fork: Generalize PF_IO_WORKER handling

Add fn and fn_arg members into struct kernel_clone_args and test for
them in copy_thread (instead of testing for PF_KTHREAD | PF_IO_WORKER).
This allows any task that wants to be a user space task that only runs
in kernel mode to use this functionality.

The code on x86 is an exception and still retains a PF_KTHREAD test
because x86 unlikely everything else handles kthreads slightly
differently than user space tasks that start with a function.

The functions that created tasks that start with a function
have been updated to set ".fn" and ".fn_arg" instead of
".stack" and ".stack_size".  These functions are fork_idle(),
create_io_thread(), kernel_thread(), and user_mode_thread().

Link: https://lkml.kernel.org/r/20220506141512.516114-4-ebiederm@xmission.com
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index 3d6b99ce5408..505aaf9fe477 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -34,6 +34,8 @@ struct kernel_clone_args {
 	int io_thread;
 	int kthread;
 	int idle;
+	int (*fn)(void *);
+	void *fn_arg;
 	struct cgroup *cgrp;
 	struct css_set *cset;
 };