Automated rollback of changelist 369686285

PiperOrigin-RevId: 371015541

5 files changed, 493 insertions, 187 deletions

diff --git a/pkg/sentry/fsimpl/gofer/BUILD b/pkg/sentry/fsimpl/gofer/BUILD
index 6d5258a9b..52879f871 100644
--- a/pkg/sentry/fsimpl/gofer/BUILD
+++ b/pkg/sentry/fsimpl/gofer/BUILD
@@ -38,6 +38,7 @@ go_library(
         "host_named_pipe.go",
         "p9file.go",
         "regular_file.go",
+        "revalidate.go",
         "save_restore.go",
         "socket.go",
         "special_file.go",
diff --git a/pkg/sentry/fsimpl/gofer/filesystem.go b/pkg/sentry/fsimpl/gofer/filesystem.go
index 4b5621043..40c9243f0 100644
--- a/pkg/sentry/fsimpl/gofer/filesystem.go
+++ b/pkg/sentry/fsimpl/gofer/filesystem.go
@@ -169,156 +169,89 @@ func (fs *filesystem) renameMuUnlockAndCheckCaching(ctx context.Context, ds **[]
 // * fs.renameMu must be locked.
 // * d.dirMu must be locked.
 // * !rp.Done().
-// * If !d.cachedMetadataAuthoritative(), then d's cached metadata must be up
-//   to date.
+// * If !d.cachedMetadataAuthoritative(), then d and all children that are
+//   part of rp must have been revalidated.
 //
 // Postconditions: The returned dentry's cached metadata is up to date.
-func (fs *filesystem) stepLocked(ctx context.Context, rp *vfs.ResolvingPath, d *dentry, mayFollowSymlinks bool, ds **[]*dentry) (*dentry, error) {
+func (fs *filesystem) stepLocked(ctx context.Context, rp *vfs.ResolvingPath, d *dentry, mayFollowSymlinks bool, ds **[]*dentry) (*dentry, bool, error) {
 	if !d.isDir() {
-		return nil, syserror.ENOTDIR
+		return nil, false, syserror.ENOTDIR
 	}
 	if err := d.checkPermissions(rp.Credentials(), vfs.MayExec); err != nil {
-		return nil, err
+		return nil, false, err
 	}
+	followedSymlink := false
 afterSymlink:
 	name := rp.Component()
 	if name == "." {
 		rp.Advance()
-		return d, nil
+		return d, followedSymlink, nil
 	}
 	if name == ".." {
 		if isRoot, err := rp.CheckRoot(ctx, &d.vfsd); err != nil {
-			return nil, err
+			return nil, false, err
 		} else if isRoot || d.parent == nil {
 			rp.Advance()
-			return d, nil
-		}
-		// We must assume that d.parent is correct, because if d has been moved
-		// elsewhere in the remote filesystem so that its parent has changed,
-		// we have no way of determining its new parent's location in the
-		// filesystem.
-		//
-		// Call rp.CheckMount() before updating d.parent's metadata, since if
-		// we traverse to another mount then d.parent's metadata is irrelevant.
-		if err := rp.CheckMount(ctx, &d.parent.vfsd); err != nil {
-			return nil, err
+			return d, followedSymlink, nil
 		}
-		if d != d.parent && !d.cachedMetadataAuthoritative() {
-			if err := d.parent.updateFromGetattr(ctx); err != nil {
-				return nil, err
-			}
+		if err := rp.CheckMount(ctx, &d.parent.vfsd); err != nil {
+			return nil, false, err
 		}
 		rp.Advance()
-		return d.parent, nil
+		return d.parent, followedSymlink, nil
 	}
-	child, err := fs.getChildLocked(ctx, rp.VirtualFilesystem(), d, name, ds)
+	child, err := fs.getChildLocked(ctx, d, name, ds)
 	if err != nil {
-		return nil, err
-	}
-	if child == nil {
-		return nil, syserror.ENOENT
+		return nil, false, err
 	}
 	if err := rp.CheckMount(ctx, &child.vfsd); err != nil {
-		return nil, err
+		return nil, false, err
 	}
 	if child.isSymlink() && mayFollowSymlinks && rp.ShouldFollowSymlink() {
 		target, err := child.readlink(ctx, rp.Mount())
 		if err != nil {
-			return nil, err
+			return nil, false, err
 		}
 		if err := rp.HandleSymlink(target); err != nil {
-			return nil, err
+			return nil, false, err
 		}
+		followedSymlink = true
 		goto afterSymlink // don't check the current directory again
 	}
 	rp.Advance()
-	return child, nil
+	return child, followedSymlink, nil
 }
 
 // getChildLocked returns a dentry representing the child of parent with the
-// given name. If no such child exists, getChildLocked returns (nil, nil).
