packet: Allow packets with only a header (but no payload)

Commit 9c707762 ("packet: make packet_snd fail on len smaller
than l2 header") added validation for the packet size in packet_snd.
This change enforces that every packet needs a header (with at least
hard_header_len bytes) plus a payload with at least one byte. Before
this change the payload was optional.

This fixes PPPoE connections which do not have a "Service" or
"Host-Uniq" configured (which is violating the spec, but is still
widely used in real-world setups). Those are currently failing with the
following message: "pppd: packet size is too short (24 <= 24)"
Signed-off-by: NMartin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -1398,7 +1398,8 @@ enum netdev_priv_flags {
  *	@dma:		DMA channel
  *	@mtu:		Interface MTU value
  *	@type:		Interface hardware type
- *	@hard_header_len: Hardware header length
+ *	@hard_header_len: Hardware header length, which means that this is the
+ *			  minimum size of a packet.
  *
  *	@needed_headroom: Extra headroom the hardware may need, but not in all
  *			  cases can this be guaranteed

net/packet/af_packet.c

@@ -2329,8 +2329,8 @@ static void tpacket_destruct_skb(struct sk_buff *skb)
 static bool ll_header_truncated(const struct net_device *dev, int len)
 {
 	/* net device doesn't like empty head */
-	if (unlikely(len <= dev->hard_header_len)) {
-		net_warn_ratelimited("%s: packet size is too short (%d <= %d)\n",
+	if (unlikely(len < dev->hard_header_len)) {
+		net_warn_ratelimited("%s: packet size is too short (%d < %d)\n",
 				     current->comm, len, dev->hard_header_len);
 		return true;
 	}