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, 4 insertions, 6 deletions

diff --git a/arch/um/kernel/process.c b/arch/um/kernel/process.c
index fd2d2361484d..181cc9aafb25 100644
--- a/arch/um/kernel/process.c
+++ b/arch/um/kernel/process.c
@@ -158,15 +158,13 @@ int copy_thread(struct task_struct * p, const struct kernel_clone_args *args)
 {
 	unsigned long clone_flags = args->flags;
 	unsigned long sp = args->stack;
-	unsigned long arg = args->stack_size;
 	unsigned long tls = args->tls;
 	void (*handler)(void);
-	int kthread = current->flags & (PF_KTHREAD | PF_IO_WORKER);
 	int ret = 0;
 
 	p->thread = (struct thread_struct) INIT_THREAD;
 
-	if (!kthread) {
+	if (!args->fn) {
 	  	memcpy(&p->thread.regs.regs, current_pt_regs(),
 		       sizeof(p->thread.regs.regs));
 		PT_REGS_SET_SYSCALL_RETURN(&p->thread.regs, 0);
@@ -178,14 +176,14 @@ int copy_thread(struct task_struct * p, const struct kernel_clone_args *args)
 		arch_copy_thread(&current->thread.arch, &p->thread.arch);
 	} else {
 		get_safe_registers(p->thread.regs.regs.gp, p->thread.regs.regs.fp);
-		p->thread.request.u.thread.proc = (int (*)(void *))sp;
-		p->thread.request.u.thread.arg = (void *)arg;
+		p->thread.request.u.thread.proc = args->fn;
+		p->thread.request.u.thread.arg = args->fn_arg;
 		handler = new_thread_handler;
 	}
 
 	new_thread(task_stack_page(p), &p->thread.switch_buf, handler);
 
-	if (!kthread) {
+	if (!args->fn) {
 		clear_flushed_tls(p);
 
 		/*