+// given name. Returns ENOENT if the child doesn't exist.
 //
 // Preconditions:
 // * fs.renameMu must be locked.
 // * parent.dirMu must be locked.
 // * parent.isDir().
 // * name is not "." or "..".
-//
-// Postconditions: If getChildLocked returns a non-nil dentry, its cached
-// metadata is up to date.
-func (fs *filesystem) getChildLocked(ctx context.Context, vfsObj *vfs.VirtualFilesystem, parent *dentry, name string, ds **[]*dentry) (*dentry, error) {
+// * dentry at name has been revalidated
+func (fs *filesystem) getChildLocked(ctx context.Context, parent *dentry, name string, ds **[]*dentry) (*dentry, error) {
 	if len(name) > maxFilenameLen {
 		return nil, syserror.ENAMETOOLONG
 	}
-	child, ok := parent.children[name]
-	if (ok && fs.opts.interop != InteropModeShared) || parent.isSynthetic() {
-		// Whether child is nil or not, it is cached information that is
-		// assumed to be correct.
+	if child, ok := parent.children[name]; ok || parent.isSynthetic() {
+		if child == nil {
+			return nil, syserror.ENOENT
+		}
 		return child, nil
 	}
-	// We either don't have cached information or need to verify that it's
-	// still correct, either of which requires a remote lookup. Check if this
-	// name is valid before performing the lookup.
-	return fs.revalidateChildLocked(ctx, vfsObj, parent, name, child, ds)
-}
 
-// Preconditions: Same as getChildLocked, plus:
-// * !parent.isSynthetic().
-func (fs *filesystem) revalidateChildLocked(ctx context.Context, vfsObj *vfs.VirtualFilesystem, parent *dentry, name string, child *dentry, ds **[]*dentry) (*dentry, error) {
-	if child != nil {
-		// Need to lock child.metadataMu because we might be updating child
-		// metadata. We need to hold the lock *before* getting metadata from the
-		// server and release it after updating local metadata.
-		child.metadataMu.Lock()
-	}
 	qid, file, attrMask, attr, err := parent.file.walkGetAttrOne(ctx, name)
-	if err != nil && err != syserror.ENOENT {
-		if child != nil {
-			child.metadataMu.Unlock()
+	if err != nil {
+		if err == syserror.ENOENT {
+			parent.cacheNegativeLookupLocked(name)
 		}
 		return nil, err
 	}
-	if child != nil {
-		if !file.isNil() && qid.Path == child.qidPath {
-			// The file at this path hasn't changed. Just update cached metadata.
-			file.close(ctx)
-			child.updateFromP9AttrsLocked(attrMask, &attr)
-			child.metadataMu.Unlock()
-			return child, nil
-		}
-		child.metadataMu.Unlock()
-		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(ctx, &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.decRefNoCaching()
-			parent.dirents = nil
-		}
-		*ds = appendDentry(*ds, child)
-	}
-	if file.isNil() {
-		// No file exists at this path now. Cache the negative lookup if
-		// allowed.
-		parent.cacheNegativeLookupLocked(name)
-		return nil, nil
-	}
+
 	// Create a new dentry representing the file.
-	child, err = fs.newDentry(ctx, file, qid, attrMask, &attr)
+	child, err := fs.newDentry(ctx, file, qid, attrMask, &attr)
 	if err != nil {
 		file.close(ctx)
 		delete(parent.children, name)
@@ -344,14 +277,22 @@ func (fs *filesystem) revalidateChildLocked(ctx context.Context, vfsObj *vfs.Vir
 // * 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) {
+	if err := fs.revalidateParentDir(ctx, rp, d, ds); err != nil {
+		return nil, err
+	}
 	for !rp.Final() {
 		d.dirMu.Lock()
-		next, err := fs.stepLocked(ctx, rp, d, true /* mayFollowSymlinks */, ds)
+		next, followedSymlink, err := fs.stepLocked(ctx, rp, d, true /* mayFollowSymlinks */, ds)
 		d.dirMu.Unlock()
 		if err != nil {
 			return nil, err
 		}
 		d = next
+		if followedSymlink {
+			if err := fs.revalidateParentDir(ctx, rp, d, ds); err != nil {
+				return nil, err
+			}
+		}
 	}
 	if !d.isDir() {
 		return nil, syserror.ENOTDIR
@@ -364,20 +305,22 @@ 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 !d.cachedMetadataAuthoritative() {
-		// Get updated metadata for rp.Start() as required by fs.stepLocked().
