Automated rollback of changelist 369325957

PiperOrigin-RevId: 369686285

5 files changed, 187 insertions, 493 deletions

diff --git a/pkg/sentry/fsimpl/gofer/BUILD b/pkg/sentry/fsimpl/gofer/BUILD
index 52879f871..6d5258a9b 100644
--- a/pkg/sentry/fsimpl/gofer/BUILD
+++ b/pkg/sentry/fsimpl/gofer/BUILD
@@ -38,7 +38,6 @@ 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 40c9243f0..4b5621043 100644
--- a/pkg/sentry/fsimpl/gofer/filesystem.go
+++ b/pkg/sentry/fsimpl/gofer/filesystem.go
@@ -169,89 +169,156 @@ 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 and all children that are
-//   part of rp must have been revalidated.
+// * 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, mayFollowSymlinks bool, ds **[]*dentry) (*dentry, bool, error) {
+func (fs *filesystem) stepLocked(ctx context.Context, rp *vfs.ResolvingPath, d *dentry, mayFollowSymlinks bool, ds **[]*dentry) (*dentry, error) {
 	if !d.isDir() {
-		return nil, false, syserror.ENOTDIR
+		return nil, syserror.ENOTDIR
 	}
 	if err := d.checkPermissions(rp.Credentials(), vfs.MayExec); err != nil {
-		return nil, false, err
+		return nil, err
 	}
-	followedSymlink := false
 afterSymlink:
 	name := rp.Component()
 	if name == "." {
 		rp.Advance()
-		return d, followedSymlink, nil
+		return d, nil
 	}
 	if name == ".." {
 		if isRoot, err := rp.CheckRoot(ctx, &d.vfsd); err != nil {
-			return nil, false, err
+			return nil, err
 		} else if isRoot || d.parent == nil {
 			rp.Advance()
-			return d, followedSymlink, nil
-		}
+			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, false, err
+			return nil, err
+		}
+		if d != d.parent && !d.cachedMetadataAuthoritative() {
+			if err := d.parent.updateFromGetattr(ctx); err != nil {
+				return nil, err
+			}
 		}
 		rp.Advance()
-		return d.parent, followedSymlink, nil
+		return d.parent, nil
 	}
-	child, err := fs.getChildLocked(ctx, d, name, ds)
+	child, err := fs.getChildLocked(ctx, rp.VirtualFilesystem(), d, name, ds)
 	if err != nil {
-		return nil, false, err
+		return nil, err
+	}
+	if child == nil {
+		return nil, syserror.ENOENT
 	}
 	if err := rp.CheckMount(ctx, &child.vfsd); err != nil {
-		return nil, false, err
+		return nil, err
 	}
 	if child.isSymlink() && mayFollowSymlinks && rp.ShouldFollowSymlink() {
 		target, err := child.readlink(ctx, rp.Mount())
 		if err != nil {
-			return nil, false, err
+			return nil, err
 		}
 		if err := rp.HandleSymlink(target); err != nil {
-			return nil, false, err
+			return nil, err
 		}
-		followedSymlink = true
 		goto afterSymlink // don't check the current directory again
 	}
 	rp.Advance()
-	return child, followedSymlink, nil
+	return child, nil
 }
 
