risk reading from the
+		// wrong source.
+		f.upperMu.Lock()
+		if f.upper == nil {
+			var err error
+			f.upper, err = overlayFile(ctx, o.upper, file.Flags())
+			if err != nil {
+				f.upperMu.Unlock()
+				log.Warningf("failed to acquire handle with flags %v: %v", file.Flags(), err)
+				return 0, syserror.EIO
+			}
+		}
+		f.upperMu.Unlock()
+		return f.upper.FileOperations.Read(ctx, f.upper, dst, offset)
+	}
+	return f.lower.FileOperations.Read(ctx, f.lower, dst, offset)
+}
+
+// Write implements FileOperations.Write.
+func (f *overlayFileOperations) Write(ctx context.Context, file *File, src usermem.IOSequence, offset int64) (int64, error) {
+	// f.upper must be non-nil. See inode_overlay.go:overlayGetFile, where the
+	// file is copied up and opened in the upper filesystem if FileFlags.Write.
+	// Write cannot be called if !FileFlags.Write, see FileOperations.Write.
+	return f.upper.FileOperations.Write(ctx, f.upper, src, offset)
+}
+
+// Fsync implements FileOperations.Fsync.
+func (f *overlayFileOperations) Fsync(ctx context.Context, file *File, start, end int64, syncType SyncType) error {
+	var err error
+	f.upperMu.Lock()
+	if f.upper != nil {
+		err = f.upper.FileOperations.Fsync(ctx, f.upper, start, end, syncType)
+	}
+	f.upperMu.Unlock()
+	if f.lower != nil {
+		// N.B. Fsync on the lower filesystem can cause writes of file
+		// attributes (i.e. access time) despite the fact that we must
+		// treat the lower filesystem as read-only.
+		//
+		// This matches the semantics of fsync(2) in Linux overlayfs.
+		err = f.lower.FileOperations.Fsync(ctx, f.lower, start, end, syncType)
+	}
+	return err
+}
+
+// Flush implements FileOperations.Flush.
+func (f *overlayFileOperations) Flush(ctx context.Context, file *File) error {
+	// Flush whatever handles we have.
+	var err error
+	f.upperMu.Lock()
+	if f.upper != nil {
+		err = f.upper.FileOperations.Flush(ctx, f.upper)
+	}
+	f.upperMu.Unlock()
+	if f.lower != nil {
+		err = f.lower.FileOperations.Flush(ctx, f.lower)
+	}
+	return err
+}
+
+// ConfigureMMap implements FileOperations.ConfigureMMap.
+func (*overlayFileOperations) ConfigureMMap(ctx context.Context, file *File, opts *memmap.MMapOpts) error {
+	o := file.Dirent.Inode.overlay
+
+	o.copyMu.RLock()
+	defer o.copyMu.RUnlock()
+
+	if !o.isMappableLocked() {
+		return syserror.ENODEV
+	}
+	// FIXME: This is a copy/paste of fsutil.GenericConfigureMMap,
+	// which we can't use because the overlay implementation is in package fs,
+	// so depending on fs/fsutil would create a circular dependency. Move
+	// overlay to fs/overlay.
+	opts.Mappable = o
+	opts.MappingIdentity = file
+	file.IncRef()
+	return nil
+}
+
+// Ioctl implements fs.FileOperations.Ioctl and always returns ENOTTY.
+func (*overlayFileOperations) Ioctl(ctx context.Context, io usermem.IO, args arch.SyscallArguments) (uintptr, error) {
+	return 0, syserror.ENOTTY
+}
+
+// readdirEntries returns a sorted map of directory entries from the
+// upper and/or lower filesystem.
+func readdirEntries(ctx context.Context, o *overlayEntry) (*SortedDentryMap, error) {
+	// Assert that there is at least one upper or lower entry.
+	if o.upper == nil && o.lower == nil {
+		panic("invalid overlayEntry, needs at least one Inode")
+	}
+	entries := make(map[string]DentAttr)
+
+	// Try the upper filesystem first.
+	if o.upper != nil {
+		var err error
+		entries, err = readdirOne(ctx, NewTransientDirent(o.upper))
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	// Try the lower filesystem next.
+	if o.lower != nil {
+		lowerEntries, err := readdirOne(ctx, NewTransientDirent(o.lower))
+		if err != nil {
+			return nil, err
+		}
+		for name, entry := range lowerEntries {
+			// Skip this name if it is a negative entry in the
+			// upper or there exists a whiteout for it.
+			if o.upper != nil {
+				if overlayHasWhiteout(o.upper, name) {
+					continue
+				}
+			}
+			// Prefer the entries from the upper filesystem
+			// when names overlap.
+			if _, ok := entries[name]; !ok {
+				entries[name] = entry
+			}
+		}
+	}
+
+	// Sort and return the entries.
+	return NewSortedDentryMap(entries), nil
+}
+
+// readdirOne reads all of the directory entries from d.
+func readdirOne(ctx context.Context, d *Dirent) (map[string]DentAttr, error) {
+	dir, err := d.Inode.GetFile(ctx, d, FileFlags{Read: true})
+	if err != nil {
+		return nil, err
+	}
+	defer dir.DecRef()
+
+	// Use a stub serializer to read the entries into memory.
+	stubSerializer := &CollectEntriesSerializer{}
+	if err := dir.Readdir(ctx, stubSerializer); err != nil {
+		return nil, err
+	}
+	// The "." and ".." entries are from the overlay Inode's Dirent, not the stub.
+	delete(stubSerializer.Entries, ".")
+	delete(stubSerializer.Entries, "..")
+	return stubSerializer.Entries, nil
+}