-		if err := d.updateFromGetattr(ctx); err != nil {
-			return nil, err
-		}
+	if err := fs.revalidatePath(ctx, rp, d, ds); err != nil {
+		return nil, err
 	}
 	for !rp.Done() {
 		d.dirMu.Lock()
-		next, err := fs.stepLocked(ctx, rp, d, true /* mayFollowSymlinks */, ds)
+		next, followedSymlink, err := fs.stepLocked(ctx, rp, d, true /* mayFollowSymlinks */, ds)
 		d.dirMu.Unlock()
 		if err != nil {
 			return nil, err
 		}
 		d = next
+		if followedSymlink {
+			if err := fs.revalidatePath(ctx, rp, d, ds); err != nil {
+				return nil, err
+			}
+		}
 	}
 	if rp.MustBeDir() && !d.isDir() {
 		return nil, syserror.ENOTDIR
@@ -397,13 +340,6 @@ func (fs *filesystem) doCreateAt(ctx context.Context, rp *vfs.ResolvingPath, dir
 	fs.renameMu.RLock()
 	defer fs.renameMuRUnlockAndCheckCaching(ctx, &ds)
 	start := rp.Start().Impl().(*dentry)
-	if !start.cachedMetadataAuthoritative() {
-		// Get updated metadata for start as required by
-		// fs.walkParentDirLocked().
-		if err := start.updateFromGetattr(ctx); err != nil {
-			return err
-		}
-	}
 	parent, err := fs.walkParentDirLocked(ctx, rp, start, &ds)
 	if err != nil {
 		return err
@@ -421,11 +357,14 @@ func (fs *filesystem) doCreateAt(ctx context.Context, rp *vfs.ResolvingPath, dir
 	if parent.isDeleted() {
 		return syserror.ENOENT
 	}
+	if err := fs.revalidateOne(ctx, rp.VirtualFilesystem(), parent, name, &ds); err != nil {
+		return err
+	}
 
 	parent.dirMu.Lock()
 	defer parent.dirMu.Unlock()
 
-	child, err := fs.getChildLocked(ctx, rp.VirtualFilesystem(), parent, name, &ds)
+	child, err := fs.getChildLocked(ctx, parent, name, &ds)
 	switch {
 	case err != nil && err != syserror.ENOENT:
 		return err
@@ -489,13 +428,6 @@ func (fs *filesystem) unlinkAt(ctx context.Context, rp *vfs.ResolvingPath, dir b
 	fs.renameMu.RLock()
 	defer fs.renameMuRUnlockAndCheckCaching(ctx, &ds)
 	start := rp.Start().Impl().(*dentry)
-	if !start.cachedMetadataAuthoritative() {
-		// Get updated metadata for start as required by
-		// fs.walkParentDirLocked().
-		if err := start.updateFromGetattr(ctx); err != nil {
-			return err
-		}
-	}
 	parent, err := fs.walkParentDirLocked(ctx, rp, start, &ds)
 	if err != nil {
 		return err
@@ -521,33 +453,32 @@ func (fs *filesystem) unlinkAt(ctx context.Context, rp *vfs.ResolvingPath, dir b
 			return syserror.EISDIR
 		}
 	}
+
 	vfsObj := rp.VirtualFilesystem()
+	if err := fs.revalidateOne(ctx, vfsObj, parent, rp.Component(), &ds); err != nil {
+		return err
+	}
+
 	mntns := vfs.MountNamespaceFromContext(ctx)
 	defer mntns.DecRef(ctx)
+
 	parent.dirMu.Lock()
 	defer parent.dirMu.Unlock()
 
-	child, ok := parent.children[name]
-	if ok && child == nil {
-		return syserror.ENOENT
-	}
-
-	sticky := atomic.LoadUint32(&parent.mode)&linux.ModeSticky != 0
-	if sticky {
-		if !ok {
-			// If the sticky bit is set, we need to retrieve the child to determine
-			// whether removing it is allowed.
-			child, err = fs.stepLocked(ctx, rp, parent, false /* mayFollowSymlinks */, &ds)
-			if err != nil {
-				return err
-			}
-		} else if child != nil && !child.cachedMetadataAuthoritative() {
-			// Make sure the dentry representing the file at name is up to date
-			// before examining its metadata.
