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client: safely handle dealloc on event dispatch 

See also the long in-code comment.

Related: OS#6302
Change-Id: I6f1c0f6a26f9cd6993dc1910a44070ec0438e636
This commit is contained in:
Neels Hofmeyr 2023-12-12 23:57:39 +01:00 committed by osmith
parent 7a6d9c2f72
commit 43eed63b09
1 changed files with 34 additions and 2 deletions
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36
src/libosmo-mgcp-client/mgcp_client_endpoint_fsm.c
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@ -533,10 +533,42 @@ static void on_success(struct osmo_mgcpc_ep_ci *ci, void *data)
mgcp_conn_peer_name(ci->got_port_info? &ci->rtp_info : NULL),
ci->notify.fi ? "" : " (not sending a notification)");
/* Below ordering is a delicate decision:
*
* We want to
* - emit the resulting event to ci->notify.fi,
* - check whether we want to tx the next pending MGCP message.
* Both these steps may terminate (=deallocate) the ep.
* So whichever one goes first may cause a use-after-free in the other.
*
* When dispatching the FSM event, we don't get an rc indicating dealloc of the FSM -- it may deallocate and we
* cannot tell. The common mechanism for that is osmo_fsm_set_dealloc_ctx(OTC_SELECT) and query the still
* allocated FSM state after termination (here we would check 'if (ci->ep != NULL)'), but we cannot assume the
* caller has actually set up an osmo_fsm_set_dealloc_ctx(). At time of writing, e.g. osmo-hnbgw does not use
* it.
*
* In osmo_mgcpc_ep_fsm_check_state_chg_after_response(), we do get an rc: false means FSM has terminated.
* On termination, the ep emits a term event to the FSM's parent.
* That may cause the notify.fi to be terminated in turn, depending on how the caller set things up.
* So: we cannot store notify.fi before, then call osmo_mgcpc_ep_fsm_check_state_chg_after_response(), and then
* emit the event, because notify.fi may have deallocated. We cannot look up whether
* osmo_mgcpc_ep_cancel_notify() has been called, because ci may have deallocated along with ci->ep.
*
* We have to skip emitting below success event in case the ep is now terminated.
* - It may be the final DLCX OK: not a problem, osmo_mgcpc_ep_ci_dlcx() has no notify args on purpose, so we do
* make all callers not set a notify event for DLCX by design. notify.fi should always be NULL when the final
* DLCX OK terminates the local endpoint state.
* - It may also be sudden termination due to a bad problem, in which case we shouldn't emit success.
* The osmo_fsm_inst.parent_term_event should suffice as feedback to the caller.
*/
if (osmo_mgcpc_ep_fsm_check_state_chg_after_response(ci->ep->fi) == false) {
/* false means, the ci->ep has been terminated. */
return;
}
if (ci->notify.fi)
osmo_fsm_inst_dispatch(ci->notify.fi, ci->notify.success, ci->notify.data);
osmo_mgcpc_ep_fsm_check_state_chg_after_response(ci->ep->fi);
}
/*! Return the MGW's local RTP port information for this connection, i.e. the local port that MGW is receiving on, as