Add vfs.MkdirOptions.ForSyntheticMountpoint.

PiperOrigin-RevId: 308143529

1 files changed, 215 insertions, 108 deletions

diff --git a/pkg/sentry/fsimpl/gofer/filesystem.go b/pkg/sentry/fsimpl/gofer/filesystem.go
index eba4aabe8..98ccb42fd 100644
--- a/pkg/sentry/fsimpl/gofer/filesystem.go
+++ b/pkg/sentry/fsimpl/gofer/filesystem.go
@@ -29,14 +29,16 @@ import (
 
 // Sync implements vfs.FilesystemImpl.Sync.
 func (fs *filesystem) Sync(ctx context.Context) error {
-	// Snapshot current dentries and special files.
+	// Snapshot current syncable dentries and special files.
 	fs.syncMu.Lock()
-	ds := make([]*dentry, 0, len(fs.dentries))
-	for d := range fs.dentries {
+	ds := make([]*dentry, 0, len(fs.syncableDentries))
+	for d := range fs.syncableDentries {
+		d.IncRef()
 		ds = append(ds, d)
 	}
 	sffds := make([]*specialFileFD, 0, len(fs.specialFileFDs))
 	for sffd := range fs.specialFileFDs {
+		sffd.vfsfd.IncRef()
 		sffds = append(sffds, sffd)
 	}
 	fs.syncMu.Unlock()
@@ -47,9 +49,6 @@ func (fs *filesystem) Sync(ctx context.Context) error {
 
 	// Sync regular files.
 	for _, d := range ds {
-		if !d.TryIncRef() {
-			continue
-		}
 		err := d.syncSharedHandle(ctx)
 		d.DecRef()
 		if err != nil && retErr == nil {
@@ -60,9 +59,6 @@ func (fs *filesystem) Sync(ctx context.Context) error {
 	// Sync special files, which may be writable but do not use dentry shared
 	// handles (so they won't be synced by the above).
 	for _, sffd := range sffds {
-		if !sffd.vfsfd.TryIncRef() {
-			continue
-		}
 		err := sffd.Sync(ctx)
 		sffd.vfsfd.DecRef()
 		if err != nil && retErr == nil {
@@ -114,8 +110,8 @@ func putDentrySlice(ds *[]*dentry) {
 // to *ds.
 //
 // Preconditions: fs.renameMu must be locked. d.dirMu must be locked.
-// !rp.Done(). If fs.opts.interop == InteropModeShared, then d's cached
-// metadata must be up to date.
+// !rp.Done(). If !d.cachedMetadataAuthoritative(), then d's cached metadata
+// must be up to date.
 //
 // Postconditions: The returned dentry's cached metadata is up to date.
 func (fs *filesystem) stepLocked(ctx context.Context, rp *vfs.ResolvingPath, d *dentry, ds **[]*dentry) (*dentry, error) {
@@ -148,7 +144,7 @@ afterSymlink:
 		if err := rp.CheckMount(&d.parent.vfsd); err != nil {
 			return nil, err
 		}
-		if fs.opts.interop == InteropModeShared && d != d.parent {
+		if d != d.parent && !d.cachedMetadataAuthoritative() {
 			_, attrMask, attr, err := d.parent.file.getAttr(ctx, dentryAttrMask())
 			if err != nil {
 				return nil, err
@@ -195,7 +191,7 @@ func (fs *filesystem) getChildLocked(ctx context.Context, vfsObj *vfs.VirtualFil
 		return nil, syserror.ENAMETOOLONG
 	}
 	child, ok := parent.children[name]
-	if ok && fs.opts.interop != InteropModeShared {
+	if (ok && fs.opts.interop != InteropModeShared) || parent.isSynthetic() {
 		// Whether child is nil or not, it is cached information that is
 		// assumed to be correct.
 		return child, nil
@@ -206,7 +202,7 @@ func (fs *filesystem) getChildLocked(ctx context.Context, vfsObj *vfs.VirtualFil
 	return fs.revalidateChildLocked(ctx, vfsObj, parent, name, child, ds)
 }
 