-			child, err = fs.revalidateChildLocked(ctx, vfsObj, parent, name, child, &ds)
-			if err != nil {
-				return err
-			}
+	// Load child if sticky bit is set because we need to determine whether
+	// deletion is allowed.
+	var child *dentry
+	if atomic.LoadUint32(&parent.mode)&linux.ModeSticky == 0 {
+		var ok bool
+		child, ok = parent.children[name]
+		if ok && child == nil {
+			// Hit a negative cached entry, child doesn't exist.
+			return syserror.ENOENT
+		}
+	} else {
+		child, _, err = fs.stepLocked(ctx, rp, parent, false /* mayFollowSymlinks */, &ds)
+		if err != nil {
+			return err
 		}
 		if err := parent.mayDelete(rp.Credentials(), child); err != nil {
 			return err
@@ -556,11 +487,7 @@ func (fs *filesystem) unlinkAt(ctx context.Context, rp *vfs.ResolvingPath, dir b
 
 	// If a child dentry exists, prepare to delete it. This should fail if it is
 	// a mount point. We detect mount points by speculatively calling
-	// PrepareDeleteDentry, which fails if child is a mount point. However, we
-	// may need to revalidate the file in this case to make sure that it has not
-	// been deleted or replaced on the remote fs, in which case the mount point
-	// will have disappeared. If calling PrepareDeleteDentry fails again on the
-	// up-to-date dentry, we can be sure that it is a mount point.
+	// PrepareDeleteDentry, which fails if child is a mount point.
 	//
 	// Also note that if child is nil, then it can't be a mount point.
 	if child != nil {
@@ -575,23 +502,7 @@ func (fs *filesystem) unlinkAt(ctx context.Context, rp *vfs.ResolvingPath, dir b
 		child.dirMu.Lock()
 		defer child.dirMu.Unlock()
 		if err := vfsObj.PrepareDeleteDentry(mntns, &child.vfsd); err != nil {
-			// We can skip revalidation in several cases:
-			// - We are not in InteropModeShared
-			// - The parent directory is synthetic, in which case the child must also
-			//   be synthetic
-			// - We already updated the child during the sticky bit check above
-			if parent.cachedMetadataAuthoritative() || sticky {
-				return err
-			}
-			child, err = fs.revalidateChildLocked(ctx, vfsObj, parent, name, child, &ds)
-			if err != nil {
-				return err
-			}
-			if child != nil {
-				if err := vfsObj.PrepareDeleteDentry(mntns, &child.vfsd); err != nil {
-					return err
-				}
-			}
+			return err
 		}
 	}
 	flags := uint32(0)
@@ -723,13 +634,6 @@ func (fs *filesystem) GetParentDentryAt(ctx context.Context, rp *vfs.ResolvingPa
 	fs.renameMu.RLock()
 	defer fs.renameMuRUnlockAndCheckCaching(ctx, &ds)
 	start := rp.Start().Impl().(*dentry)
-	if !start.cachedMetadataAuthoritative() {
-		// Get updated metadata for start as required by
-		// fs.walkParentDirLocked().
-		if err := start.updateFromGetattr(ctx); err != nil {
-			return nil, err
-		}
-	}
 	d, err := fs.walkParentDirLocked(ctx, rp, start, &ds)
 	if err != nil {
 		return nil, err
@@ -830,7 +734,7 @@ func (fs *filesystem) MknodAt(ctx context.Context, rp *vfs.ResolvingPath, opts v
 		// to creating a synthetic one, i.e. one that is kept entirely in memory.
 
 		// Check that we're not overriding an existing file with a synthetic one.
-		_, err = fs.stepLocked(ctx, rp, parent, true, ds)
+		_, _, err = fs.stepLocked(ctx, rp, parent, true, ds)
 		switch {
 		case err == nil:
 			// Step succeeded, another file exists.
@@ -891,12 +795,6 @@ func (fs *filesystem) OpenAt(ctx context.Context, rp *vfs.ResolvingPath, opts vf
 	defer unlock()
 
 	start := rp.Start().Impl().(*dentry)
-	if !start.cachedMetadataAuthoritative() {
-		// Get updated metadata for start as required by fs.stepLocked().
