[B.A.T.M.A.N.] [PATCH] batman-adv: compat: fix null pointer exception for kernels < 3.9

[Linus Lüssing]

The compat code of the new multicast patchset leads to null pointer
derefernces for kernels 3.9 in netdev_master_upper_dev_get_rcu(). This
is because the initially NULL is assigned to upper, which is equal to
dev. dev is dereferenced one line later, though, leading to a crash.

Fixing this by assigning NULL only when we are sure that the according
pointer is not going to be dereferenced anymore.

Introduced by: 532cadf26cfbb1099ef31fae9ccafcbbfc37b9b5
("batman-adv: Multicast Listener Announcements via Translation Table")

Reported-by: Marek Lindner <mareklind...@neomailbox.ch>
Signed-off-by: Linus Lüssing <linus.luess...@web.de>
---
 compat.h |   10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/compat.h b/compat.h
index 7a3d235..7beba36 100644
--- a/compat.h
+++ b/compat.h
@@ -162,12 +162,13 @@ static inline int batadv_param_set_copystring(const char 
*val,
 #define NET_ADDR_RANDOM 0
 
 #define netdev_master_upper_dev_get_rcu(dev) \
-       NULL; \
+       upper; \
        if (dev->br_port ? 1 : 0) { \
                rcu_read_unlock(); \
                dev_hold(dev); \
                return dev; \
-       }
+       } else \
+               dev = NULL;
 
 #endif /* < KERNEL_VERSION(2, 6, 36) */
 
@@ -371,12 +372,13 @@ static int __batadv_interface_tx(struct sk_buff *skb, \
 
 #ifndef netdev_master_upper_dev_get_rcu
 #define netdev_master_upper_dev_get_rcu(dev) \
-       NULL; \
+       upper; \
        if (dev->priv_flags & IFF_BRIDGE_PORT) { \
                rcu_read_unlock(); \
                dev_hold(dev); \
                return dev; \
-       }
+       } else \
+               dev = NULL;
 
 #endif /* netdev_master_upper_dev_get_rcu */
 
-- 
1.7.10.4