-// Preconditions: As for getChildLocked.
+// Preconditions: As for getChildLocked. !parent.isSynthetic().
 func (fs *filesystem) revalidateChildLocked(ctx context.Context, vfsObj *vfs.VirtualFilesystem, parent *dentry, name string, child *dentry, ds **[]*dentry) (*dentry, error) {
 	qid, file, attrMask, attr, err := parent.file.walkGetAttrOne(ctx, name)
 	if err != nil && err != syserror.ENOENT {
@@ -220,24 +216,41 @@ func (fs *filesystem) revalidateChildLocked(ctx context.Context, vfsObj *vfs.Vir
 			child.updateFromP9Attrs(attrMask, &attr)
 			return child, nil
 		}
-		// The file at this path has changed or no longer exists. Remove
-		// the stale dentry from the tree, and re-evaluate its caching
-		// status (i.e. if it has 0 references, drop it).
+		if file.isNil() && child.isSynthetic() {
+			// We have a synthetic file, and no remote file has arisen to
+			// replace it.
+			return child, nil
+		}
+		// The file at this path has changed or no longer exists. Mark the
+		// dentry invalidated, and re-evaluate its caching status (i.e. if it
+		// has 0 references, drop it). Wait to update parent.children until we
+		// know what to replace the existing dentry with (i.e. one of the
+		// returns below), to avoid a redundant map access.
 		vfsObj.InvalidateDentry(&child.vfsd)
+		if child.isSynthetic() {
+			// Normally we don't mark invalidated dentries as deleted since
+			// they may still exist (but at a different path), and also for
+			// consistency with Linux. However, synthetic files are guaranteed
+			// to become unreachable if their dentries are invalidated, so
+			// treat their invalidation as deletion.
+			child.setDeleted()
+			parent.syntheticChildren--
+			child.decRefLocked()
+			parent.dirents = nil
+		}
 		*ds = appendDentry(*ds, child)
 	}
 	if file.isNil() {
 		// No file exists at this path now. Cache the negative lookup if
 		// allowed.
-		if fs.opts.interop != InteropModeShared {
-			parent.cacheNegativeChildLocked(name)
-		}
+		parent.cacheNegativeLookupLocked(name)
 		return nil, nil
 	}
 	// Create a new dentry representing the file.
 	child, err = fs.newDentry(ctx, file, qid, attrMask, &attr)
 	if err != nil {
 		file.close(ctx)
+		delete(parent.children, name)
 		return nil, err
 	}
 	parent.cacheNewChildLocked(child, name)
@@ -252,8 +265,9 @@ func (fs *filesystem) revalidateChildLocked(ctx context.Context, vfsObj *vfs.Vir
 // rp.Start().Impl().(*dentry)). It does not check that the returned directory
 // is searchable by the provider of rp.
 //
-// Preconditions: fs.renameMu must be locked. !rp.Done(). If fs.opts.interop ==
-// InteropModeShared, then d's cached metadata must be up to date.
+// Preconditions: fs.renameMu must be locked. !rp.Done(). If
+// !d.cachedMetadataAuthoritative(), then d's cached metadata must be up to
+// date.
 func (fs *filesystem) walkParentDirLocked(ctx context.Context, rp *vfs.ResolvingPath, d *dentry, ds **[]*dentry) (*dentry, error) {
 	for !rp.Final() {
 		d.dirMu.Lock()
@@ -275,7 +289,7 @@ func (fs *filesystem) walkParentDirLocked(ctx context.Context, rp *vfs.Resolving
 // Preconditions: fs.renameMu must be locked.
 func (fs *filesystem) resolveLocked(ctx context.Context, rp *vfs.ResolvingPath, ds **[]*dentry) (*dentry, error) {
 	d := rp.Start().Impl().(*dentry)
-	if fs.opts.interop == InteropModeShared {
+	if !d.cachedMetadataAuthoritative() {
 		// Get updated metadata for rp.Start() as required by fs.stepLocked().
 		if err := d.updateFromGetattr(ctx); err != nil {
 			return nil, err
@@ -297,16 +311,17 @@ func (fs *filesystem) resolveLocked(ctx context.Context, rp *vfs.ResolvingPath,
 }
 
