diff options
Diffstat (limited to 'pkg/sentry/fs/copy_up.go')
| -rw-r--r-- | pkg/sentry/fs/copy_up.go | 414 |
1 files changed, 414 insertions, 0 deletions
diff --git a/pkg/sentry/fs/copy_up.go b/pkg/sentry/fs/copy_up.go new file mode 100644 index 000000000..ea74d0efd --- /dev/null +++ b/pkg/sentry/fs/copy_up.go @@ -0,0 +1,414 @@ +// 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 ( + "fmt" + "io" + "sync" + + "gvisor.googlesource.com/gvisor/pkg/log" + "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" +) + +// copyUp copies a file in an overlay from a lower filesystem to an +// upper filesytem so that the file can be modified in the upper +// filesystem. Copying a file involves several steps: +// +// - All parent directories of the file are created in the upper +// filesystem if they don't exist there. For instance: +// +// upper /dir0 +// lower /dir0/dir1/file +// +// copyUp of /dir0/dir1/file creates /dir0/dir1 in order to create +// /dir0/dir1/file. +// +// - The file content is copied from the lower file to the upper +// file. For symlinks this is the symlink target. For directories, +// upper directory entries are merged with lower directory entries +// so there is no need to copy any entries. +// +// - A subset of file attributes of the lower file are set on the +// upper file. These are the file owner, the file timestamps, +// and all non-overlay extended attributes. copyUp will fail if +// the upper filesystem does not support the setting of these +// attributes. +// +// The file's permissions are set when the file is created and its +// size will be brought up to date when its contents are copied. +// Notably no attempt is made to bring link count up to date because +// hard links are currently not preserved across overlay filesystems. +// +// - Memory mappings of the lower file are invalidated and memory +// references are transferred to the upper file. From this point on, +// memory mappings of the file will be backed by content in the upper +// filesystem. +// +// Synchronization: +// +// copyUp synchronizes with rename(2) using renameMu to ensure that +// parentage does not change while a file is being copied. In the context +// of rename(2), copyUpLockedForRename should be used to avoid deadlock on +// renameMu. +// +// The following operations synchronize with copyUp using copyMu: +// +// - InodeOperations, i.e. to ensure that looking up a directory takes +// into account new upper filesystem directories created by copy up, +// which subsequently can be modified. +// +// - FileOperations, i.e. to ensure that reading from a file does not +// continue using a stale, lower filesystem handle when the file is +// written to. +// +// Lock ordering: Dirent.mu -> Inode.overlay.copyMu -> Inode.mu. +// +// Caveats: +// +// If any step in copying up a file fails, copyUp cleans the upper +// filesystem of any partially up-to-date file. If this cleanup fails, +// the overlay may be in an unacceptable, inconsistent state, so copyUp +// panics. If copyUp fails because any step (above) fails, a generic +// error is returned. +// +// copyUp currently makes no attempt to optimize copying up file content. +// For large files, this means that copyUp blocks until the entire file +// is copied synchronously. +func copyUp(ctx context.Context, d *Dirent) error { + renameMu.RLock() + defer renameMu.RUnlock() + return copyUpLockedForRename(ctx, d) +} + +// copyUpLockedForRename is the same as copyUp except that it does not lock +// renameMu. +// +// It copies each component of d that does not yet exist in the upper +// filesystem. If d already exists in the upper filesystem, it is a no-op. +// +// Any error returned indicates a failure to copy all of d. This may +// leave the upper filesystem filled with any number of parent directories +// but the upper filesystem will never be in an inconsistent state. +// +// Preconditions: +// - d.Inode.overlay is non-nil. +func copyUpLockedForRename(ctx context.Context, d *Dirent) error { + for { + // Did we race with another copy up or does there + // already exist something in the upper filesystem + // for d? + d.Inode.overlay.copyMu.Lock() + if d.Inode.overlay.upper != nil { + d.Inode.overlay.copyMu.Unlock() + // Done, d is in the upper filesystem. + return nil + } + d.Inode.overlay.copyMu.Unlock() + + // Find the next component to copy up. We will work our way + // down to the last component of d and finally copy it. + next := findNextCopyUp(ctx, d) + + // Attempt to copy. + if err := doCopyUp(ctx, next); err != nil { + return err + } + } +} + +// findNextCopyUp finds the next component of d from root that does not +// yet exist in the upper filesystem. The parent of this component is +// also returned, which is the root of the overlay in the worst