sgsn: Re-add searching for MM ctx based on TLLI / P-TMSI matches

If an MM context cannot be found based on BBSGP info and a RA UPDATE
REQUEST is received, try to find an MM context with an P-TMSI from
which the TLLI could have been derived. This also checks, whether the
routing area matches.

This is similar to the old behaviour removed by the commits
"sgsn: Only look at TLLIs in sgsn_mm_ctx_by_tlli" and
"sgsn: Remove tlli_foreign2local", except that this will only
be done for RA UPDATE REQUESTs now.

Sponsored-by: On-Waves ehf

2 files changed, 48 insertions, 3 deletions

diff --git a/openbsc/src/gprs/gprs_gmm.c b/openbsc/src/gprs/gprs_gmm.c
index 5aea76f88..d93ba3f77 100644
--- a/openbsc/src/gprs/gprs_gmm.c
+++ b/openbsc/src/gprs/gprs_gmm.c
@@ -1172,13 +1172,33 @@ static int gsm48_rx_gmm_ra_upd_req(struct sgsn_mm_ctx *mmctx, struct msgb *msg,
 		 * if the TLLI matches foreign_tlli (P-TMSI). Note that this
 		 * is an optimization to avoid the RA reject (impl detached)
 		 * below, which will cause a new attach cycle. */
-	}
-
-	if (!mmctx || !gprs_ra_id_equals(&mmctx->ra, &old_ra_id) ||
+		/* Look-up the MM context based on old RA-ID and TLLI */
+		mmctx = sgsn_mm_ctx_by_tlli_and_ptmsi(msgb_tlli(msg), &old_ra_id);
+		if (mmctx) {
+			LOGMMCTXP(LOGL_INFO, mmctx,
+				"Looked up by matching TLLI and P_TMSI. "
+				"BSSGP TLLI: %08x, P-TMSI: %08x (%08x), "
+				"TLLI: %08x (%08x), RA: %d-%d-%d-%d\n",
+				msgb_tlli(msg),
+				mmctx->p_tmsi, mmctx->p_tmsi_old,
+				mmctx->tlli, mmctx->tlli_new,
+				mmctx->ra.mcc, mmctx->ra.mnc,
+				mmctx->ra.lac, mmctx->ra.rac);
+
+			mmctx->mm_state = GMM_COMMON_PROC_INIT;
+		}
+	} else if (!gprs_ra_id_equals(&mmctx->ra, &old_ra_id) ||
 		mmctx->mm_state == GMM_DEREGISTERED)
 	{
 		/* We cannot use the mmctx */
+		LOGMMCTXP(LOGL_INFO, mmctx,
+			"The MM context cannot be used, RA: %d-%d-%d-%d\n",
+			mmctx->ra.mcc, mmctx->ra.mnc,
+			mmctx->ra.lac, mmctx->ra.rac);
+		mmctx = NULL;
+	}
 
+	if (!mmctx) {
 		/* send a XID reset to re-set all LLC sequence numbers
 		 * in the MS */
 		LOGMMCTXP(LOGL_NOTICE, mmctx, "LLC XID RESET\n");
diff --git a/openbsc/src/gprs/gprs_sgsn.c b/openbsc/src/gprs/gprs_sgsn.c
index f71066d8e..b7bda49ba 100644
--- a/openbsc/src/gprs/gprs_sgsn.c
+++ b/openbsc/src/gprs/gprs_sgsn.c
@@ -105,6 +105,31 @@ struct sgsn_mm_ctx *sgsn_mm_ctx_by_tlli(uint32_t tlli,
 	return NULL;
 }
 
+struct sgsn_mm_ctx *sgsn_mm_ctx_by_tlli_and_ptmsi(uint32_t tlli,
+					const struct gprs_ra_id *raid)
+{
+	struct sgsn_mm_ctx *ctx;
+	int tlli_type;
+
+	/* TODO: Also check the P_TMSI signature to be safe. That signature
+	 * should be different (at least with a sufficiently high probability)
+	 * after SGSN restarts and for multiple SGSN instances.
+	 */
+
+	tlli_type = gprs_tlli_type(tlli);
+	if (tlli_type != TLLI_FOREIGN && tlli_type != TLLI_LOCAL)
+		return NULL;
+
+	llist_for_each_entry(ctx, &sgsn_mm_ctxts, list) {
+		if ((gprs_tmsi2tlli(ctx->p_tmsi, tlli_type) == tlli ||
+		     gprs_tmsi2tlli(ctx->p_tmsi_old, tlli_type) == tlli) &&
+		    gprs_ra_id_equals(raid, &ctx->ra))
+			return ctx;
+	}
+
+	return NULL;
+}
+
 struct sgsn_mm_ctx *sgsn_mm_ctx_by_ptmsi(uint32_t p_tmsi)
 {
 	struct sgsn_mm_ctx *ctx;