 // doCreateAt checks that creating a file at rp is permitted, then invokes
-// create to do so.
+// createInRemoteDir (if the parent directory is a real remote directory) or
+// createInSyntheticDir (if the parent directory is synthetic) to do so.
 //
 // Preconditions: !rp.Done(). For the final path component in rp,
 // !rp.ShouldFollowSymlink().
-func (fs *filesystem) doCreateAt(ctx context.Context, rp *vfs.ResolvingPath, dir bool, create func(parent *dentry, name string) error) error {
+func (fs *filesystem) doCreateAt(ctx context.Context, rp *vfs.ResolvingPath, dir bool, createInRemoteDir func(parent *dentry, name string) error, createInSyntheticDir func(parent *dentry, name string) error) error {
 	var ds *[]*dentry
 	fs.renameMu.RLock()
 	defer fs.renameMuRUnlockAndCheckCaching(&ds)
 	start := rp.Start().Impl().(*dentry)
-	if fs.opts.interop == InteropModeShared {
+	if !start.cachedMetadataAuthoritative() {
 		// Get updated metadata for start as required by
 		// fs.walkParentDirLocked().
 		if err := start.updateFromGetattr(ctx); err != nil {
@@ -340,6 +355,20 @@ func (fs *filesystem) doCreateAt(ctx context.Context, rp *vfs.ResolvingPath, dir
 	defer mnt.EndWrite()
 	parent.dirMu.Lock()
 	defer parent.dirMu.Unlock()
+	if parent.isSynthetic() {
+		if child := parent.children[name]; child != nil {
+			return syserror.EEXIST
+		}
+		if createInSyntheticDir == nil {
+			return syserror.EPERM
+		}
+		if err := createInSyntheticDir(parent, name); err != nil {
+			return err
+		}
+		parent.touchCMtime()
+		parent.dirents = nil
+		return nil
+	}
 	if fs.opts.interop == InteropModeShared {
 		// The existence of a dentry at name would be inconclusive because the
 		// file it represents may have been deleted from the remote filesystem,
@@ -348,21 +377,21 @@ func (fs *filesystem) doCreateAt(ctx context.Context, rp *vfs.ResolvingPath, dir
 		// will fail with EEXIST like we would have. If the RPC succeeds, and a
 		// stale dentry exists, the dentry will fail revalidation next time
 		// it's used.
-		return create(parent, name)
+		return createInRemoteDir(parent, name)
 	}
 	if child := parent.children[name]; child != nil {
 		return syserror.EEXIST
 	}
 	// No cached dentry exists; however, there might still be an existing file
 	// at name. As above, we attempt the file creation RPC anyway.
-	if err := create(parent, name); err != nil {
+	if err := createInRemoteDir(parent, name); err != nil {
 		return err
 	}
+	if child, ok := parent.children[name]; ok && child == nil {
+		// Delete the now-stale negative dentry.
+		delete(parent.children, name)
+	}
 	parent.touchCMtime()
-	// Either parent.children[name] doesn't exist (in which case this is a
-	// no-op) or is nil (in which case this erases the now-stale information
-	// that the file doesn't exist).
-	delete(parent.children, name)
 	parent.dirents = nil
 	return nil
 }
@@ -373,7 +402,7 @@ func (fs *filesystem) unlinkAt(ctx context.Context, rp *vfs.ResolvingPath, dir b
 	fs.renameMu.RLock()
 	defer fs.renameMuRUnlockAndCheckCaching(&ds)
 	start := rp.Start().Impl().(*dentry)
-	if fs.opts.interop == InteropModeShared {
+	if !start.cachedMetadataAuthoritative() {
 		// Get updated metadata for start as required by