-		if err := start.updateFromGetattr(ctx); err != nil {
-			return nil, err
-		}
-	}
 	if rp.Done() {
 		// Reject attempts to open mount root directory with O_CREAT.
 		if mayCreate && rp.MustBeDir() {
@@ -905,6 +803,12 @@ func (fs *filesystem) OpenAt(ctx context.Context, rp *vfs.ResolvingPath, opts vf
 		if mustCreate {
 			return nil, syserror.EEXIST
 		}
+		if !start.cachedMetadataAuthoritative() {
+			// Refresh dentry's attributes before opening.
+			if err := start.updateFromGetattr(ctx); err != nil {
+				return nil, err
+			}
+		}
 		start.IncRef()
 		defer start.DecRef(ctx)
 		unlock()
@@ -926,9 +830,12 @@ afterTrailingSymlink:
 	if mayCreate && rp.MustBeDir() {
 		return nil, syserror.EISDIR
 	}
+	if err := fs.revalidateOne(ctx, rp.VirtualFilesystem(), parent, rp.Component(), &ds); err != nil {
+		return nil, err
+	}
 	// Determine whether or not we need to create a file.
 	parent.dirMu.Lock()
-	child, err := fs.stepLocked(ctx, rp, parent, false /* mayFollowSymlinks */, &ds)
+	child, _, err := fs.stepLocked(ctx, rp, parent, false /* mayFollowSymlinks */, &ds)
 	if err == syserror.ENOENT && mayCreate {
 		if parent.isSynthetic() {
 			parent.dirMu.Unlock()
@@ -1297,18 +1204,23 @@ func (fs *filesystem) RenameAt(ctx context.Context, rp *vfs.ResolvingPath, oldPa
 	if err := oldParent.checkPermissions(creds, vfs.MayWrite|vfs.MayExec); err != nil {
 		return err
 	}
+
 	vfsObj := rp.VirtualFilesystem()
+	if err := fs.revalidateOne(ctx, vfsObj, newParent, newName, &ds); err != nil {
+		return err
+	}
+	if err := fs.revalidateOne(ctx, vfsObj, oldParent, oldName, &ds); err != nil {
+		return err
+	}
+
 	// We need a dentry representing the renamed file since, if it's a
 	// directory, we need to check for write permission on it.
 	oldParent.dirMu.Lock()
 	defer oldParent.dirMu.Unlock()
-	renamed, err := fs.getChildLocked(ctx, vfsObj, oldParent, oldName, &ds)
+	renamed, err := fs.getChildLocked(ctx, oldParent, oldName, &ds)
 	if err != nil {
 		return err
 	}
-	if renamed == nil {
-		return syserror.ENOENT
-	}
 	if err := oldParent.mayDelete(creds, renamed); err != nil {
 		return err
 	}
@@ -1337,8 +1249,8 @@ func (fs *filesystem) RenameAt(ctx context.Context, rp *vfs.ResolvingPath, oldPa
 	if newParent.isDeleted() {
 		return syserror.ENOENT
 	}
-	replaced, err := fs.getChildLocked(ctx, rp.VirtualFilesystem(), newParent, newName, &ds)
-	if err != nil {
+	replaced, err := fs.getChildLocked(ctx, newParent, newName, &ds)
+	if err != nil && err != syserror.ENOENT {
 		return err
 	}
 	var replacedVFSD *vfs.Dentry
diff --git a/pkg/sentry/fsimpl/gofer/gofer.go b/pkg/sentry/fsimpl/gofer/gofer.go
index fb42c5f62..21692d2ac 100644
--- a/pkg/sentry/fsimpl/gofer/gofer.go
+++ b/pkg/sentry/fsimpl/gofer/gofer.go
@@ -32,9 +32,9 @@
 //   specialFileFD.mu
 //     specialFileFD.bufMu
 //
-// Locking dentry.dirMu in multiple dentries requires that either ancestor
-// dentries are locked before descendant dentries, or that filesystem.renameMu
-// is locked for writing.
