arp: Don't mix incoming and outgoing buffers

Don't use the memory of the incoming packet to build the outgoing buffer
as it can be memory of the TX queue in the case of vhost-user.

Moreover with vhost-user, the packet can be split across several
iovec and it's easier to rebuild it in a buffer than updating an
existing iovec array.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

1 files changed, 55 insertions, 29 deletions

diff --git a/arp.c b/arp.c
index fc482bb..9f1fede 100644
--- a/arp.c
+++ b/arp.c
@@ -31,56 +31,82 @@
 #include "tap.h"
 
 /**
- * arp() - Check if this is a supported ARP message, reply as needed
+ * ignore_arp() - Check if we should ignore this ARP message
  * @c:		Execution context
- * @p:		Packet pool, single packet with Ethernet buffer
+ * @ah:		ARP header
+ * @am:		ARP message
  *
- * Return: 1 if handled, -1 on failure
+ * Return: true if the ARP message should be ignored, false otherwise
  */
-int arp(const struct ctx *c, const struct pool *p)
+static bool ignore_arp(const struct ctx *c,
+		       const struct arphdr *ah, const struct arpmsg *am)
 {
-	unsigned char swap[4];
-	struct ethhdr *eh;
-	struct arphdr *ah;
-	struct arpmsg *am;
-	size_t l2len;
-
-	eh = packet_get(p, 0, 0,			 sizeof(*eh), NULL);
-	ah = packet_get(p, 0, sizeof(*eh),		 sizeof(*ah), NULL);
-	am = packet_get(p, 0, sizeof(*eh) + sizeof(*ah), sizeof(*am), NULL);
-
-	if (!eh || !ah || !am)
-		return -1;
-
 	if (ah->ar_hrd != htons(ARPHRD_ETHER)	||
 	    ah->ar_pro != htons(ETH_P_IP)	||
 	    ah->ar_hln != ETH_ALEN		||
 	    ah->ar_pln != 4			||
 	    ah->ar_op  != htons(ARPOP_REQUEST))
-		return 1;
+		return true;
 
 	/* Discard announcements, but not 0.0.0.0 "probes" */
 	if (memcmp(am->sip, &in4addr_any, sizeof(am->sip)) &&
 	    !memcmp(am->sip, am->tip, sizeof(am->sip)))
-		return 1;
+		return true;
 
 	/* Don't resolve the guest's assigned address, either. */
 	if (!memcmp(am->tip, &c->ip4.addr, sizeof(am->tip)))
+		return true;
+
+	return false;
+}
+
+/**
+ * arp() - Check if this is a supported ARP message, reply as needed
+ * @c:		Execution context
+ * @p:		Packet pool, single packet with Ethernet buffer
+ *
+ * Return: 1 if handled, -1 on failure
+ */
+int arp(const struct ctx *c, const struct pool *p)
+{
+	struct {
+		struct ethhdr eh;
+		struct arphdr ah;
+		struct arpmsg am;
+	} __attribute__((__packed__)) resp;
+	const struct ethhdr *eh;
+	const struct arphdr *ah;
+	const struct arpmsg *am;
+
+	eh = packet_get(p, 0, 0,			 sizeof(*eh), NULL);
+	ah = packet_get(p, 0, sizeof(*eh),		 sizeof(*ah), NULL);
+	am = packet_get(p, 0, sizeof(*eh) + sizeof(*ah), sizeof(*am), NULL);
+
+	if (!eh || !ah || !am)
+		return -1;
+
+	if (ignore_arp(c, ah, am))
 		return 1;
 
-	ah->ar_op = htons(ARPOP_REPLY);
-	memcpy(am->tha,		am->sha,	sizeof(am->tha));
-	memcpy(am->sha,		c->our_tap_mac,	sizeof(am->sha));
+	/* Ethernet header */
+	resp.eh.h_proto = htons(ETH_P_ARP);
+	memcpy(resp.eh.h_dest, eh->h_source, sizeof(resp.eh.h_dest));
+	memcpy(resp.eh.h_source, c->our_tap_mac, sizeof(resp.eh.h_source));
 
-	memcpy(swap,		am->tip,	sizeof(am->tip));
-	memcpy(am->tip,		am->sip,	sizeof(am->tip));
-	memcpy(am->sip,		swap,		sizeof(am->sip));
+	/* ARP header */
+	resp.ah.ar_op = htons(ARPOP_REPLY);
+	resp.ah.ar_hrd = ah->ar_hrd;
+	resp.ah.ar_pro = ah->ar_pro;
+	resp.ah.ar_hln = ah->ar_hln;
+	resp.ah.ar_pln = ah->ar_pln;
 
-	l2len = sizeof(*eh) + sizeof(*ah) + sizeof(*am);
-	memcpy(eh->h_dest,	eh->h_source,	sizeof(eh->h_dest));
-	memcpy(eh->h_source,	c->our_tap_mac,	sizeof(eh->h_source));
+	/* ARP message */
+	memcpy(resp.am.sha,		c->our_tap_mac,	sizeof(resp.am.sha));
+	memcpy(resp.am.sip,		am->tip,	sizeof(resp.am.sip));
+	memcpy(resp.am.tha,		am->sha,	sizeof(resp.am.tha));
+	memcpy(resp.am.tip,		am->sip,	sizeof(resp.am.tip));
 
-	tap_send_single(c, eh, l2len);
+	tap_send_single(c, &resp, sizeof(resp));
 
 	return 1;
 }