[PATCH v11 11/11] seccomp: implement SECCOMP_FILTER_FLAG_TSYNC

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 9065d2c79c56..2125b83ccfd4 100644
+/**
+ * seccomp_can_sync_threads: checks if all threads can be synchronized
+ *
+ * Expects sighand and cred_guard_mutex locks to be held.
+ *
+ * Returns 0 on success, -ve on error, or the pid of a thread which was
+ * either not in the correct seccomp mode or it did not have an ancestral
+ * seccomp filter.
+ */
+static inline pid_t seccomp_can_sync_threads(void)
+{
+       struct task_struct *thread, *caller;
+
+       BUG_ON(!mutex_is_locked(&current->signal->cred_guard_mutex));
+       BUG_ON(!spin_is_locked(&current->sighand->siglock));
+
+       if (current->seccomp.mode != SECCOMP_MODE_FILTER)
+               return -EACCES;

+
+       /* Validate all threads being eligible for synchronization. */
+       caller = current;
+       for_each_thread(caller, thread) {
+               pid_t failed;
+
+               if (thread->seccomp.mode == SECCOMP_MODE_DISABLED ||
+                   (thread->seccomp.mode == SECCOMP_MODE_FILTER &&
+                    is_ancestor(thread->seccomp.filter,
+                                caller->seccomp.filter)))
+                       continue;
+
+               /* Return the first thread that cannot be synchronized. */
+               failed = task_pid_vnr(thread);
+               /* If the pid cannot be resolved, then return -ESRCH */
+               if (unlikely(WARN_ON(failed == 0)))
+                       failed = -ESRCH;
+               return failed;
+       }
+
+       return 0;
+}