gvisor: don't allocate a new credential object on fork

A credential object is immutable, so we don't need to copy it for a new
task.

PiperOrigin-RevId: 239519266
Change-Id: I0632f641fdea9554779ac25d84bee4231d0d18f2

pkg/sentry/kernel/task_clone.go

@@ -252,7 +252,7 @@ func (t *Task) Clone(opts *CloneOptions) (ThreadID, *SyscallControl, error) {
 		TaskContext:             tc,
 		FSContext:               fsc,
 		FDMap:                   fds,
-		Credentials:             creds.Fork(),
+		Credentials:             creds,
 		Niceness:                t.Niceness(),
 		NetworkNamespaced:       t.netns,
 		AllowedCPUMask:          t.CPUMask(),

pkg/sentry/kernel/task_identity.go

@@ -372,6 +372,7 @@ func (t *Task) DropBoundingCapability(cp linux.Capability) error {
 	if !t.creds.HasCapability(linux.CAP_SETPCAP) {
 		return syserror.EPERM
 	}
+	t.creds = t.creds.Fork() // See doc for creds.
 	t.creds.BoundingCaps &^= auth.CapabilitySetOf(cp)
 	return nil
 }