batman-adv: compat: fix null pointer exception for kernels < 3.9

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

On 16/02/14 13:01, Linus Lüssing wrote:
> @@ -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;
>  

Following your patch the code in multicast.c will become:

172         do {
173                 upper = upper;
			if (dev->priv_flags & IFF_BRIDGE_PORT) {
				rcu_read_unlock();
				dev_hold(dev);
				return dev;
			} else
				dev = NULL;
174         } while (upper && !(upper->priv_flags & IFF_EBRIDGE));

am I wrong or this is going to break the while? I think there is a
missing '}'.


What about a simplified version like this:

 #define netdev_master_upper_dev_get_rcu(dev) \
-	NULL; \
-	if (dev->priv_flags & IFF_BRIDGE_PORT) { \
+	({if (dev->priv_flags & IFF_BRIDGE_PORT) { \
 		rcu_read_unlock(); \
 		dev_hold(dev); \
 		return dev; \
-	}
+	}\
+	NULL;})


Cheers,
  

On 16/02/14 14:39, Antonio Quartulli wrote:
> On 16/02/14 13:01, Linus Lüssing wrote:
>> @@ -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;
>>  
> 
> Following your patch the code in multicast.c will become:
> 
> 172         do {
> 173                 upper = upper;
> 			if (dev->priv_flags & IFF_BRIDGE_PORT) {
> 				rcu_read_unlock();
> 				dev_hold(dev);
> 				return dev;
> 			} else
> 				dev = NULL;
> 174         } while (upper && !(upper->priv_flags & IFF_EBRIDGE));
> 
> am I wrong or this is going to break the while? I think there is a
> missing '}'.
> 

I was wrong. I just dreamt of a '{' after "else".
Forget about this comment.

Cheers,
  

On Sunday 16 February 2014 13:01:02 Linus Lüssing wrote:
> 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 <mareklindner@neomailbox.ch>
> Signed-off-by: Linus Lüssing <linus.luessing@web.de>
> ---
>  compat.h |   10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)

Applied in revision 65d8217.

Thanks,
Marek