fix PACCH paging: don't return early in case of NULL TBF

Commit b78a4a6dfef217c538d45949a6ae725e22a36b05 tried to fix a NULL dereference
error, but apparently was overly eager to return, because it looked like all
code paths would dereference the tbf.

In fact the code path further above, for msg != NULL, has "always" dereferenced
the tbf, but the lower code path, the one effecting the paging, has only
started to dereference tbf since shortly before the overly eager fix: in
da7250ad2c1cd5ddc7d3c6e10435a00b357ef8f7, to "update the dl ctrl msg counter
for ms". It seems that this tbf dereference in the paging path is bogus and the
cause for the segfault that made me write the early exit fix.

Fix that fix:

Do not exit early if tbf == NULL, stay in there to be able to reach the paging
path below.

In case of a message to be sent, assume that tbf is present, and verify: print
an error message and abort if there is a msg but no tbf, so that we will see
the error if I'm wrong there. If a tbf is missing, free the msg.

In case of no message, go on to send pending pagings, but do not attempt to
count ctrl messages for a tbf -- IIUC there will never be a tbf if we're
paging.

This should avoid segfaults while keeping PACCH paging intact.

Tweak a comment for and add a blank line above the paging section.

Related: OS#2176 CID#158969
Change-Id: Ib79f4a945e211a13ac7d1e511cc37b0940ac6202

1 files changed, 8 insertions, 7 deletions

diff --git a/src/gprs_rlcmac_sched.cpp b/src/gprs_rlcmac_sched.cpp
index 3b940f47..97ee53e5 100644
--- a/src/gprs_rlcmac_sched.cpp
+++ b/src/gprs_rlcmac_sched.cpp
@@ -178,11 +178,14 @@ static struct msgb *sched_select_ctrl_msg(
 		}
 	}
 
-	if (!tbf)
-		return NULL;
-
 	/* any message */
 	if (msg) {
+		if (!tbf) {
+			LOGP(DRLCMACSCHED, LOGL_ERROR,
+			     "Control message to be scheduled, but no TBF (TRX=%d, TS=%d)\n", trx, ts);
+			msgb_free(msg);
+			return NULL;
+		}
 		tbf->rotate_in_list();
 		LOGP(DRLCMACSCHED, LOGL_DEBUG, "Scheduling control "
 			"message at RTS for %s (TRX=%d, TS=%d)\n",
@@ -191,14 +194,12 @@ static struct msgb *sched_select_ctrl_msg(
 		tbf->ms()->update_dl_ctrl_msg();
 		return msg;
 	}
-	/* schedule PACKET PAGING REQUEST */
+
+	/* schedule PACKET PAGING REQUEST, if any are pending */
 	msg = pdch->packet_paging_request();
 	if (msg) {
 		LOGP(DRLCMACSCHED, LOGL_DEBUG, "Scheduling paging request "
 			"message at RTS for (TRX=%d, TS=%d)\n", trx, ts);
-
-		/* Updates the dl ctrl msg counter for ms */
-		tbf->ms()->update_dl_ctrl_msg();
 		return msg;
 	}