driver core: check 'start' argument in bus iterators

bus_for_each_dev() and bus_find_device() both take a 'start' argument to start the iteration at a specific list entry. However, this list entry might already been detached by the time these functions are called.
So we need to check if the arguments are still valid.

+       if (start && !klist_node_attached(&start->p->knode_bus))
+               return -ENODEV;
+
       klist_iter_init_node(&bus->p->klist_devices, &i, (start ? &start->p->knode_bus : NULL));

drivers/base/bus.c int bus_for_each_dev(struct bus_type *bus, struct device *start, · line 304

Quoted 11 lines · Hannes Reinecke

bus_for_each_dev() and bus_find_device() both take a 'start' argument to start the iteration at a specific list entry. However, this list entry might already been detached by the time these functions are called.
So we need to check if the arguments are still valid.

drivers/base/bus.c +3 -0

+       if (start && !klist_node_attached(&start->p->knode_bus))
+               return -ENODEV;
+
       klist_iter_init_node(&bus->p->klist_devices, &i,
                            (start ? &start->p->knode_bus : NULL));

Shouldn't we do this check in klist_iter_init_node()? After a kref_get(), and return -ENODEV from there if the node it as asked to start is gone?

This check otherwise still has the window between the check and the use of it, only very unlikely to hit now, hasn't it?