 		// fs.walkParentDirLocked().
 		if err := start.updateFromGetattr(ctx); err != nil {
@@ -421,8 +450,10 @@ func (fs *filesystem) unlinkAt(ctx context.Context, rp *vfs.ResolvingPath, dir b
 	// only revalidating the dentry if that fails (indicating that the existing
 	// dentry is a mount point).
 	if child != nil {
+		child.dirMu.Lock()
+		defer child.dirMu.Unlock()
 		if err := vfsObj.PrepareDeleteDentry(mntns, &child.vfsd); err != nil {
-			if fs.opts.interop != InteropModeShared {
+			if parent.cachedMetadataAuthoritative() {
 				return err
 			}
 			child, err = fs.revalidateChildLocked(ctx, vfsObj, parent, name, child, &ds)
@@ -437,13 +468,37 @@ func (fs *filesystem) unlinkAt(ctx context.Context, rp *vfs.ResolvingPath, dir b
 		}
 	}
 	flags := uint32(0)
+	// If a dentry exists, use it for best-effort checks on its deletability.
 	if dir {
-		if child != nil && !child.isDir() {
-			vfsObj.AbortDeleteDentry(&child.vfsd)
-			return syserror.ENOTDIR
+		if child != nil {
+			// child must be an empty directory.
+			if child.syntheticChildren != 0 {
+				// This is definitely not an empty directory, irrespective of
+				// fs.opts.interop.
+				vfsObj.AbortDeleteDentry(&child.vfsd)
+				return syserror.ENOTEMPTY
+			}
+			// If InteropModeShared is in effect and the first call to
+			// PrepareDeleteDentry above succeeded, then child wasn't
+			// revalidated (so we can't expect its file type to be correct) and
+			// individually revalidating its children (to confirm that they
+			// still exist) would be a waste of time.
+			if child.cachedMetadataAuthoritative() {
+				if !child.isDir() {
+					vfsObj.AbortDeleteDentry(&child.vfsd)
+					return syserror.ENOTDIR
+				}
+				for _, grandchild := range child.children {
+					if grandchild != nil {
+						vfsObj.AbortDeleteDentry(&child.vfsd)
+						return syserror.ENOTEMPTY
+					}
+				}
+			}
 		}
 		flags = linux.AT_REMOVEDIR
 	} else {
+		// child must be a non-directory file.
 		if child != nil && child.isDir() {
 			vfsObj.AbortDeleteDentry(&child.vfsd)
 			return syserror.EISDIR
@@ -455,28 +510,36 @@ func (fs *filesystem) unlinkAt(ctx context.Context, rp *vfs.ResolvingPath, dir b
 			return syserror.ENOTDIR
 		}
 	}
-	err = parent.file.unlinkAt(ctx, name, flags)
-	if err != nil {
-		if child != nil {
-			vfsObj.AbortDeleteDentry(&child.vfsd)
-		}
-		return err
-	}
-	if fs.opts.interop != InteropModeShared {
-		parent.touchCMtime()
-		if dir {
-			parent.decLinks()
+	if parent.isSynthetic() {
+		if child == nil {
+			return syserror.ENOENT
 		}
-		parent.cacheNegativeChildLocked(name)
-		parent.dirents = nil
 	} else {
-		delete(parent.children, name)
+		err = parent.file.unlinkAt(ctx, name, flags)
+		if err != nil {
+			if child != nil {
+				vfsObj.AbortDeleteDentry(&child.vfsd)
+			}
+			return err
+		}
 	}
 	if child != nil {
-		child.setDeleted()
 		vfsObj.CommitDeleteDentry(&child.vfsd)
+		child.setDeleted()
+		if child.isSynthetic() {
+			parent.syntheticChildren--
+			child.decRefLocked()
+		}
 		ds = appendDentry(ds, child)
 	}
+	parent.cacheNegativeLookupLocked(name)
+	if parent.cachedMetadataAuthoritative() {
+		parent.dirents = nil
+		parent.touchCMtime()
+		if dir {
+			parent.decLinks()
+		}
+	}
 	return nil
 }
 
