[vfs] overlayfs: Do not unlink non-existing whiteout during file creation.

We can reuse information about whether a whiteout exists on a given file path
from stepLocked when creating a file at that path. This helps save an Unlink
call to the upper filesystem if the whiteout does NOT exist (common case).

Plumbs this information from lookupLocked() -> getChildLocked() -> stepLocked().
This also helped save a Lookup in RenameAt().

Fixes #1199

PiperOrigin-RevId: 341105351

1 files changed, 7 insertions, 0 deletions

diff --git a/pkg/sentry/fsimpl/overlay/overlay.go b/pkg/sentry/fsimpl/overlay/overlay.go
index f6c58f2e7..73130bc8d 100644
--- a/pkg/sentry/fsimpl/overlay/overlay.go
+++ b/pkg/sentry/fsimpl/overlay/overlay.go
@@ -696,6 +696,13 @@ func (d *dentry) topLayer() vfs.VirtualDentry {
 	return vd
 }
 
+func (d *dentry) topLookupLayer() lookupLayer {
+	if d.upperVD.Ok() {
+		return lookupLayerUpper
+	}
+	return lookupLayerLower
+}
+
 func (d *dentry) checkPermissions(creds *auth.Credentials, ats vfs.AccessTypes) error {
 	return vfs.GenericCheckPermissions(creds, ats, linux.FileMode(atomic.LoadUint32(&d.mode)), auth.KUID(atomic.LoadUint32(&d.uid)), auth.KGID(atomic.LoadUint32(&d.gid)))
 }