[PATCH net v2] net: Reset MAC header for direct packet transmission

Reset MAC header in case of using dev_direct_xmit(), e.g. by specifying
PACKET_QDISC_BYPASS. This is needed, because other code such as the HSR layer
expects the MAC header to be correctly set.

This has been observed using the following setup:

|$ ip link add name hsr0 type hsr slave1 lan0 slave2 lan1 supervision 45 version 1
|$ ifconfig hsr0 up
|$ ./test hsr0

The test binary is using mmap'ed sockets and is specifying the
PACKET_QDISC_BYPASS socket option.

This patch resolves the following warning on a non-patched kernel:

|[  112.725394] ------------[ cut here ]------------
|[  112.731418] WARNING: CPU: 1 PID: 257 at net/hsr/hsr_forward.c:560 hsr_forward_skb+0x484/0x568
|[  112.739962] net/hsr/hsr_forward.c:560: Malformed frame (port_src hsr0)

The MAC header is also reset unconditionally in case of PACKET_QDISC_BYPASS is
not used at the top of __dev_queue_xmit().

Fixes: d346a3fae3ff ("packet: introduce PACKET_QDISC_BYPASS socket option")
Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
---

Changes since v1:

 * Move skb_reset_mac_header() to __dev_direct_xmit()
 * Add Fixes tag
 * Target net tree

Previous versions:

 * https://lkml.kernel.org/netdev/20210326154835.21296-1-kurt@linutronix.de/

net/core/dev.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/core/dev.c b/net/core/dev.c
index b4c67a5be606..b5088223dc57 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c

@@ -4297,6 +4297,8 @@ int __dev_direct_xmit(struct sk_buff *skb, u16 queue_id)
 		     !netif_carrier_ok(dev)))
 		goto drop;
 
+	skb_reset_mac_header(skb);
+
 	skb = validate_xmit_skb_list(skb, dev, &again);
 	if (skb != orig_skb)
 		goto drop;

-- 
2.20.1