initArgs must hold a reference on the Root if it is not nil.

The contract in ExecArgs says that a reference on ExecArgs.Root must be held
for the lifetime of the struct, but the caller is free to drop the ref after
that.

As a result, proc.Exec must take an additional ref on Root when it constructs
the CreateProcessArgs, since that holds a pointer to Root as well. That ref is
dropped in CreateProcess.

PiperOrigin-RevId: 212828348
Change-Id: I7f44a612f337ff51a02b873b8a845d3119408707

pkg/sentry/control/proc.go

@@ -132,6 +132,11 @@ func (proc *Proc) execAsync(args *ExecArgs) (*kernel.ThreadGroup, error) {
 		IPCNamespace:            proc.Kernel.RootIPCNamespace(),
 		AbstractSocketNamespace: proc.Kernel.RootAbstractSocketNamespace(),
 	}
+	if initArgs.Root != nil {
+		// initArgs must hold a reference on Root. This ref is dropped
+		// in CreateProcess.
+		initArgs.Root.IncRef()
+	}
 	ctx := initArgs.NewContext(proc.Kernel)
 
 	if initArgs.Filename == "" {