 // getChildLocked returns a dentry representing the child of parent with the
-// given name. Returns ENOENT if the child doesn't exist.
+// given name. If no such child exists, getChildLocked returns (nil, nil).
 //
 // Preconditions:
 // * fs.renameMu must be locked.
 // * parent.dirMu must be locked.
 // * parent.isDir().
 // * name is not "." or "..".
-// * dentry at name has been revalidated
-func (fs *filesystem) getChildLocked(ctx context.Context, parent *dentry, name string, ds **[]*dentry) (*dentry, error) {
+//
+// 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) {
 	if len(name) > maxFilenameLen {
 		return nil, syserror.ENAMETOOLONG
 	}
-	if child, ok := parent.children[name]; ok || parent.isSynthetic() {
-		if child == nil {
-			return nil, syserror.ENOENT
-		}
+	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.
 		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 {
-		if err == syserror.ENOENT {
-			parent.cacheNegativeLookupLocked(name)
+	if err != nil && err != syserror.ENOENT {
+		if child != nil {
+			child.metadataMu.Unlock()
 		}
 		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)
@@ -277,22 +344,14 @@ func (fs *filesystem) getChildLocked(ctx context.Context, parent *dentry, name s
 // * 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, followedSymlink, err := fs.stepLocked(ctx, rp, d, true /* mayFollowSymlinks */, ds)
+		next, 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
@@ -305,22 +364,20 @@ 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 err := fs.revalidatePath(ctx, rp, d, ds); err != nil {
-		return nil, err
+	if !d.cachedMetadataAuthoritative() {
+		// Get updated metadata for rp.Start() as required by fs.stepLocked().
+		if err := d.updateFromGetattr(ctx); err != nil {
+			return nil, err
+		}
 	}
 	for !rp.Done() {
 		d.dirMu.Lock()
-		next, followedSymlink, err := fs.stepLocked(ctx, rp, d, true /* mayFollowSymlinks */, ds)
+		next, 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
@@ -340,6 +397,13 @@ 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
@@ -357,14 +421,11 @@ 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, parent, name, &ds)
+	child, err := fs.getChildLocked(ctx, rp.VirtualFilesystem(), parent, name, &ds)
 	switch {
 	case err != nil && err != syserror.ENOENT:
 		return err
@@ -428,6 +489,13 @@ 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
@@ -453,32 +521,33 @@ 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()
 
-	// 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
+	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
+			}
 		}
 		if err := parent.mayDelete(rp.Credentials(), child); err != nil {
 			return err
@@ -487,7 +556,11 @@ 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.
+	// 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.
 	//
 	// Also note that if child is nil, then it can't be a mount point.
 	if child != nil {
@@ -502,7 +575,23 @@ 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 {
-			return err
+			// 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
+				}
+			}
 		}
 	}
 	flags := uint32(0)
@@ -634,6 +723,13 @@ 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
@@ -734,7 +830,7 @@ 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.
@@ -795,6 +891,12 @@ 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() {
@@ -803,12 +905,6 @@ 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()
@@ -830,12 +926,9 @@ 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()
@@ -1204,23 +1297,18 @@ 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, oldParent, oldName, &ds)
+	renamed, err := fs.getChildLocked(ctx, vfsObj, oldParent, oldName, &ds)
 	if err != nil {
 		return err
 	}
+	if renamed == nil {
+		return syserror.ENOENT
+	}
 	if err := oldParent.mayDelete(creds, renamed); err != nil {
 		return err
 	}
@@ -1249,8 +1337,8 @@ func (fs *filesystem) RenameAt(ctx context.Context, rp *vfs.ResolvingPath, oldPa
 	if newParent.isDeleted() {
 		return syserror.ENOENT
 	}
-	replaced, err := fs.getChildLocked(ctx, newParent, newName, &ds)
-	if err != nil && err != syserror.ENOENT {
+	replaced, err := fs.getChildLocked(ctx, rp.VirtualFilesystem(), newParent, newName, &ds)
+	if err != nil {
 		return err
 	}
 	var replacedVFSD *vfs.Dentry
diff --git a/pkg/sentry/fsimpl/gofer/gofer.go b/pkg/sentry/fsimpl/gofer/gofer.go
index 21692d2ac..fb42c5f62 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 and dentry.metadataMu in multiple dentries requires that
-// either ancestor dentries are locked before descendant dentries, or that
-// filesystem.renameMu is locked for writing.
+// Locking dentry.dirMu 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 b0a429d42..21b4a96fe 100644
--- a/pkg/sentry/fsimpl/gofer/p9file.go
+++ b/pkg/sentry/fsimpl/gofer/p9file.go
@@ -238,10 +238,3 @@ 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
deleted file mode 100644
index 8f81f0822..000000000
--- a/pkg/sentry/fsimpl/gofer/revalidate.go
+++ /dev/null
@@ -1,386 +0,0 @@
-// 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]
-}