[RFC] batman-adv: dirty hack to recompute mac_len in the rx path

  It is possible that the mac_len is not properly exported because of
strange device configuration (this behaviour has been observed when
using batman-adv on top of a vlan interface). Therefore it is needed to
explicitly recompute it at the very beginning of the rx path.

This is done by appending the recompute function to the skb_share_mac()
function, hence the "dirty hack" in the subject. We expect this problem to
be fixed in linux 3.8 and above.

Reported-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
---

this is a rewrite of Antonios patch
"batman-adv: recompute mac_len at the beginning of the rx path". It is
intended to fix the issue for out-of-kernel releases, without hurting
the in-kernel code too much. Obviously, this patch won't go upstream
into the kernel.
---
 compat.h |   15 +++++++++++++++
 1 file changed, 15 insertions(+)
  

On Tuesday, October 02, 2012 04:53:28 Simon Wunderlich wrote:
> +#define skb_share_check(skb, b) \
> +       skb_share_check(skb, b); \
> +       if (skb) \
> +               skb_reset_mac_len(skb)
> +
> +#endif /* < KERNEL_VERSION(3, 8, 0) */

Has this patch been tested ? Our skb_share_check() call is this:
skb = skb_share_check(skb, GFP_ATOMIC);

Now we replace this function call with 2 function calls and 2 return values ?


Cheers,
Marek
  

On Monday 01 October 2012 22:53:28 Simon Wunderlich wrote:
> It is possible that the mac_len is not properly exported because of
> strange device configuration (this behaviour has been observed when
> using batman-adv on top of a vlan interface). Therefore it is needed to
> explicitly recompute it at the very beginning of the rx path.
> 
> This is done by appending the recompute function to the skb_share_mac()
> function, hence the "dirty hack" in the subject. We expect this problem to
> be fixed in linux 3.8 and above.
> 
> Reported-by: Antonio Quartulli <ordex@autistici.org>
> Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
> ---
> 
> this is a rewrite of Antonios patch
> "batman-adv: recompute mac_len at the beginning of the rx path". It is
> intended to fix the issue for out-of-kernel releases, without hurting
> the in-kernel code too much. Obviously, this patch won't go upstream
> into the kernel.
> ---
>  compat.h |   15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/compat.h b/compat.h
> index 14969e0..dca9685 100644
> --- a/compat.h
> +++ b/compat.h
> @@ -159,4 +159,19 @@ static inline void eth_hw_addr_random(struct net_device
> *dev)
> 
>  #endif /* < KERNEL_VERSION(3, 5, 0) */
> 
> +#if LINUX_VERSION_CODE < KERNEL_VERSION(3, 8, 0)
> +
> +/* hack for not correctly set mac_len. This may happen for some special
> + * configurations like batman-adv on VLANs.
> + *
> + * This is pretty dirty, but we only use skb_share_check() in main.c right
> + * before mac_len is checked, and the recomputation shouldn't hurt too
> much. + */
> +#define skb_share_check(skb, b) \
> +	skb_share_check(skb, b); \
> +	if (skb) \
> +		skb_reset_mac_len(skb)
> +
> +#endif /* < KERNEL_VERSION(3, 8, 0) */
> +
>  #endif /* _NET_BATMAN_ADV_COMPAT_H_ */

Can we try a more sane solution like

#define skb_share_check(skb, b) \
	({ \
		struct sk_buff *_t_skb; \
		_t_skb = skb_share_check(skb, b); \
		if (_t_skb) \
			skb_reset_mac_len(_t_skb); \
		_t_skb; \
	})

Please test whether this thing really works and compiles. I just wrote it 
doing something else and never compiled it.

Kind regards,
	Sven
  

On Sunday, October 14, 2012 20:46:57 Simon Wunderlich wrote:
> It is possible that the mac_len is not properly exported because of
> strange device configuration (this behaviour has been observed when
> using batman-adv on top of a vlan interface). Therefore it is needed to
> explicitly recompute it at the very beginning of the rx path.
> 
> This is done by appending the recompute function to the skb_share_mac()
> function, hence the "dirty hack" in the subject. We expect this problem to
> be fixed in linux 3.8 and above.
> 
> Reported-by: Antonio Quartulli <ordex@autistici.org>
> Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
> ---
> 
> this is a rewrite of Antonios patch
> "batman-adv: recompute mac_len at the beginning of the rx path". It is
> intended to fix the issue for out-of-kernel releases, without hurting
> the in-kernel code too much. Obviously, this patch won't go upstream
> into the kernel.
> 
> This version includes Svens "more sane" version, and also has implements
> skb_reset_mac_len() for kernels prior to 3.0.
> ---
>  compat.h |   24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)

Applied in revision a6ad857.

Thanks,
Marek