+// Locking dentry.dirMu and dentry.metadataMu in multiple dentries requires that
+// either ancestor dentries are locked before descendant dentries, or that
+// filesystem.renameMu is locked for writing.
 package gofer
 
 import (
diff --git a/pkg/sentry/fsimpl/gofer/p9file.go b/pkg/sentry/fsimpl/gofer/p9file.go
index 21b4a96fe..b0a429d42 100644
--- a/pkg/sentry/fsimpl/gofer/p9file.go
+++ b/pkg/sentry/fsimpl/gofer/p9file.go
@@ -238,3 +238,10 @@ func (f p9file) connect(ctx context.Context, flags p9.ConnectFlags) (*fd.FD, err
 	ctx.UninterruptibleSleepFinish(false)
 	return fdobj, err
 }
+
+func (f p9file) multiGetAttr(ctx context.Context, names []string) ([]p9.FullStat, error) {
+	ctx.UninterruptibleSleepStart(false)
+	stats, err := f.file.MultiGetAttr(names)
+	ctx.UninterruptibleSleepFinish(false)
+	return stats, err
+}
diff --git a/pkg/sentry/fsimpl/gofer/revalidate.go b/pkg/sentry/fsimpl/gofer/revalidate.go
new file mode 100644
index 000000000..8f81f0822
--- /dev/null
+++ b/pkg/sentry/fsimpl/gofer/revalidate.go
@@ -0,0 +1,386 @@
+// Copyright 2021 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 gofer
+
+import (
+	"gvisor.dev/gvisor/pkg/context"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+	"gvisor.dev/gvisor/pkg/sync"
+)
+
+type errPartialRevalidation struct{}
+
+// Error implements error.Error.
+func (errPartialRevalidation) Error() string {
+	return "partial revalidation"
+}
+
+type errRevalidationStepDone struct{}
+
+// Error implements error.Error.
+func (errRevalidationStepDone) Error() string {
+	return "stop revalidation"
+}
+
+// revalidatePath checks cached dentries for external modification. File
+// attributes are refreshed and cache is invalidated in case the dentry has been
+// deleted, or a new file/directory created in its place.
+//
+// Revalidation stops at symlinks and mount points. The caller is responsible
+// for revalidating again after symlinks are resolved and after changing to
+// different mounts.
+//
+// Preconditions:
+// * fs.renameMu must be locked.
+func (fs *filesystem) revalidatePath(ctx context.Context, rpOrig *vfs.ResolvingPath, start *dentry, ds **[]*dentry) error {
+	// Revalidation is done even if start is synthetic in case the path is
+	// something like: ../non_synthetic_file.
+	if fs.opts.interop != InteropModeShared {
+		return nil
+	}
+
+	// Copy resolving path to walk the path for revalidation.
+	rp := rpOrig.Copy()
+	err := fs.revalidate(ctx, rp, start, rp.Done, ds)
+	rp.Release(ctx)
+	return err
+}
+
+// revalidateParentDir does the same as revalidatePath, but stops at the parent.
+//
+// Preconditions:
+// * fs.renameMu must be locked.
+func (fs *filesystem) revalidateParentDir(ctx context.Context, rpOrig *vfs.ResolvingPath, start *dentry, ds **[]*dentry) error {
+	// Revalidation is done even if start is synthetic in case the path is
+	// something like: ../non_synthetic_file and parent is non synthetic.
+	if fs.opts.interop != InteropModeShared {
+		return nil
+	}
+
+	// Copy resolving path to walk the path for revalidation.
+	rp := rpOrig.Copy()
+	err := fs.revalidate(ctx, rp, start, rp.Final, ds)
+	rp.Release(ctx)
+	return err
+}
+
+// revalidateOne does the same as revalidatePath, but checks a single dentry.
+//
+// Preconditions:
+// * fs.renameMu must be locked.
+func (fs *filesystem) revalidateOne(ctx context.Context, vfsObj *vfs.VirtualFilesystem, parent *dentry, name string, ds **[]*dentry) error {
+	// Skip revalidation for interop mode different than InteropModeShared or
+	// if the parent is synthetic (child must be synthetic too, but it cannot be
+	// replaced without first replacing the parent).
+	if parent.cachedMetadataAuthoritative() {
+		return nil
+	}
+
+	parent.dirMu.Lock()
+	child, ok := parent.children[name]
+	parent.dirMu.Unlock()
+	if !ok {
+		return nil
+	}
+
+	state := makeRevalidateState(parent)
+	defer state.release()
+
+	state.add(name, child)
+	return fs.revalidateHelper(ctx, vfsObj, state, ds)
+}
+
+// revalidate revalidates path components in rp until done returns true, or
+// until a mount point or symlink is reached. It may send multiple MultiGetAttr
+// calls to the gofer to handle ".." in the path.
+//
+// Preconditions:
+// * fs.renameMu must be locked.
+// * InteropModeShared is in effect.
+func (fs *filesystem) revalidate(ctx context.Context, rp *vfs.ResolvingPath, start *dentry, done func() bool, ds **[]*dentry) error {
+	state := makeRevalidateState(start)
+	defer state.release()
+
+	// Skip synthetic dentries because the start dentry cannot be replaced in case
+	// it has been created in the remote file system.
