Event Notification

A controlling process can track certain events that occur within the scope of the runtime linker that. These events are:

RD_PREINIT

The runtime linker has loaded and relocated all the dynamic objects and is about to start calling the .init sections of each object loaded.

RD_POSTINIT

The runtime linker has finished calling all of the .init sections and is about to transfer control to the primary executable.

RD_DLACTIVITY

The runtime linker has been invoked to either load or unload a dynamic object.

These events can be monitored using the following interface, defined in sys/link.h and rtld_db.h.

typedef enum {
        RD_NONE = 0,
        RD_PREINIT,
        RD_POSTINIT,
        RD_DLACTIVITY
} rd_event_e;
 
/*
 * Ways that the event notification can take place:
 */
typedef enum {
        RD_NOTIFY_BPT,
        RD_NOTIFY_AUTOBPT,
        RD_NOTIFY_SYSCALL
} rd_notify_e;
 
/*
 * Information on ways that the event notification can take place:
 */
typedef struct rd_notify {
        rd_notify_e     type;
        union {
                psaddr_t        bptaddr;
                long            syscallno;
        } u;
} rd_notify_t;

The following functions track events.

rd_event_enable()

This function enables (1) or disables (0) event monitoring.

rd_err_e rd_event_enable(struct rd_agent * rdap, int onoff);

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Note –

Presently, for performance reasons, the runtime linker ignores event disabling. The controlling process should not assume that a given break-point can not be reached because of the last call to this routine.

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rd_event_addr()

This function specifies how the controlling program is notified of a given event.

rd_err_e rd_event_addr(rd_agent_t * rdap, rd_event_e event,
        rd_notify_t * notify);

Depending on the event type, the notification of the controlling process takes place by calling a benign, cheap system call that is identified by notify->u.syscallno, or executing a break point at the address specified by notify->u.bptaddr. The controlling process is responsible for tracing the system call or place the actual break-point.

When an event has occurred, additional information can be obtained by this interface, defined in rtld_db.h.

typedef enum {
        RD_NOSTATE = 0,
        RD_CONSISTENT,
        RD_ADD,
        RD_DELETE
} rd_state_e;
 
typedef struct rd_event_msg {
        rd_event_e      type;
        union {
                rd_state_e      state;
        } u;
} rd_event_msg_t;

The rd_state_e values are:

RD_NOSTATE

There is no additional state information available.

RD_CONSISTANT

The link-maps are in a stable state and can be examined.

RD_ADD

A dynamic object is in the process of being loaded and the link-maps are not in a stable state. They should not be examined until the RD_CONSISTANT state is reached.

RD_DELETE

A dynamic object is in the process of being deleted and the link-maps are not in a stable state. They should not be examined until the RD_CONSISTANT state is reached.

The rd_event_getmsg() function is used to obtain this event state information.

rd_event_getmsg()

This function provides additional information concerning an event.

rd_err_e rd_event_getmsg(struct rd_agent * rdap, rd_event_msg_t * msg);

The following table shows the possible state for each of the different event types.