stackleak: allow to specify arch specific stackleak poison function

Factor out the code that fills the stack with the stackleak poison value
in order to allow architectures to provide a faster implementation.

Acked-by: Vasily Gorbik <[e-mail]>
Signed-off-by: Heiko Carstens <[e-mail]>
Acked-by: Mark Rutland <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Vasily Gorbik <[e-mail]>

kernel/stackleak.c

···
 #define skip_erasing()	false
 #endif /* CONFIG_STACKLEAK_RUNTIME_DISABLE */
 
+#ifndef __stackleak_poison
+static __always_inline void __stackleak_poison(unsigned long erase_low,
+					       unsigned long erase_high,
+					       unsigned long poison)
+{
+	while (erase_low < erase_high) {
+		*(unsigned long *)erase_low = poison;
+		erase_low += sizeof(unsigned long);
+	}
+}
+#endif
+
 static __always_inline void __stackleak_erase(bool on_task_stack)
 {
 	const unsigned long task_stack_low = stackleak_task_low_bound(current);
···
 	else
 		erase_high = task_stack_high;
 
-	while (erase_low < erase_high) {
-		*(unsigned long *)erase_low = STACKLEAK_POISON;
-		erase_low += sizeof(unsigned long);
-	}
+	__stackleak_poison(erase_low, erase_high, STACKLEAK_POISON);
 
 	/* Reset the 'lowest_stack' value for the next syscall */
 	current->lowest_stack = task_stack_high;