[vfs2] Destroy all tmpfs files when the filesystem is released.

In addition to fixing reference leaks, this change also releases memory used
by regular tmpfs files once the containing filesystem is released.

PiperOrigin-RevId: 336833111

pkg/sentry/fsimpl/tmpfs/tmpfs.go

@@ -74,6 +74,8 @@ type filesystem struct {
 	mu sync.RWMutex `state:"nosave"`
 
 	nextInoMinusOne uint64 // accessed using atomic memory operations
+
+	root *dentry
 }
 
 // Name implements vfs.FilesystemType.Name.
@@ -197,6 +199,7 @@ func (fstype FilesystemType) GetFilesystem(ctx context.Context, vfsObj *vfs.Virt
 		fs.vfsfs.DecRef(ctx)
 		return nil, nil, fmt.Errorf("invalid tmpfs root file type: %#o", rootFileType)
 	}
+	fs.root = root
 	return &fs.vfsfs, &root.vfsd, nil
 }
 
@@ -208,6 +211,37 @@ func NewFilesystem(ctx context.Context, vfsObj *vfs.VirtualFilesystem, creds *au
 // Release implements vfs.FilesystemImpl.Release.
 func (fs *filesystem) Release(ctx context.Context) {
 	fs.vfsfs.VirtualFilesystem().PutAnonBlockDevMinor(fs.devMinor)
+	fs.mu.Lock()
+	if fs.root.inode.isDir() {
+		fs.root.releaseChildrenLocked(ctx)
+	}
+	fs.mu.Unlock()
+}
+
+// releaseChildrenLocked is called on the mount point by filesystem.Release() to
+// destroy all objects in the mount. It performs a depth-first walk of the
+// filesystem and "unlinks" everything by decrementing link counts
+// appropriately. There should be no open file descriptors when this is called,
+// so each inode should only have one outstanding reference that is removed once
+// its link count hits zero.
+//
+// Note that we do not update filesystem state precisely while tearing down (for
+// instance, the child maps are ignored)--we only care to remove all remaining
+// references so that every filesystem object gets destroyed. Also note that we
+// do not need to trigger DecRef on the mount point itself or any child mount;
+// these are taken care of by the destructor of the enclosing MountNamespace.
+//
+// Precondition: filesystem.mu is held.
+func (d *dentry) releaseChildrenLocked(ctx context.Context) {
+	dir := d.inode.impl.(*directory)
+	for _, child := range dir.childMap {
+		if child.inode.isDir() {
+			child.releaseChildrenLocked(ctx)
+			child.inode.decLinksLocked(ctx) // link for child/.
+			dir.inode.decLinksLocked(ctx)   // link for child/..
+		}
+		child.inode.decLinksLocked(ctx) // link for child
+	}
 }
 
 // immutable