gsm/ipa: Add segmentation callback

Add segmentation callback to be used by the streaming backend of libosmo-netif

Related: OS#5753, OS#5751
Change-Id: I3a639e6896cc3b3fc8e9b2e1a58254710efa0d3f

include/osmocom/gsm/ipa.h

@@ -79,3 +79,6 @@ struct msgb *ipa_msg_alloc(int headroom);
 
 int ipa_msg_recv(int fd, struct msgb **rmsg);
 int ipa_msg_recv_buffered(int fd, struct msgb **rmsg, struct msgb **tmp_msg);
+
+/* Callback for segmenting TCP stream data into IPA packets */
+int ipa_segmentation_cb(struct msgb *msg);

src/gsm/ipa.c

@@ -30,6 +30,7 @@
 #include <stdint.h>
 #include <errno.h>
 #include <stdlib.h>
+#include <inttypes.h>
 
 #include <sys/types.h>
 
@@ -718,4 +719,32 @@ struct msgb *ipa_msg_alloc(int headroom)
 	return nmsg;
 }
 
+/*! Segmentation callback used by libosmo-netif streaming backend
+ *  See definition of `struct osmo_io_ops` for callback semantics
+ *  \param[in] msg	Original `struct msgb` received via osmo_io
+ *  \returns		The total packet length indicated by the first header,
+ *			otherwise
+ *			-EAGAIN,  if the header has not been read yet,
+ *			-ENOBUFS, if the header declares a payload too large
+ */
+int ipa_segmentation_cb(struct msgb *msg)
+{
+	const struct ipaccess_head *hh = (const struct ipaccess_head *) msg->data;
+	size_t payload_len, total_len;
+	size_t available = msgb_length(msg) + msgb_tailroom(msg);
+	if (msgb_length(msg) < sizeof(*hh)) {
+		/* Haven't even read the entire header */
+		return -EAGAIN;
+	}
+	payload_len = osmo_ntohs(hh->len);
+	total_len = sizeof(*hh) + payload_len;
+	if (OSMO_UNLIKELY(available < total_len)) {
+		LOGP(DLINP, LOGL_ERROR, "Not enough space left in message buffer. "
+					"Have %zu octets, but need %zu\n",
+					available, total_len);
+		return -ENOBUFS;
+	}
+	return total_len;
+}
+
 /*! @} */

src/gsm/libosmogsm.map

@@ -689,6 +689,7 @@ ipa_ccm_id_get_parse;
 ipa_ccm_id_resp_parse;
 ipa_ccm_make_id_resp;
 ipa_ccm_make_id_resp_from_req;
+ipa_segmentation_cb;
 ipa_msg_alloc;
 ipa_msg_recv;
 ipa_msg_recv_buffered;