add caller-owns-msgb variant osmo_sccp_user_sap_down_nofree()

Add osmo_sccp_user_sap_down_nofree(), which is identical to
osmo_sccp_user_sap_down(), but doesn't imply a msgb_free().

To implement that, sccp_sclc_user_sap_down_nofree() with the same msgb
semantics is required.

Rationale:

Avoiding msgb leaks is easiest if the caller retains ownership of the msgb.
Take this hypothetical chain where leaks are obviously avoided:

  void send()
  {
  	msg = msgb_alloc();
	dispatch(msg);
	msgb_free(msg);
  }

  void dispatch(msg)
  {
  	osmo_fsm_inst_dispatch(fi, msg);
  }

  void fi_on_event(fi, data)
  {
	if (socket_is_ok)
		socket_write((struct msgb*)data);
  }

  void socket_write(msgb)
  {
  	if (!ok1)
		return;
	if (ok2) {
		if (!ok3)
			return;
		write(sock, msg->data);
	}
  }

However, if the caller passes ownership down to the msgb consumer, things
become nightmarishly complex:

  void send()
  {
  	msg = msgb_alloc();
	rc = dispatch(msg);
	/* dispatching event failed? */
	if (rc)
		msgb_free(msg);
  }

  int dispatch(msg)
  {
  	if (osmo_fsm_inst_dispatch(fi, msg))
		return -1;
	if (something_else())
		return -1; // <-- double free!
  }

  void fi_on_event(fi, data)
  {
	if (socket_is_ok) {
		socket_write((struct msgb*)data);
	else
		/* socket didn't consume? */
		msgb_free(data);
  }

  int socket_write(msgb)
  {
  	if (!ok1)
		return -1; // <-- leak!
	if (ok2) {
		if (!ok3)
			goto out;
		write(sock, msg->data);
	}
  out:
        msgb_free(msg);
	return -2;
  }

If any link in this call chain fails to be aware of the importance to return a
failed RC or to free a msgb if the chain is broken, or to not return a failed
RC if the msgb is consumed, we have a hidden msgb leak or double free.

This is the case with osmo_sccp_user_sap_down(). In new osmo-msc, passing data
through various FSM instances, there is high potential for leak/double-free
bugs. A very large brain is required to track down every msgb path.

osmo_sccp_user_sap_down_nofree() makes this problem trivial to solve even for
humans.

Change-Id: Ic818efa78b90f727e1a94c18b60d9a306644f340

1 files changed, 1 insertions, 0 deletions

diff --git a/include/osmocom/sigtran/sccp_sap.h b/include/osmocom/sigtran/sccp_sap.h
index 84d762c..f64b7aa 100644
--- a/include/osmocom/sigtran/sccp_sap.h
+++ b/include/osmocom/sigtran/sccp_sap.h
@@ -263,6 +263,7 @@ osmo_sccp_user_bind(struct osmo_sccp_instance *inst, const char *name,
 		    osmo_prim_cb prim_cb, uint16_t ssn);
 
 int osmo_sccp_user_sap_down(struct osmo_sccp_user *scu, struct osmo_prim_hdr *oph);
+int osmo_sccp_user_sap_down_nofree(struct osmo_sccp_user *scu, struct osmo_prim_hdr *oph);
 
 struct osmo_ss7_instance *
 osmo_sccp_addr_by_name(struct osmo_sccp_addr *dest_addr,