+	if !start.isSynthetic() {
+		state.add("", start)
+	}
+
+done:
+	for cur := start; !done(); {
+		var err error
+		cur, err = fs.revalidateStep(ctx, rp, cur, state)
+		if err != nil {
+			switch err.(type) {
+			case errPartialRevalidation:
+				if err := fs.revalidateHelper(ctx, rp.VirtualFilesystem(), state, ds); err != nil {
+					return err
+				}
+
+				// Reset state to release any remaining locks and restart from where
+				// stepping stopped.
+				state.reset()
+				state.start = cur
+
+				// Skip synthetic dentries because the start dentry cannot be replaced in
+				// case it has been created in the remote file system.
+				if !cur.isSynthetic() {
+					state.add("", cur)
+				}
+
+			case errRevalidationStepDone:
+				break done
+
+			default:
+				return err
+			}
+		}
+	}
+	return fs.revalidateHelper(ctx, rp.VirtualFilesystem(), state, ds)
+}
+
+// revalidateStep walks one element of the path and updates revalidationState
+// with the dentry if needed. It may also stop the stepping or ask for a
+// partial revalidation. Partial revalidation requires the caller to revalidate
+// the current revalidationState, release all locks, and resume stepping.
+// In case a symlink is hit, revalidation stops and the caller is responsible
+// for calling revalidate again after the symlink is resolved. Revalidation may
+// also stop for other reasons, like hitting a child not in the cache.
+//
+// Returns:
+// * (dentry, nil): step worked, continue stepping.`
+// * (dentry, errPartialRevalidation): revalidation should be done with the
+//     state gathered so far. Then continue stepping with the remainder of the
+//     path, starting at `dentry`.
+// * (nil, errRevalidationStepDone): revalidation doesn't need to step any
+//     further. It hit a symlink, a mount point, or an uncached dentry.
+//
+// Preconditions:
+// * fs.renameMu must be locked.
+// * !rp.Done().
+// * InteropModeShared is in effect (assumes no negative dentries).
+func (fs *filesystem) revalidateStep(ctx context.Context, rp *vfs.ResolvingPath, d *dentry, state *revalidateState) (*dentry, error) {
+	switch name := rp.Component(); name {
+	case ".":
+		// Do nothing.
+
+	case "..":
+		// Partial revalidation is required when ".." is hit because metadata locks
+		// can only be acquired from parent to child to avoid deadlocks.
+		if isRoot, err := rp.CheckRoot(ctx, &d.vfsd); err != nil {
+			return nil, errRevalidationStepDone{}
+		} else if isRoot || d.parent == nil {
+			rp.Advance()
+			return d, errPartialRevalidation{}
+		}
+		// We must assume that d.parent is correct, because if d has been moved
+		// elsewhere in the remote filesystem so that its parent has changed,
+		// we have no way of determining its new parent's location in the
+		// filesystem.
+		//
+		// Call rp.CheckMount() before updating d.parent's metadata, since if
+		// we traverse to another mount then d.parent's metadata is irrelevant.
+		if err := rp.CheckMount(ctx, &d.parent.vfsd); err != nil {
+			return nil, errRevalidationStepDone{}
+		}
+		rp.Advance()
+		return d.parent, errPartialRevalidation{}
+
+	default:
+		d.dirMu.Lock()
+		child, ok := d.children[name]
+		d.dirMu.Unlock()
+		if !ok {
+			// child is not cached, no need to validate any further.
+			return nil, errRevalidationStepDone{}
+		}
+
+		state.add(name, child)
+
+		// Symlink must be resolved before continuing with revalidation.
+		if child.isSymlink() {
+			return nil, errRevalidationStepDone{}
+		}
+
+		d = child
+	}
+
+	rp.Advance()
+	return d, nil
+}
+
+// revalidateHelper calls the gofer to stat all dentries in `state`. It will
+// update or invalidate dentries in the cache based on the result.
+//
+// Preconditions:
+// * fs.renameMu must be locked.
+// * InteropModeShared is in effect.
+func (fs *filesystem) revalidateHelper(ctx context.Context, vfsObj *vfs.VirtualFilesystem, state *revalidateState, ds **[]*dentry) error {
+	if len(state.names) == 0 {
+		return nil
+	}
+	// Lock metadata on all dentries *before* getting attributes for them.