case. +func findNextCopyUp(ctx context.Context, d *Dirent) *Dirent { + next := d + for parent := next.parent; ; /* checked in-loop */ /* updated in-loop */ { + // Does this parent have a non-nil upper Inode? + parent.Inode.overlay.copyMu.RLock() + if parent.Inode.overlay.upper != nil { + parent.Inode.overlay.copyMu.RUnlock() + // Note that since we found an upper, it is stable. + return next + } + parent.Inode.overlay.copyMu.RUnlock() + + // Continue searching for a parent with a non-nil + // upper Inode. + next = parent + parent = next.parent + } +} + +func doCopyUp(ctx context.Context, d *Dirent) error { + // Wait to get exclusive access to the upper Inode. + d.Inode.overlay.copyMu.Lock() + defer d.Inode.overlay.copyMu.Unlock() + if d.Inode.overlay.upper != nil { + // We raced with another doCopyUp, no problem. + return nil + } + + // Perform the copy. + return copyUpLocked(ctx, d.parent, d) +} + +// copyUpLocked creates a copy of next in the upper filesystem of parent. +// +// copyUpLocked must be called with d.Inode.overlay.copyMu locked. +// +// Returns a generic error on failure. +// +// Preconditions: +// - parent.Inode.overlay.upper must be non-nil. +// - next.Inode.overlay.copyMu must be locked writable. +// - next.Inode.overlay.lower must be non-nil. +// - upper filesystem must support setting file ownership and timestamps. +func copyUpLocked(ctx context.Context, parent *Dirent, next *Dirent) error { + // Extract the attributes of the file we wish to copy. + attrs, err := next.Inode.overlay.lower.UnstableAttr(ctx) + if err != nil { + log.Warningf("copy up failed to get lower attributes: %v", err) + return syserror.EIO + } + + var childUpperInode *Inode + parentUpper := parent.Inode.overlay.upper + + // Create the file in the upper filesystem and get an Inode for it. + switch next.Inode.StableAttr.Type { + case RegularFile: + childFile, err := parentUpper.Create(ctx, RootFromContext(ctx), next.name, FileFlags{Read: true, Write: true}, attrs.Perms) + if err != nil { + log.Warningf("copy up failed to create file: %v", err) + return syserror.EIO + } + defer childFile.DecRef() + childUpperInode = childFile.Dirent.Inode + + case Directory: + if err := parentUpper.CreateDirectory(ctx, RootFromContext(ctx), next.name, attrs.Perms); err != nil { + log.Warningf("copy up failed to create directory: %v", err) + return syserror.EIO + } + childUpper, err := parentUpper.Lookup(ctx, next.name) + if err != nil { + log.Warningf("copy up failed to lookup directory: %v", err) + cleanupUpper(ctx, parentUpper, next.name) + return syserror.EIO + } + defer childUpper.DecRef() + childUpperInode = childUpper.Inode + + case Symlink: + childLower := next.Inode.overlay.lower + link, err := childLower.Readlink(ctx) + if err != nil { + log.Warningf("copy up failed to read symlink value: %v", err) + return syserror.EIO + } + if err := parentUpper.CreateLink(ctx, RootFromContext(ctx), link, next.name); err != nil { + log.Warningf("copy up failed to create symlink: %v", err) + return syserror.EIO + } + childUpper, err := parentUpper.Lookup(ctx, next.name) + if err != nil { + log.Warningf("copy up failed to lookup symlink: %v", err) + cleanupUpper(ctx, parentUpper, next.name) + return syserror.EIO + } + defer childUpper.DecRef() + childUpperInode = childUpper.Inode + + default: + return syserror.EINVAL + } + + // Bring file attributes up to date. This does not include size, which will be + // brought up to date with copyContentsLocked. + if err := copyAttributesLocked(ctx, childUpperInode, next.Inode.overlay.lower); err != nil { + log.Warningf("copy up failed to copy up attributes: %v", err) + cleanupUpper(ctx, parentUpper, next.name) + return syserror.EIO + } + + // Copy the entire file. + if err := copyContentsLocked(ctx, childUpperInode, next.Inode.overlay.lower, attrs.Size); err != nil { + log.Warningf("copy up failed to copy up contents: %v", err) + cleanupUpper(ctx, parentUpper, next.name) + return syserror.EIO + } + + lowerMappable := next.Inode.overlay.lower.Mappable() + upperMappable := childUpperInode.Mappable() + if lowerMappable != nil && upperMappable == nil { + log.Warningf("copy up failed: cannot ensure memory mapping coherence") + cleanupUpper(ctx, parentUpper, next.name) + return syserror.EIO + } + + // Propagate memory mappings to the upper Inode. + next.Inode.overlay.mapsMu.Lock() + defer next.Inode.overlay.mapsMu.Unlock() + if upperMappable != nil { + // Remember which mappings we added so we can remove them on failure. + allAdded := make(map[memmap.MappableRange]memmap.MappingsOfRange) + for seg := next.Inode.overlay.mappings.FirstSegment(); seg.Ok(); seg = seg.NextSegment() { + added := make(memmap.MappingsOfRange) + for m := range seg.Value() { + if err := upperMappable.AddMapping(ctx, m.MappingSpace, m.AddrRange, seg.Start()); err != nil { + for m := range added { + upperMappable.RemoveMapping(ctx, m.MappingSpace, m.AddrRange, seg.Start()) + } + for mr, mappings := range allAdded { + for m := range mappings { + upperMappable.RemoveMapping(ctx, m.MappingSpace, m.AddrRange, mr.Start) + } + } + return err + } + added[m] = struct{}{} + } + allAdded[seg.Range()] = added + } + } + + // Take a reference on the upper Inode (transferred to + // next.Inode.overlay.upper) and make new translations use it. + next.Inode.overlay.dataMu.Lock() + childUpperInode.IncRef() + next.Inode.overlay.upper = childUpperInode + next.Inode.overlay.dataMu.Unlock() + + // Invalidate existing translations through the lower Inode. + next.Inode.overlay.mappings.InvalidateAll(memmap.InvalidateOpts{}) + + // Remove existing memory mappings from the lower Inode. + if lowerMappable != nil { + for seg := next.Inode.overlay.mappings.FirstSegment(); seg.Ok(); seg = seg.NextSegment() { + for m := range seg.Value() { + lowerMappable.RemoveMapping(ctx, m.MappingSpace, m.AddrRange, seg.Start()) + } + } + } + + return nil +} + +// cleanupUpper removes name from parent, and panics if it is unsuccessful. +func cleanupUpper(ctx context.Context, parent *Inode, name string) { + if err := parent.InodeOperations.Remove(ctx, parent, name); err != nil { + // Unfortunately we don't have much choice. We shouldn't + // willingly give the caller access to a nonsense filesystem. + panic(fmt.Sprintf("overlay filesystem is in an inconsistent state: failed to remove %q from upper filesystem: %v", name, err)) + } +} + +// copyUpBuffers is a buffer pool for copying file content. The buffer +// size is the same used by io.Copy. +var copyUpBuffers = sync.Pool{New: func() interface{} { return make([]byte, 8*usermem.PageSize) }} + +// copyContentsLocked copies the contents of lower to upper. It panics if +// less than size bytes can be copied. +func copyContentsLocked(ctx context.Context, upper *Inode, lower *Inode, size int64) error { + // We don't support copying up for anything other than regular files. + if lower.StableAttr.Type != RegularFile { + return nil + } + + // Get a handle to the upper filesystem, which we will write to. + upperFile, err := overlayFile(ctx, upper, FileFlags{Write: true}) + if err != nil { + return err + } + defer upperFile.DecRef() + + // Get a handle to the lower filesystem, which we will read from. + lowerFile, err := overlayFile(ctx, lower, FileFlags{Read: true}) + if err != nil { + return err + } + defer lowerFile.DecRef() + + // Use a buffer pool to minimize allocations. + buf := copyUpBuffers.Get().([]byte) + defer copyUpBuffers.Put(buf) + + // Transfer the contents. + // + // One might be able to optimize this by doing parallel reads, parallel writes and reads, larger + // buffers, etc. But we really don't know anything about the underlying implementation, so these + // optimizations could be self-defeating. So we leave this as simple as possible. + var offset int64 + for { + nr, err := lowerFile.FileOperations.Read(ctx, lowerFile, usermem.BytesIOSequence(buf), offset) + if err != nil && err != io.EOF { + return err + } + if nr == 0 { + if offset != size { + // Same as in cleanupUpper, we cannot live + // with ourselves if we do anything less. + panic(fmt.Sprintf("filesystem is in an inconsistent state: wrote only %d bytes of %d sized file", offset, size)) + } + return nil + } + nw, err := upperFile.FileOperations.Write(ctx, upperFile, usermem.BytesIOSequence(buf[:nr]), offset) + if err != nil { + return err + } + offset += nw + } +} + +// copyAttributesLocked copies a subset of lower's attributes to upper, +// specifically owner, timestamps (except of status change time), and +// extended attributes. Notably no attempt is made to copy link count. +// Size and permissions are set on upper when the file content is copied +// and when the file is created respectively. +func copyAttributesLocked(ctx context.Context, upper *Inode, lower *Inode) error { + // Extract attributes fro the lower filesystem. + lowerAttr, err := lower.UnstableAttr(ctx) + if err != nil { + return err + } + lowerXattr, err := lower.Listxattr() + if err != nil && err != syserror.EOPNOTSUPP { + return err + } + + // Set the attributes on the upper filesystem. + if err := upper.InodeOperations.SetOwner(ctx, upper, lowerAttr.Owner); err != nil { + return err + } + if err := upper.InodeOperations.SetTimestamps(ctx, upper, TimeSpec{ + ATime: lowerAttr.AccessTime, + MTime: lowerAttr.ModificationTime, + }); err != nil { + return err + } + for name := range lowerXattr { + value, err := lower.Getxattr(name) + if err != nil { + return err + } + if err := upper.InodeOperations.Setxattr(upper, name, value); err != nil { + return err + } + } + return nil +} |