pktgen: Dont leak kernel memory

While playing with pktgen, I realized IP ID was not filled and a
random value was taken, possibly leaking 2 bytes of kernel memory.
 
We can use an increasing ID, this can help diagnostics anyway.

Also clear packet payload, instead of leaking kernel memory.
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/pktgen.c

@@ -335,6 +335,7 @@ struct pktgen_dev {
 	__u32 cur_src_mac_offset;
 	__be32 cur_saddr;
 	__be32 cur_daddr;
+	__u16 ip_id;
 	__u16 cur_udp_dst;
 	__u16 cur_udp_src;
 	__u16 cur_queue_map;
@@ -2630,6 +2631,8 @@ static struct sk_buff *fill_packet_ipv4(struct net_device *odev,
 	iph->protocol = IPPROTO_UDP;	/* UDP */
 	iph->saddr = pkt_dev->cur_saddr;
 	iph->daddr = pkt_dev->cur_daddr;
+	iph->id = htons(pkt_dev->ip_id);
+	pkt_dev->ip_id++;
 	iph->frag_off = 0;
 	iplen = 20 + 8 + datalen;
 	iph->tot_len = htons(iplen);
@@ -2641,24 +2644,26 @@ static struct sk_buff *fill_packet_ipv4(struct net_device *odev,
 	skb->dev = odev;
 	skb->pkt_type = PACKET_HOST;
 
-	if (pkt_dev->nfrags <= 0)
+	if (pkt_dev->nfrags <= 0) {
 		pgh = (struct pktgen_hdr *)skb_put(skb, datalen);
-	else {
+		memset(pgh + 1, 0, datalen - sizeof(struct pktgen_hdr));
+	} else {
 		int frags = pkt_dev->nfrags;
-		int i;
+		int i, len;
 
 		pgh = (struct pktgen_hdr *)(((char *)(udph)) + 8);
 
 		if (frags > MAX_SKB_FRAGS)
 			frags = MAX_SKB_FRAGS;
 		if (datalen > frags * PAGE_SIZE) {
-			skb_put(skb, datalen - frags * PAGE_SIZE);
+			len = datalen - frags * PAGE_SIZE;
+			memset(skb_put(skb, len), 0, len);
 			datalen = frags * PAGE_SIZE;
 		}
 
 		i = 0;
 		while (datalen > 0) {
-			struct page *page = alloc_pages(GFP_KERNEL, 0);
+			struct page *page = alloc_pages(GFP_KERNEL | __GFP_ZERO, 0);
 			skb_shinfo(skb)->frags[i].page = page;
 			skb_shinfo(skb)->frags[i].page_offset = 0;
 			skb_shinfo(skb)->frags[i].size =