Fix memmap.MappingIdentity.Device/InodeID() lock ordering.

For vfs.FileDescriptions for which FileDescriptionOptions.UseDentryMetadata is
true, memmap.MappingIdentity.Device/InodeID() => FileDescription.Stat() =>
FilesystemImpl.StatAt() takes fsimpl locks for path traversal, which violates
the lock ordering and is unnecessary since no path is being traversed. Fix this
by carving out a special case where FilesystemImpl.Stat() (and
FileDescriptionImpl.Stat()) are required to meet the lock ordering requirements
of memmap.MappingIdentity.Device/InodeID(), and implement that special case by
skipping path traversal (and gofer revalidation) locks when not required.

PiperOrigin-RevId: 698608924

pkg/sentry/fsimpl/gofer/filesystem.go

@@ -1578,6 +1578,12 @@ func (fs *filesystem) SetStatAt(ctx context.Context, rp *vfs.ResolvingPath, opts
 
 // StatAt implements vfs.FilesystemImpl.StatAt.
 func (fs *filesystem) StatAt(ctx context.Context, rp *vfs.ResolvingPath, opts vfs.StatOptions) (linux.Statx, error) {
+	if rp.Done() && opts.Sync == linux.AT_STATX_DONT_SYNC {
+		var stat linux.Statx
+		rp.Start().Impl().(*dentry).statTo(&stat)
+		return stat, nil
+	}
+
 	var ds *[]*dentry
 	fs.renameMu.RLock()
 	defer fs.renameMuRUnlockAndCheckCaching(ctx, &ds)

pkg/sentry/fsimpl/kernfs/filesystem.go

@@ -915,6 +915,10 @@ func (fs *Filesystem) SetStatAt(ctx context.Context, rp *vfs.ResolvingPath, opts
 
 // StatAt implements vfs.FilesystemImpl.StatAt.
 func (fs *Filesystem) StatAt(ctx context.Context, rp *vfs.ResolvingPath, opts vfs.StatOptions) (linux.Statx, error) {
+	if rp.Done() && opts.Sync == linux.AT_STATX_DONT_SYNC {
+		return rp.Start().Impl().(*Dentry).inode.Stat(ctx, fs.VFSFilesystem(), opts)
+	}
+
 	fs.mu.RLock()
 	defer fs.processDeferredDecRefs(ctx)
 	defer fs.mu.RUnlock()

pkg/sentry/fsimpl/overlay/filesystem.go

@@ -1548,17 +1548,24 @@ func (d *dentry) setStatLocked(ctx context.Context, rp *vfs.ResolvingPath, opts
 
 // StatAt implements vfs.FilesystemImpl.StatAt.
 func (fs *filesystem) StatAt(ctx context.Context, rp *vfs.ResolvingPath, opts vfs.StatOptions) (linux.Statx, error) {
-	var ds *[]*dentry
-	fs.renameMu.RLock()
-	defer fs.renameMuRUnlockAndCheckDrop(ctx, &ds)
-	d, err := fs.resolveLocked(ctx, rp, &ds)
-	if err != nil {
-		return linux.Statx{}, err
+	var d *dentry
+	if rp.Done() {
+		d = rp.Start().Impl().(*dentry)
+	} else {
+		var ds *[]*dentry
+		fs.renameMu.RLock()
+		defer fs.renameMuRUnlockAndCheckDrop(ctx, &ds)
+		var err error
+		d, err = fs.resolveLocked(ctx, rp, &ds)
+		if err != nil {
+			return linux.Statx{}, err
+		}
 	}
 
 	var stat linux.Statx
 	if layerMask := opts.Mask &^ statInternalMask; layerMask != 0 {
 		layerVD := d.topLayer()
+		var err error
 		stat, err = fs.vfsfs.VirtualFilesystem().StatAt(ctx, fs.creds, &vfs.PathOperation{
 			Root:  layerVD,
 			Start: layerVD,

pkg/sentry/fsimpl/tmpfs/filesystem.go

@@ -757,11 +757,17 @@ func (fs *filesystem) SetStatAt(ctx context.Context, rp *vfs.ResolvingPath, opts
 
 // StatAt implements vfs.FilesystemImpl.StatAt.
 func (fs *filesystem) StatAt(ctx context.Context, rp *vfs.ResolvingPath, opts vfs.StatOptions) (linux.Statx, error) {
-	fs.mu.RLock()
-	defer fs.mu.RUnlock()
-	d, err := resolveLocked(ctx, rp)
-	if err != nil {
-		return linux.Statx{}, err
+	var d *dentry
+	if rp.Done() {
+		d = rp.Start().Impl().(*dentry)
+	} else {
+		fs.mu.RLock()
+		defer fs.mu.RUnlock()
+		var err error
+		d, err = resolveLocked(ctx, rp)
+		if err != nil {
+			return linux.Statx{}, err
+		}
 	}
 	var stat linux.Statx
 	d.inode.statTo(&stat)

pkg/sentry/vfs/file_description.go

@@ -334,6 +334,9 @@ type FileDescriptionImpl interface {
 	OnClose(ctx context.Context) error
 
 	// Stat returns metadata for the file represented by the FileDescription.
+	//
+	// If opts.Sync == linux.AT_STATX_SYNC_DONT_SYNC, Stat cannot take locks
+	// preceding memmap.MappingIdentity locks.
 	Stat(ctx context.Context, opts StatOptions) (linux.Statx, error)
 
 	// SetStat updates metadata for the file represented by the

pkg/sentry/vfs/filesystem.go

@@ -365,6 +365,10 @@ type FilesystemImpl interface {
 	SetStatAt(ctx context.Context, rp *ResolvingPath, opts SetStatOptions) error
 
 	// StatAt returns metadata for the file at rp.
+	//
+	// If rp.Done() (i.e. rp refers to the dentry rp.Start()) and opts.Sync ==
+	// linux.AT_STATX_DONT_SYNC, StatAt cannot take locks preceding
+	// memmap.MappingIdentity locks.
 	StatAt(ctx context.Context, rp *ResolvingPath, opts StatOptions) (linux.Statx, error)
 
 	// StatFSAt returns metadata for the filesystem containing the file at rp.