ax25: Use kernel universal linked list to implement ax25_dev_list

The origin ax25_dev_list implements its own single linked list,
which is complicated and error-prone. For example, when deleting
the node of ax25_dev_list in ax25_dev_device_down(), we have to
operate on the head node and other nodes separately.

This patch uses kernel universal linked list to replace original
ax25_dev_list, which make the operation of ax25_dev_list easier.

We should do "dev->ax25_ptr = ax25_dev;" and "dev->ax25_ptr = NULL;"
while holding the spinlock, otherwise the ax25_dev_device_up() and
ax25_dev_device_down() could race.

Suggested-by: Dan Carpenter <[email protected]>
Signed-off-by: Duoming Zhou <[email protected]>
Reviewed-by: Dan Carpenter <[email protected]>
Link: https://lore.kernel.org/r/85bba3af651ca0e1a519da8d0d715b949891171c.1715247018.git.duoming@zju.edu.cn
Signed-off-by: Jakub Kicinski <[email protected]>

include/net/ax25.h

@@ -216,7 +216,7 @@ typedef struct {
 struct ctl_table;
 
 typedef struct ax25_dev {
- struct ax25_dev *next;
+ struct list_head list;
 
  struct net_device *dev;
  netdevice_tracker dev_tracker;
@@ -330,7 +330,6 @@ int ax25_addr_size(const ax25_digi *);
 void ax25_digi_invert(const ax25_digi *, ax25_digi *);
 
 /* ax25_dev.c */
-extern ax25_dev *ax25_dev_list;
 extern spinlock_t ax25_dev_lock;
 
 #if IS_ENABLED(CONFIG_AX25)

net/ax25/ax25_dev.c

@@ -22,19 +22,20 @@
 #include <net/sock.h>
 #include <linux/uaccess.h>
 #include <linux/fcntl.h>
+#include <linux/list.h>
 #include <linux/mm.h>
 #include <linux/interrupt.h>
 #include <linux/init.h>
 
-ax25_dev *ax25_dev_list;
+static LIST_HEAD(ax25_dev_list);
 DEFINE_SPINLOCK(ax25_dev_lock);
 
 ax25_dev *ax25_addr_ax25dev(ax25_address *addr)
 {
  ax25_dev *ax25_dev, *res = NULL;
 
  spin_lock_bh(&ax25_dev_lock);
- for (ax25_dev = ax25_dev_list; ax25_dev != NULL; ax25_dev = ax25_dev->next)
+ list_for_each_entry(ax25_dev, &ax25_dev_list, list)
  if (ax25cmp(addr, (const ax25_address *)ax25_dev->dev->dev_addr) == 0) {
  res = ax25_dev;
  ax25_dev_hold(ax25_dev);
@@ -59,7 +60,6 @@ void ax25_dev_device_up(struct net_device *dev)
  }
 
  refcount_set(&ax25_dev->refcount, 1);
- dev->ax25_ptr = ax25_dev;
  ax25_dev->dev = dev;
  netdev_hold(dev, &ax25_dev->dev_tracker, GFP_KERNEL);
  ax25_dev->forward = NULL;
@@ -85,8 +85,8 @@ void ax25_dev_device_up(struct net_device *dev)
 #endif
 
  spin_lock_bh(&ax25_dev_lock);
- ax25_dev->next = ax25_dev_list;
- ax25_dev_list = ax25_dev;
+ list_add(&ax25_dev->list, &ax25_dev_list);
+ dev->ax25_ptr  = ax25_dev;
  spin_unlock_bh(&ax25_dev_lock);
  ax25_dev_hold(ax25_dev);
 
@@ -111,32 +111,25 @@ void ax25_dev_device_down(struct net_device *dev)
  /*
  * Remove any packet forwarding that points to this device.
  */
- for (s = ax25_dev_list; s != NULL; s = s->next)
+ list_for_each_entry(s, &ax25_dev_list, list)
  if (s->forward == dev)
  s->forward = NULL;
 
- if ((s = ax25_dev_list) == ax25_dev) {
- ax25_dev_list = s->next;
- goto unlock_put;
- }
-
- while (s != NULL && s->next != NULL) {
- if (s->next == ax25_dev) {
- s->next = ax25_dev->next;
+ list_for_each_entry(s, &ax25_dev_list, list) {
+ if (s == ax25_dev) {
+ list_del(&s->list);
  goto unlock_put;
  }
-
- s = s->next;
  }
- spin_unlock_bh(&ax25_dev_lock);
  dev->ax25_ptr = NULL;
+ spin_unlock_bh(&ax25_dev_lock);
  ax25_dev_put(ax25_dev);
  return;
 
 unlock_put:
+ dev->ax25_ptr = NULL;
  spin_unlock_bh(&ax25_dev_lock);
  ax25_dev_put(ax25_dev);
- dev->ax25_ptr = NULL;
  netdev_put(dev, &ax25_dev->dev_tracker);
  ax25_dev_put(ax25_dev);
 }
@@ -200,16 +193,13 @@ struct net_device *ax25_fwd_dev(struct net_device *dev)
  */
 void __exit ax25_dev_free(void)
 {
- ax25_dev *s, *ax25_dev;
+ ax25_dev *s, *n;
 
  spin_lock_bh(&ax25_dev_lock);
- ax25_dev = ax25_dev_list;
- while (ax25_dev != NULL) {
- s = ax25_dev;
- netdev_put(ax25_dev->dev, &ax25_dev->dev_tracker);
- ax25_dev = ax25_dev->next;
+ list_for_each_entry_safe(s, n, &ax25_dev_list, list) {
+ netdev_put(s->dev, &s->dev_tracker);
+ list_del(&s->list);
  kfree(s);
  }
- ax25_dev_list = NULL;
  spin_unlock_bh(&ax25_dev_lock);
 }