[PATCH v2 06/12] packet: Don't hard code maximum packet size to UINT16_MAX

[David Gibson <david@gibson.dropbear.id.au>]

We verify that every packet we store in a pool - and every partial packet
we retreive from it has a length no longer than UINT16_MAX.  This
originated in the older packet pool implementation which stored packet
lengths in a uint16_t.  Now, that packets are represented by a struct
iovec with its size_t length, this check serves only as a sanity / security
check that we don't have some wildly out of range length due to a bug
elsewhere.

However, UINT16_MAX (65535) isn't quite enough, because the "packet" as
stored in the pool is in fact an entire frame including both L2 and any
backend specific headers.  We can exceed this in passt mode, even with the
default MTU: 65520 bytes of IP datagram + 14 bytes of Ethernet header +
4 bytes of qemu stream length header = 65538 bytes.

Introduce our own define for the maximum length of a packet in the pool and
set it slightly larger, allowing 128 bytes for L2 and/or other backend
specific headers.  We'll use different amounts of that depending on the
tap backend, but since this is just a sanity check, the bound doesn't need
to be 100% tight.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 packet.c | 4 ++--
 packet.h | 7 +++++++
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/packet.c b/packet.c
index 0330b548..bcac0375 100644
--- a/packet.c
+++ b/packet.c
@@ -83,7 +83,7 @@ void packet_add_do(struct pool *p, size_t len, const char *start,
 	if (packet_check_range(p, start, len, func, line))
 		return;
 
-	if (len > UINT16_MAX) {
+	if (len > PACKET_MAX_LEN) {
 		trace("add packet length %zu, %s:%i", len, func, line);
 		return;
 	}
@@ -119,7 +119,7 @@ void *packet_get_do(const struct pool *p, size_t idx, size_t offset,
 		return NULL;
 	}
 
-	if (len > UINT16_MAX) {
+	if (len > PACKET_MAX_LEN) {
 		if (func) {
 			trace("packet data length %zu, %s:%i",
 			      len, func, line);
diff --git a/packet.h b/packet.h
index bdc07fef..05d37695 100644
--- a/packet.h
+++ b/packet.h
@@ -6,6 +6,13 @@
 #ifndef PACKET_H
 #define PACKET_H
 
+/* Maximum size of a single packet in a pool, including all headers.  Sized to
+ * allow a maximum size IP datagram (65535) plus some extra space or L2 and
+ * backend specific headers.  This is just for sanity checking, so doesn't need
+ * to be a tight limit.
+ */
+#define PACKET_MAX_LEN	ROUND_UP(IP_MAX_MTU + 128, sizeof(unsigned long))
+
 /**
  * struct pool - Generic pool of packets stored in a buffer
  * @buf:	Buffer storing packet descriptors,
-- 
@@ -6,6 +6,13 @@
 #ifndef PACKET_H
 #define PACKET_H
 
+/* Maximum size of a single packet in a pool, including all headers.  Sized to
+ * allow a maximum size IP datagram (65535) plus some extra space or L2 and
+ * backend specific headers.  This is just for sanity checking, so doesn't need
+ * to be a tight limit.
+ */
+#define PACKET_MAX_LEN	ROUND_UP(IP_MAX_MTU + 128, sizeof(unsigned long))
+
 /**
  * struct pool - Generic pool of packets stored in a buffer
  * @buf:	Buffer storing packet descriptors,
-- 
2.47.1