@@ -554,7 +617,7 @@ func (fs *filesystem) GetParentDentryAt(ctx context.Context, rp *vfs.ResolvingPa
 	fs.renameMu.RLock()
 	defer fs.renameMuRUnlockAndCheckCaching(&ds)
 	start := rp.Start().Impl().(*dentry)
-	if fs.opts.interop == InteropModeShared {
+	if !start.cachedMetadataAuthoritative() {
 		// Get updated metadata for start as required by
 		// fs.walkParentDirLocked().
 		if err := start.updateFromGetattr(ctx); err != nil {
@@ -577,20 +640,32 @@ func (fs *filesystem) LinkAt(ctx context.Context, rp *vfs.ResolvingPath, vd vfs.
 		}
 		// 9P2000.L supports hard links, but we don't.
 		return syserror.EPERM
-	})
+	}, nil)
 }
 
 // MkdirAt implements vfs.FilesystemImpl.MkdirAt.
 func (fs *filesystem) MkdirAt(ctx context.Context, rp *vfs.ResolvingPath, opts vfs.MkdirOptions) error {
+	creds := rp.Credentials()
 	return fs.doCreateAt(ctx, rp, true /* dir */, func(parent *dentry, name string) error {
-		creds := rp.Credentials()
 		if _, err := parent.file.mkdir(ctx, name, (p9.FileMode)(opts.Mode), (p9.UID)(creds.EffectiveKUID), (p9.GID)(creds.EffectiveKGID)); err != nil {
-			return err
+			if !opts.ForSyntheticMountpoint || err == syserror.EEXIST {
+				return err
+			}
+			ctx.Infof("Failed to create remote directory %q: %v; falling back to synthetic directory", name, err)
+			parent.createSyntheticDirectoryLocked(name, opts.Mode, creds.EffectiveKUID, creds.EffectiveKGID)
 		}
 		if fs.opts.interop != InteropModeShared {
 			parent.incLinks()
 		}
 		return nil
+	}, func(parent *dentry, name string) error {
+		if !opts.ForSyntheticMountpoint {
+			// Can't create non-synthetic files in synthetic directories.
+			return syserror.EPERM
+		}
+		parent.createSyntheticDirectoryLocked(name, opts.Mode, creds.EffectiveKUID, creds.EffectiveKGID)
+		parent.incLinks()
+		return nil
 	})
 }
 
@@ -600,7 +675,7 @@ func (fs *filesystem) MknodAt(ctx context.Context, rp *vfs.ResolvingPath, opts v
 		creds := rp.Credentials()
 		_, err := parent.file.mknod(ctx, name, (p9.FileMode)(opts.Mode), opts.DevMajor, opts.DevMinor, (p9.UID)(creds.EffectiveKUID), (p9.GID)(creds.EffectiveKGID))
 		return err
-	})
+	}, nil)
 }
 
 // OpenAt implements vfs.FilesystemImpl.OpenAt.
@@ -620,7 +695,7 @@ func (fs *filesystem) OpenAt(ctx context.Context, rp *vfs.ResolvingPath, opts vf
 	defer fs.renameMuRUnlockAndCheckCaching(&ds)
 
 	start := rp.Start().Impl().(*dentry)
-	if fs.opts.interop == InteropModeShared {
+	if !start.cachedMetadataAuthoritative() {
 		// Get updated metadata for start as required by fs.stepLocked().
 		if err := start.updateFromGetattr(ctx); err != nil {
 			return nil, err
@@ -643,6 +718,10 @@ afterTrailingSymlink:
 	parent.dirMu.Lock()
 	child, err := fs.stepLocked(ctx, rp, parent, &ds)
 	if err == syserror.ENOENT && mayCreate {
+		if parent.isSynthetic() {
+			parent.dirMu.Unlock()
+			return nil, syserror.EPERM
+		}
 		fd, err := parent.createAndOpenChildLocked(ctx, rp, &opts)
 		parent.dirMu.Unlock()
 		return fd, err
@@ -702,8 +781,10 @@ func (d *dentry) openLocked(ctx context.Context, rp *vfs.ResolvingPath, opts *vf
 		if opts.Flags&linux.O_DIRECT != 0 {
 			return nil, syserror.EINVAL
 		}
-		if err := d.ensureSharedHandle(ctx, ats&vfs.MayRead != 0, false /* write */, false /* trunc */); err != nil {
-			return nil, err
+		if !d.isSynthetic() {
+			if err := d.ensureSharedHandle(ctx, ats&vfs.MayRead != 0, false /* write */, false /* trunc */); err != nil {
+				return nil, err
+			}
 		}
 		fd := &directoryFD{}
 		if err := fd.vfsfd.Init(fd, opts.Flags, mnt, &d.vfsd, &vfs.FileDescriptionOptions{}); err != nil {
@@ -733,6 +814,7 @@ func (d *dentry) openLocked(ctx context.Context, rp *vfs.ResolvingPath, opts *vf
 }
 
