[PATCH libnetfilter_queue 1/1] src: Add alternative function to pktb_alloc to avoid malloc / free overhead

[Duncan Roe <duncan_roe@xxxxxxxxxxxxxxx>]

New function pktb_usebuf() works like pktb_alloc() except it has 2 extra
arguments, being a user-supplied buffer and its length.
Performance testing is ongoing, so the 5% improvement claimed in the
documentation is conservative.

Updated:

 include/libnetfilter_queue/pktbuff.h: Add pktb_usebuf() prototype

 src/extra/pktbuff.c: Implement pktb_usebuf()

Signed-off-by: Duncan Roe <duncan_roe@xxxxxxxxxxxxxxx>
---
 include/libnetfilter_queue/pktbuff.h |  2 +
 src/extra/pktbuff.c                  | 82 ++++++++++++++++++++++++++++++++++++
 2 files changed, 84 insertions(+)

diff --git a/include/libnetfilter_queue/pktbuff.h b/include/libnetfilter_queue/pktbuff.h
index b15ee1e..ec75727 100644
--- a/include/libnetfilter_queue/pktbuff.h
+++ b/include/libnetfilter_queue/pktbuff.h
@@ -4,6 +4,8 @@
 struct pkt_buff;
 
 struct pkt_buff *pktb_alloc(int family, void *data, size_t len, size_t extra);
+struct pkt_buff *pktb_usebuf(int family, void *data, size_t len, size_t extra,
+			     void *buf, size_t bufsize);
 void pktb_free(struct pkt_buff *pktb);
 
 uint8_t *pktb_data(struct pkt_buff *pktb);
diff --git a/src/extra/pktbuff.c b/src/extra/pktbuff.c
index c4f3da3..774e4ab 100644
--- a/src/extra/pktbuff.c
+++ b/src/extra/pktbuff.c
@@ -96,6 +96,88 @@ struct pkt_buff *pktb_alloc(int family, void *data, size_t len, size_t extra)
 	return pktb;
 }
 
+/**
+ * pktb_usebuf - make a packet buffer from an existing buffer
+ * \param family Indicate what family. Currently supported families are
+ * AF_BRIDGE, AF_INET & AF_INET6.
+ * \param data Pointer to packet data
+ * \param len Packet length
+ * \param extra Extra memory in the tail to be allocated (for mangling)
+ * \param buf Existing buffer to use
+ * \param bufsize Size of _buf_
+ *
+ * This function builds a userspace packet buffer inside a supplied buffer.
+ * The packet buffer contains the packet data and some extra memory room in the
+ * tail (if requested).
+ *
+ * This function uses less CPU cycles than pktb_alloc() + pktb_free().
+ * Users can expect a decrease in overall CPU time in the order of 5%.
+ *
+ * \return Pointer to a userspace packet buffer aligned within _buf_ or NULL on
+ * failure.
+ * \par Errors
+ * __ENOMEM__ _bufsize_ is too small to accomodate _len_
+ * \n
+ * __EPROTONOSUPPORT__ _family_ was __AF_BRIDGE__ and this is not an IP packet
+ * (v4 or v6)
+ * \warning Do __not__ call pktb_free() on the returned pointer
+ */
+EXPORT_SYMBOL
+struct pkt_buff *pktb_usebuf(int family, void *data, size_t len, size_t extra,
+			     void *buf, size_t bufsize)
+{
+	struct pkt_buff *pktb;
+	void *pkt_data;
+
+	/* Better make sure alignment is correct. */
+	size_t extra2 =
+		(size_t)buf & 0x7 ? (~((size_t)buf & 0x7) & 0x7) + 1 : 0;
+
+	pktb = buf + extra2;
+	if (extra2 + sizeof(struct pkt_buff) + len + extra > bufsize)
+	{
+		errno = ENOMEM;
+		return NULL;
+	}
+	memset(pktb, 0, sizeof(struct pkt_buff) + len + extra);
+
+	pkt_data = (uint8_t *)pktb + sizeof(struct pkt_buff);
+	memcpy(pkt_data, data, len);
+
+	pktb->len = len;
+	pktb->data_len = len + extra;
+
+	pktb->head = pkt_data;
+	pktb->data = pkt_data;
+	pktb->tail = pktb->head + len;
+
+	switch(family) {
+	case AF_INET:
+	case AF_INET6:
+		pktb->network_header = pktb->data;
+		break;
+	case AF_BRIDGE: {
+		struct ethhdr *ethhdr = (struct ethhdr *)pktb->data;
+
+		pktb->mac_header = pktb->data;
+
+		switch(ethhdr->h_proto) {
+		case ETH_P_IP:
+		case ETH_P_IPV6:
+			pktb->network_header = pktb->data + ETH_HLEN;
+			break;
+		default:
+			/* This protocol is unsupported. */
+			errno = EPROTONOSUPPORT;
+			free(pktb);
+			return NULL;
+		}
+		break;
+	}
+	}
+	return pktb;
+}
+
 /**
  * pktb_data - get pointer to network packet
  * \param pktb Pointer to userspace packet buffer
-- 
2.14.5