+	state.lockAllMetadata()
+	stats, err := state.start.file.multiGetAttr(ctx, state.names)
+	if err != nil {
+		return err
+	}
+
+	i := -1
+	for d := state.popFront(); d != nil; d = state.popFront() {
+		i++
+		found := i < len(stats)
+		if i == 0 && len(state.names[0]) == 0 {
+			if found && !d.isSynthetic() {
+				// First dentry is where the search is starting, just update attributes
+				// since it cannot be replaced.
+				d.updateFromP9AttrsLocked(stats[i].Valid, &stats[i].Attr)
+			}
+			d.metadataMu.Unlock()
+			continue
+		}
+
+		// Note that synthetic dentries will always fails the comparison check
+		// below.
+		if !found || d.qidPath != stats[i].QID.Path {
+			d.metadataMu.Unlock()
+			if !found && d.isSynthetic() {
+				// We have a synthetic file, and no remote file has arisen to replace
+				// it.
+				return 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). The dentry will be reloaded next time it's
+			// accessed.
+			vfsObj.InvalidateDentry(ctx, &d.vfsd)
+
+			name := state.names[i]
+			d.parent.dirMu.Lock()
+
+			if d.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.
+				d.setDeleted()
+				d.decRefNoCaching()
+				*ds = appendDentry(*ds, d)
+
+				d.parent.syntheticChildren--
+				d.parent.dirents = nil
+			}
+
+			// Since the dirMu was released and reacquired, re-check that the
+			// parent's child with this name is still the same. Do not touch it if
+			// it has been replaced with a different one.
+			if child := d.parent.children[name]; child == d {
+				// Invalidate dentry so it gets reloaded next time it's accessed.
+				delete(d.parent.children, name)
+			}
+			d.parent.dirMu.Unlock()
+
+			return nil
+		}
+
+		// The file at this path hasn't changed. Just update cached metadata.
+		d.updateFromP9AttrsLocked(stats[i].Valid, &stats[i].Attr)
+		d.metadataMu.Unlock()
+	}
+
+	return nil
+}
+
+// revalidateStatePool caches revalidateState instances to save array
+// allocations for dentries and names.
+var revalidateStatePool = sync.Pool{
+	New: func() interface{} {
+		return &revalidateState{}
+	},
+}
+
+// revalidateState keeps state related to a revalidation request. It keeps track
+// of {name, dentry} list being revalidated, as well as metadata locks on the
+// dentries. The list must be in ancestry order, in other words `n` must be
+// `n-1` child.
+type revalidateState struct {
+	// start is the dentry where to start the attributes search.
+	start *dentry
+
+	// List of names of entries to refresh attributes. Names length must be the
+	// same as detries length. They are kept in separate slices because names is
+	// used to call File.MultiGetAttr().
+	names []string
+
+	// dentries is the list of dentries that correspond to the names above.
+	// dentry.metadataMu is acquired as each dentry is added to this list.
+	dentries []*dentry
+
+	// locked indicates if metadata lock has been acquired on dentries.
+	locked bool
+}
+
+func makeRevalidateState(start *dentry) *revalidateState {
+	r := revalidateStatePool.Get().(*revalidateState)
+	r.start = start
+	return r
+}
+
+// release must be called after the caller is done with this object. It releases
+// all metadata locks and resources.
+func (r *revalidateState) release() {
+	r.reset()
+	revalidateStatePool.Put(r)
+}
+
+// Preconditions:
+// * d is a descendant of all dentries in r.dentries.
+func (r *revalidateState) add(name string, d *dentry) {
+	r.names = append(r.names, name)
+	r.dentries = append(r.dentries, d)
+}
+
+func (r *revalidateState) lockAllMetadata() {
+	for _, d := range r.dentries {
+		d.metadataMu.Lock()
+	}
+	r.locked = true
+}
+
+func (r *revalidateState) popFront() *dentry {
+	if len(r.dentries) == 0 {
+		return nil
+	}
+	d := r.dentries[0]
+	r.dentries = r.dentries[1:]
+	return d
+}
+
+// reset releases all metadata locks and resets all fields to allow this
+// instance to be reused.
+func (r *revalidateState) reset() {
+	if r.locked {
+		// Unlock any remaining dentries.
+		for _, d := range r.dentries {
+			d.metadataMu.Unlock()
+		}
+		r.locked = false
+	}
+	r.start = nil
+	r.names = r.names[:0]
+	r.dentries = r.dentries[:0]
+}