 // Preconditions: d.fs.renameMu must be locked. d.dirMu must be locked.
+// !d.isSynthetic().
 func (d *dentry) createAndOpenChildLocked(ctx context.Context, rp *vfs.ResolvingPath, opts *vfs.OpenOptions) (*vfs.FileDescription, error) {
 	if err := d.checkPermissions(rp.Credentials(), vfs.MayWrite); err != nil {
 		return nil, err
@@ -811,7 +893,7 @@ func (d *dentry) createAndOpenChildLocked(ctx context.Context, rp *vfs.Resolving
 	child.refs = 1
 	// Insert the dentry into the tree.
 	d.cacheNewChildLocked(child, name)
-	if d.fs.opts.interop != InteropModeShared {
+	if d.cachedMetadataAuthoritative() {
 		d.touchCMtime()
 		d.dirents = nil
 	}
@@ -888,7 +970,7 @@ func (fs *filesystem) RenameAt(ctx context.Context, rp *vfs.ResolvingPath, oldPa
 	defer mnt.EndWrite()
 
 	oldParent := oldParentVD.Dentry().Impl().(*dentry)
-	if fs.opts.interop == InteropModeShared {
+	if !oldParent.cachedMetadataAuthoritative() {
 		if err := oldParent.updateFromGetattr(ctx); err != nil {
 			return err
 		}
@@ -933,35 +1015,22 @@ func (fs *filesystem) RenameAt(ctx context.Context, rp *vfs.ResolvingPath, oldPa
 	if newParent.isDeleted() {
 		return syserror.ENOENT
 	}
-	replaced := newParent.children[newName]
-	// This is similar to unlinkAt, except:
-	//
-	// - If a dentry exists for the file to be replaced, we revalidate it
-	// unconditionally (instead of only if PrepareRenameDentry fails) for
-	// simplicity.
-	//
-	// - If rp.MustBeDir(), then we need a dentry representing the replaced
-	// file regardless to confirm that it's a directory.
-	if replaced != nil || rp.MustBeDir() {
-		replaced, err = fs.getChildLocked(ctx, rp.VirtualFilesystem(), newParent, newName, &ds)
-		if err != nil {
-			return err
-		}
-		if replaced != nil {
-			if replaced.isDir() {
-				if !renamed.isDir() {
-					return syserror.EISDIR
-				}
-			} else {
-				if rp.MustBeDir() || renamed.isDir() {
-					return syserror.ENOTDIR
-				}
-			}
-		}
+	replaced, err := fs.getChildLocked(ctx, rp.VirtualFilesystem(), newParent, newName, &ds)
+	if err != nil {
+		return err
 	}
 	var replacedVFSD *vfs.Dentry
 	if replaced != nil {
 		replacedVFSD = &replaced.vfsd
+		if replaced.isDir() {
+			if !renamed.isDir() {
+				return syserror.EISDIR
+			}
+		} else {
+			if rp.MustBeDir() || renamed.isDir() {
+				return syserror.ENOTDIR
+			}
+		}
 	}
 
