Cache directory entries in the overlay

Currently, the overlay dirCache is only used for a single logical use of
getdents. i.e., it is discard when the FD is closed or seeked back to
the beginning.

But the initial work of getting the directory contents can be quite
expensive (particularly sorting large directories), so we should keep it
as long as possible.

This is very similar to the readdirCache in fs/gofer.

Since the upper filesystem does not have to allow caching readdir
entries, the new CacheReaddir MountSourceOperations method controls this
behavior.

This caching should be trivially movable to all Inodes if desired,
though that adds an additional copy step for non-overlay Inodes.
(Overlay Inodes already do the extra copy).

PiperOrigin-RevId: 255477592

2 files changed, 8 insertions, 1 deletions

diff --git a/pkg/sentry/fs/tty/dir.go b/pkg/sentry/fs/tty/dir.go
index 58a26742c..1d128532b 100644
--- a/pkg/sentry/fs/tty/dir.go
+++ b/pkg/sentry/fs/tty/dir.go
@@ -307,7 +307,7 @@ type dirFileOperations struct {
 var _ fs.FileOperations = (*dirFileOperations)(nil)
 
 // IterateDir implements DirIterator.IterateDir.
-func (df *dirFileOperations) IterateDir(ctx context.Context, dirCtx *fs.DirCtx, offset int) (int, error) {
+func (df *dirFileOperations) IterateDir(ctx context.Context, d *fs.Dirent, dirCtx *fs.DirCtx, offset int) (int, error) {
 	df.di.mu.Lock()
 	defer df.di.mu.Unlock()
 
diff --git a/pkg/sentry/fs/tty/fs.go b/pkg/sentry/fs/tty/fs.go
index 1bbfbb90a..edee56c12 100644
--- a/pkg/sentry/fs/tty/fs.go
+++ b/pkg/sentry/fs/tty/fs.go
@@ -94,6 +94,13 @@ func (superOperations) Keep(*fs.Dirent) bool {
 	return false
 }
 
+// CacheReaddir implements fs.DirentOperations.CacheReaddir.
+//
+// CacheReaddir returns false because entries change on master operations.
+func (superOperations) CacheReaddir() bool {
+	return false
+}
+
 // ResetInodeMappings implements MountSourceOperations.ResetInodeMappings.
 func (superOperations) ResetInodeMappings() {}