fork: Duplicate task_struct before stack allocation

alloc_thread_stack_node() already populates the task_struct::stack
member except on IA64. The stack pointer is saved and populated again
because IA64 needs it and arch_dup_task_struct() overwrites it.

Allocate thread's stack after task_struct has been duplicated as a
preparation for further changes.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Andy Lutomirski <luto@kernel.org>
Link: https://lore.kernel.org/r/20220217102406.3697941-3-bigeasy@linutronix.de

Author: Sebastian Andrzej Siewior

kernel/fork.c

@@ -888,6 +888,10 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
 	if (!tsk)
 		return NULL;
 
+	err = arch_dup_task_struct(tsk, orig);
+	if (err)
+		goto free_tsk;
+
 	stack = alloc_thread_stack_node(tsk, node);
 	if (!stack)
 		goto free_tsk;
@@ -897,8 +901,6 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
 
 	stack_vm_area = task_stack_vm_area(tsk);
 
-	err = arch_dup_task_struct(tsk, orig);
-
 	/*
 	 * arch_dup_task_struct() clobbers the stack-related fields.  Make
 	 * sure they're properly initialized before using any stack-related
@@ -912,9 +914,6 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
 	refcount_set(&tsk->stack_refcount, 1);
 #endif
 
-	if (err)
-		goto free_stack;
-
 	err = scs_prepare(tsk, node);
 	if (err)
 		goto free_stack;