Kernel threads abuse set_child_tid. Historically that has been fine as set_child_tid was initialized after the kernel thread had been forked. Unfortunately storing struct kthread in set_child_tid after the thread is running makes struct kthread being unusable for storing result codes of the thread. When set_child_tid is set to struct kthread during fork that results in schedule_tail writing the thread id to the beggining of struct kthread (if put_user does not realize it is a kernel address). Solve this by skipping the put_user for all kthreads.
Unified · Split
| @@ -4908,7 +4908,7 @@ asmlinkage __visible void schedule_tail(struct task_struct *prev) · 4907 unmodified lines | ||
| 4908 | 4908 | finish_task_switch(prev); |
| 4909 | 4909 | preempt_enable(); |
| 4910 | 4910 | |
| 4911 | − if (current->set_child_tid) | |
| 4911 | + if (!(current->flags & PF_KTHREAD) && current->set_child_tid) | |
| 4912 | 4912 | put_user(task_pid_vnr(current), current->set_child_tid); |
| 4913 | 4913 | |
| 4914 | 4914 | calculate_sigpending(); |