 	if oldParent == newParent && oldName == newName {
@@ -972,27 +1041,47 @@ func (fs *filesystem) RenameAt(ctx context.Context, rp *vfs.ResolvingPath, oldPa
 	if err := vfsObj.PrepareRenameDentry(mntns, &renamed.vfsd, replacedVFSD); err != nil {
 		return err
 	}
-	if err := renamed.file.rename(ctx, newParent.file, newName); err != nil {
-		vfsObj.AbortRenameDentry(&renamed.vfsd, replacedVFSD)
-		return err
+
+	// Update the remote filesystem.
+	if !renamed.isSynthetic() {
+		if err := renamed.file.rename(ctx, newParent.file, newName); err != nil {
+			vfsObj.AbortRenameDentry(&renamed.vfsd, replacedVFSD)
+			return err
+		}
+	} else if replaced != nil && !replaced.isSynthetic() {
+		// We are replacing an existing real file with a synthetic one, so we
+		// need to unlink the former.
+		flags := uint32(0)
+		if replaced.isDir() {
+			flags = linux.AT_REMOVEDIR
+		}
+		if err := newParent.file.unlinkAt(ctx, newName, flags); err != nil {
+			vfsObj.AbortRenameDentry(&renamed.vfsd, replacedVFSD)
+			return err
+		}
 	}
-	if fs.opts.interop != InteropModeShared {
-		oldParent.cacheNegativeChildLocked(oldName)
-		oldParent.dirents = nil
-		newParent.dirents = nil
-		if renamed.isDir() {
-			oldParent.decLinks()
-			newParent.incLinks()
+
+	// Update the dentry tree.
+	vfsObj.CommitRenameReplaceDentry(&renamed.vfsd, replacedVFSD)
+	if replaced != nil {
+		replaced.setDeleted()
+		if replaced.isSynthetic() {
+			newParent.syntheticChildren--
+			replaced.decRefLocked()
 		}
-		oldParent.touchCMtime()
-		newParent.touchCMtime()
-		renamed.touchCtime()
-	} else {
-		delete(oldParent.children, oldName)
+		ds = appendDentry(ds, replaced)
 	}
+	oldParent.cacheNegativeLookupLocked(oldName)
+	// We don't use newParent.cacheNewChildLocked() since we don't want to mess
+	// with reference counts and queue oldParent for checkCachingLocked if the
+	// parent isn't actually changing.
 	if oldParent != newParent {
-		appendDentry(ds, oldParent)
+		ds = appendDentry(ds, oldParent)
 		newParent.IncRef()
+		if renamed.isSynthetic() {
+			oldParent.syntheticChildren--
+			newParent.syntheticChildren++
+		}
 	}
 	renamed.parent = newParent
 	renamed.name = newName
@@ -1000,11 +1089,25 @@ func (fs *filesystem) RenameAt(ctx context.Context, rp *vfs.ResolvingPath, oldPa
 		newParent.children = make(map[string]*dentry)
 	}
 	newParent.children[newName] = renamed
-	if replaced != nil {
-		replaced.setDeleted()
-		appendDentry(ds, replaced)
+
+	// Update metadata.
+	if renamed.cachedMetadataAuthoritative() {
+		renamed.touchCtime()
+	}
+	if oldParent.cachedMetadataAuthoritative() {
+		oldParent.dirents = nil
+		oldParent.touchCMtime()
+		if renamed.isDir() {
+			oldParent.decLinks()
+		}
+	}
+	if newParent.cachedMetadataAuthoritative() {
+		newParent.dirents = nil
+		newParent.touchCMtime()
+		if renamed.isDir() {
+			newParent.incLinks()
+		}
 	}
-	vfsObj.CommitRenameReplaceDentry(&renamed.vfsd, replacedVFSD)
 	return nil
 }
 
@@ -1051,6 +1154,10 @@ func (fs *filesystem) StatFSAt(ctx context.Context, rp *vfs.ResolvingPath) (linu
 	if err != nil {
 		return linux.Statfs{}, err
 	}
+	// If d is synthetic, invoke statfs on the first ancestor of d that isn't.
+	for d.isSynthetic() {
+		d = d.parent
+	}
 	fsstat, err := d.file.statFS(ctx)
 	if err != nil {
 		return linux.Statfs{}, err
@@ -1080,7 +1187,7 @@ func (fs *filesystem) SymlinkAt(ctx context.Context, rp *vfs.ResolvingPath, targ
 		creds := rp.Credentials()
 		_, err := parent.file.symlink(ctx, target, name, (p9.UID)(creds.EffectiveKUID), (p9.GID)(creds.EffectiveKGID))
 		return err
-	})
+	}, nil)
 }
 
 // UnlinkAt implements vfs.FilesystemImpl.UnlinkAt.