Implement mount(2) and umount2(2) for VFS2.

This is mostly syscall plumbing, VFS2 already implements the internals of
mounts. In addition to the syscall defintions, the following mount-related
mechanisms are updated:

- Implement MS_NOATIME for VFS2, but only for tmpfs and goferfs. The other VFS2
  filesystems don't implement node-level timestamps yet.

- Implement the 'mode', 'uid' and 'gid' mount options for VFS2's tmpfs.

- Plumb mount namespace ownership, which is necessary for checking appropriate
  capabilities during mount(2).

Updates #1035

PiperOrigin-RevId: 315035352

1 files changed, 2 insertions, 1 deletions

diff --git a/pkg/sentry/vfs/genericfstree/genericfstree.go b/pkg/sentry/vfs/genericfstree/genericfstree.go
index 286510195..8882fa84a 100644
--- a/pkg/sentry/vfs/genericfstree/genericfstree.go
+++ b/pkg/sentry/vfs/genericfstree/genericfstree.go
@@ -43,7 +43,7 @@ type Dentry struct {
 // IsAncestorDentry returns true if d is an ancestor of d2; that is, d is
 // either d2's parent or an ancestor of d2's parent.
 func IsAncestorDentry(d, d2 *Dentry) bool {
-	for {
+	for d2 != nil { // Stop at root, where d2.parent == nil.
 		if d2.parent == d {
 			return true
 		}
@@ -52,6 +52,7 @@ func IsAncestorDentry(d, d2 *Dentry) bool {
 		}
 		d2 = d2.parent
 	}
+	return false
 }
 
 // ParentOrSelf returns d.parent. If d.parent is nil, ParentOrSelf returns d.