[PATCH batadv v2 3/3] batman-adv: mcast: linearize skbuff for packet generation

batadv_mcast_forw_packet() and batadv_mcast_forw_packet() is not only
called by the unsharing+linearizing batadv_recv_mcast_packet() handler.
When it is called by batadv_mcast_forw_mcsend() then it will be unshared
but not linearized. The SKB_LINEAR_ASSERT() can therefore cause a fatal
BUG().

The linearization should happen during the expansion of the head because
the call chain because the scrape function can be hit already during the
initial batadv_mcast_forw_mode() selection code:

* batadv_interface_tx
* batadv_mcast_forw_mode
* batadv_mcast_forw_mode_by_count()
* batadv_mcast_forw_push()
  -> calls batadv_mcast_forw_expand_head() before everything else
* batadv_mcast_forw_push_tvlvs()
* batadv_mcast_forw_push_dests()
* batadv_mcast_forw_push_adjust_padding()
* batadv_mcast_forw_scrape()

Reported-by: Sashiko <[email protected]>
Fixes: 8ed36122d709 ("batman-adv: mcast: implement multicast packet reception and forwarding")
Signed-off-by: Sven Eckelmann <[email protected]>
---
 net/batman-adv/multicast_forw.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/batman-adv/multicast_forw.c b/net/batman-adv/multicast_forw.c
index c5b8e2e3..d8374ef5 100644
--- a/net/batman-adv/multicast_forw.c
+++ b/net/batman-adv/multicast_forw.c
@@ -1103,6 +1103,10 @@ static int batadv_mcast_forw_expand_head(struct batadv_priv *bat_priv,
 	if (skb_cow(skb, hdr_size) < 0)
 		return -ENOMEM;
 
+	/* batadv_mcast_forw_scrape() + batadv_mcast_forw_packet() require linearized skb */
+	if (skb_linearize(skb) < 0)
+		return -ENOMEM;
+
 	return 0;
 }
 

-- 
2.47.3