From 4fbd0728ac5e0aa33a7f5e2c5189751b4baa94b5 Mon Sep 17 00:00:00 2001
From: Ayush Ranjan <ayushranjan@google.com>
Date: Thu, 23 Jul 2020 09:43:05 -0700
Subject: [vfs2][gofer] Fix update attributes race condition.

We were getting the file attributes before locking the metadataMu which was
causing stale updates to the file attributes.

Fixes OpenTest_AppendConcurrentWrite.

Updates #2923

PiperOrigin-RevId: 322804438
---
 pkg/sentry/fsimpl/gofer/filesystem.go | 17 +++++++++++++----
 pkg/sentry/fsimpl/gofer/gofer.go      | 11 +++++++----
 2 files changed, 20 insertions(+), 8 deletions(-)

(limited to 'pkg/sentry')

diff --git a/pkg/sentry/fsimpl/gofer/filesystem.go b/pkg/sentry/fsimpl/gofer/filesystem.go
index cd5f5049e..491495b16 100644
--- a/pkg/sentry/fsimpl/gofer/filesystem.go
+++ b/pkg/sentry/fsimpl/gofer/filesystem.go
@@ -150,11 +150,9 @@ afterSymlink:
 			return nil, err
 		}
 		if d != d.parent && !d.cachedMetadataAuthoritative() {
-			_, attrMask, attr, err := d.parent.file.getAttr(ctx, dentryAttrMask())
-			if err != nil {
+			if err := d.parent.updateFromGetattr(ctx); err != nil {
 				return nil, err
 			}
-			d.parent.updateFromP9Attrs(attrMask, &attr)
 		}
 		rp.Advance()
 		return d.parent, nil
@@ -209,17 +207,28 @@ func (fs *filesystem) getChildLocked(ctx context.Context, vfsObj *vfs.VirtualFil
 
 // 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) {
+	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()
+		}
 		return nil, err
 	}
 	if child != nil {
 		if !file.isNil() && inoFromPath(qid.Path) == child.ino {
 			// The file at this path hasn't changed. Just update cached metadata.
 			file.close(ctx)
-			child.updateFromP9Attrs(attrMask, &attr)
+			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.
diff --git a/pkg/sentry/fsimpl/gofer/gofer.go b/pkg/sentry/fsimpl/gofer/gofer.go
index b74d489a0..ba7f650b3 100644
--- a/pkg/sentry/fsimpl/gofer/gofer.go
+++ b/pkg/sentry/fsimpl/gofer/gofer.go
@@ -785,8 +785,8 @@ func (d *dentry) cachedMetadataAuthoritative() bool {
 
 // updateFromP9Attrs is called to update d's metadata after an update from the
 // remote filesystem.
-func (d *dentry) updateFromP9Attrs(mask p9.AttrMask, attr *p9.Attr) {
-	d.metadataMu.Lock()
+// Precondition: d.metadataMu must be locked.
+func (d *dentry) updateFromP9AttrsLocked(mask p9.AttrMask, attr *p9.Attr) {
 	if mask.Mode {
 		if got, want := uint32(attr.Mode.FileType()), d.fileType(); got != want {
 			d.metadataMu.Unlock()
@@ -822,7 +822,6 @@ func (d *dentry) updateFromP9Attrs(mask p9.AttrMask, attr *p9.Attr) {
 	if mask.Size {
 		d.updateFileSizeLocked(attr.Size)
 	}
-	d.metadataMu.Unlock()
 }
 
 // Preconditions: !d.isSynthetic()
@@ -834,6 +833,10 @@ func (d *dentry) updateFromGetattr(ctx context.Context) error {
 		file            p9file
 		handleMuRLocked bool
 	)
+	// d.metadataMu must be locked *before* we getAttr so that we do not end up
+	// updating stale attributes in d.updateFromP9AttrsLocked().
+	d.metadataMu.Lock()
+	defer d.metadataMu.Unlock()
 	d.handleMu.RLock()
 	if !d.handle.file.isNil() {
 		file = d.handle.file
@@ -849,7 +852,7 @@ func (d *dentry) updateFromGetattr(ctx context.Context) error {
 	if err != nil {
 		return err
 	}
-	d.updateFromP9Attrs(attrMask, &attr)
+	d.updateFromP9AttrsLocked(attrMask, &attr)
 	return nil
 }
 
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