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Fixups and clarifying comments for msgb tail modifications

Added fixups for msgb len field whenever the tail is modified
Also, added some clarifying comments

Change-Id: Ia725edbeafe26bd2ea9b5a1810d0b26bc79d84db
---
M src/lower_mac/tetra_lower_mac.c
M src/tetra_llc.c
M src/tetra_upper_mac.c
3 files changed, 30 insertions(+), 9 deletions(-)

git pull ssh://gerrit.osmocom.org:29418/osmo-tetra refs/changes/00/34000/1

diff --git a/src/lower_mac/tetra_lower_mac.c b/src/lower_mac/tetra_lower_mac.c
index 21998f7..f8e7810 100644
--- a/src/lower_mac/tetra_lower_mac.c
+++ b/src/lower_mac/tetra_lower_mac.c
@@ -325,23 +325,26 @@
 
 	int pdu_bits = 0;
 	uint32_t offset = 0;
-	uint8_t *orig_head = msg->head;
-	uint8_t *orig_tail = msg->tail;
+	uint8_t *orig_head = msg->head; /* The true start of the timeslot */
+	uint8_t *orig_tail = msg->tail; /* The true end of the timeslot */
 	while (offset < tbp->type1_bits - 16) {
 		/* send Rx time along with the TMV-UNITDATA.ind primitive */
 		memcpy(&tup->tdma_time, &tcd->time, sizeof(tup->tdma_time));
+
+		/* Parse MAC element in timeslot (just one, possibly more later in the loop) */
 		pdu_bits = upper_mac_prim_recv(&ttp->oph, tms);
 
-		if (pdu_bits < 0) {
-			/* -1 is returned when pdu fills slot or length could not be determined */
+		/* Check if we are done (-1 returned) */
+		if (pdu_bits < 0)
 			break;
-		}
 
-		/* Increment head and l1h ptrs and reset tail to end of msg (may be altered by removing FCS) */
+		/* Not done */
+		/* Increment head and l1h ptrs */
+		/* Reset tail to end of msg (may be altered by removing FCS) */
 		offset += pdu_bits;
-		msg->head = orig_head + offset;
-		msg->tail = orig_tail;
-		msg->len = msg->tail - msg->head;
+		msg->head = orig_head + offset;		/* New head is old head plus parsed len from prev msg */
+		msg->tail = orig_tail;			/* Restore original tail */
+		msg->len = msg->tail - msg->head;	/* Fixup len */
 		msg->l1h = msg->head;
 		msg->l2h = 0;
 		msg->l3h = 0;
diff --git a/src/tetra_llc.c b/src/tetra_llc.c
index b8a378d..cdbfea0 100644
--- a/src/tetra_llc.c
+++ b/src/tetra_llc.c
@@ -122,6 +122,7 @@
 	tetra_llc_pdu_parse(&lpp, msg->l2h, len);
 	msg->l3h = lpp.tl_sdu;
 	msg->tail = msg->l3h + lpp.tl_sdu_len; // Strips off FCS (if present)
+	msg->len = msg->tail - msg->head;
 
 	printf("TM-SDU(%s)", tetra_get_llc_pdut_dec_name(lpp.pdu_type));
 	if (lpp.have_fcs) {
diff --git a/src/tetra_upper_mac.c b/src/tetra_upper_mac.c
index 94a58ce..3b2092b 100644
--- a/src/tetra_upper_mac.c
+++ b/src/tetra_upper_mac.c
@@ -175,12 +175,14 @@
 	} else {
 		pdu_bits = rsd.macpdu_length * 8;	/* Length given */
 		msg->tail = msg->head + pdu_bits;
+		msg->len = msg->tail - msg->head;
 	}
 
 	/* Strip fill bits */
 	if (rsd.fill_bits) {
 		int num_fill_bits = get_num_fill_bits(msg->l1h, msgb_l1len(msg));
 		msg->tail -= num_fill_bits;
+		msg->len = msg->tail - msg->head;
 	}
 
 	/* Decrypt buffer if encrypted and key available */
@@ -301,6 +303,7 @@
 		if (fillbits_present) {
 			int num_fill_bits = get_num_fill_bits(msg->l1h, msgb_l1len(msg));
 			msg->tail -= num_fill_bits;
+			msg->len = msg->tail - msg->head;
 		}
 
 		/* Decrypt (if required) */
@@ -349,9 +352,11 @@
 
 		/* Determine msg len, strip fill bits if any */
 		msg->tail = msg->head + length_indicator * 8;
+		msg->len = msg->tail - msg->head;
 		if (fillbits_present) {
 			num_fill_bits = get_num_fill_bits(msg->l1h, msgb_l1len(msg));
 			msg->tail -= num_fill_bits;
+			msg->len = msg->tail - msg->head;
 		}
 
 		/* Decrypt (if required) */

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