On Fri, May 21, 2021 at 2:32 PM Jann Horn [off-list ref] wrote:

On Fri, May 21, 2021 at 9:09 PM Andrei Vagin [off-list ref] wrote:

quoted

On Thu, May 20, 2021 at 11:36:09AM -0700, Peter Oskolkov wrote:

quoted

@@ -67,7 +137,75 @@ SYSCALL_DEFINE4(umcg_register_task, u32, api_version, u32, flags, u32, group_id,
  */
 SYSCALL_DEFINE1(umcg_unregister_task, u32, flags)
 {
-     return -ENOSYS;
+     struct umcg_task_data *utd;
+     int ret = -EINVAL;
+
+     rcu_read_lock();
+     utd = rcu_dereference(current->umcg_task_data);
+
+     if (!utd || flags)
+             goto out;
+
+     task_lock(current);
+     rcu_assign_pointer(current->umcg_task_data, NULL);
+     task_unlock(current);
+
+     ret = 0;
+
+out:
+     rcu_read_unlock();
+     if (!ret && utd) {
+             synchronize_rcu();

synchronize_rcu is expensive. Do we really need to call it here? Can we
use kfree_rcu?

Where is task->umcg_task_data freed when a task is destroyed?

or executed - the umcg stuff includes a userspace pointer, so it
probably shouldn't normally be kept around across execve?

Ack - thanks for these and other comments. Please keep them coming.
I'll address them in v0.2.

Thanks,
Peter