[apple/xnu.git] / osfmk / kern / ipc_mig.c

/*
 * Copyright (c) 2000-2004 Apple Computer, Inc. All rights reserved.
 *
 * @APPLE_OSREFERENCE_LICENSE_HEADER_START@
 * 
 * This file contains Original Code and/or Modifications of Original Code
 * as defined in and that are subject to the Apple Public Source License
 * Version 2.0 (the 'License'). You may not use this file except in
 * compliance with the License. The rights granted to you under the License
 * may not be used to create, or enable the creation or redistribution of,
 * unlawful or unlicensed copies of an Apple operating system, or to
 * circumvent, violate, or enable the circumvention or violation of, any
 * terms of an Apple operating system software license agreement.
 * 
 * Please obtain a copy of the License at
 * http://www.opensource.apple.com/apsl/ and read it before using this file.
 * 
 * The Original Code and all software distributed under the License are
 * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
 * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
 * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
 * Please see the License for the specific language governing rights and
 * limitations under the License.
 * 
 * @APPLE_OSREFERENCE_LICENSE_HEADER_END@
 */
/*
 * @OSF_COPYRIGHT@
 */
/* 
 * Mach Operating System
 * Copyright (c) 1991,1990 Carnegie Mellon University
 * All Rights Reserved.
 * 
 * Permission to use, copy, modify and distribute this software and its
 * documentation is hereby granted, provided that both the copyright
 * notice and this permission notice appear in all copies of the
 * software, derivative works or modified versions, and any portions
 * thereof, and that both notices appear in supporting documentation.
 * 
 * CARNEGIE MELLON ALLOWS FREE USE OF THIS SOFTWARE IN ITS "AS IS"
 * CONDITION.  CARNEGIE MELLON DISCLAIMS ANY LIABILITY OF ANY KIND FOR
 * ANY DAMAGES WHATSOEVER RESULTING FROM THE USE OF THIS SOFTWARE.
 * 
 * Carnegie Mellon requests users of this software to return to
 * 
 *  Software Distribution Coordinator  or  Software.Distribution@CS.CMU.EDU
 *  School of Computer Science
 *  Carnegie Mellon University
 *  Pittsburgh PA 15213-3890
 * 
 * any improvements or extensions that they make and grant Carnegie Mellon
 * the rights to redistribute these changes.
 */
/*
 */

#include <mach/boolean.h>
#include <mach/port.h>
#include <mach/mig.h>
#include <mach/mig_errors.h>
#include <mach/mach_types.h>
#include <mach/mach_traps.h>

#include <kern/ipc_tt.h>
#include <kern/ipc_mig.h>
#include <kern/kalloc.h>
#include <kern/task.h>
#include <kern/thread.h>
#include <kern/ipc_kobject.h>
#include <kern/misc_protos.h>

#include <ipc/port.h>
#include <ipc/ipc_kmsg.h>
#include <ipc/ipc_entry.h>
#include <ipc/ipc_object.h>
#include <ipc/ipc_mqueue.h>
#include <ipc/ipc_space.h>
#include <ipc/ipc_port.h>
#include <ipc/ipc_pset.h>
#include <ipc/ipc_notify.h>
#include <vm/vm_map.h>

#include <libkern/OSAtomic.h>

/*
 *      Routine:        mach_msg_send_from_kernel
 *      Purpose:
 *              Send a message from the kernel.
 *
 *              This is used by the client side of KernelUser interfaces
 *              to implement SimpleRoutines.  Currently, this includes
 *              memory_object messages.
 *      Conditions:
 *              Nothing locked.
 *      Returns:
 *              MACH_MSG_SUCCESS        Sent the message.
 *              MACH_SEND_INVALID_DEST  Bad destination port.
 *              MACH_MSG_SEND_NO_BUFFER Destination port had inuse fixed bufer
 *                                      or destination is above kernel limit
 */

#if IKM_SUPPORT_LEGACY

#undef mach_msg_send_from_kernel
mach_msg_return_t mach_msg_send_from_kernel(
        mach_msg_header_t       *msg,
        mach_msg_size_t         send_size);

mach_msg_return_t
mach_msg_send_from_kernel(
        mach_msg_header_t       *msg,
        mach_msg_size_t         send_size)
{
        ipc_kmsg_t kmsg;
        mach_msg_return_t mr;

        mr = ipc_kmsg_get_from_kernel(msg, send_size, &kmsg);
        if (mr != MACH_MSG_SUCCESS)
                return mr;

        mr = ipc_kmsg_copyin_from_kernel_legacy(kmsg);
        if (mr != MACH_MSG_SUCCESS) {
                ipc_kmsg_free(kmsg);
                return mr;
        }               

        mr = ipc_kmsg_send_always(kmsg);
        if (mr != MACH_MSG_SUCCESS) {
                ipc_kmsg_destroy(kmsg);
        }

        return mr;
}

#endif /* IKM_SUPPORT_LEGACY */

mach_msg_return_t
mach_msg_send_from_kernel_proper(
        mach_msg_header_t       *msg,
        mach_msg_size_t         send_size)
{
        ipc_kmsg_t kmsg;
        mach_msg_return_t mr;

        mr = ipc_kmsg_get_from_kernel(msg, send_size, &kmsg);
        if (mr != MACH_MSG_SUCCESS)
                return mr;

        mr = ipc_kmsg_copyin_from_kernel(kmsg);
        if (mr != MACH_MSG_SUCCESS) {
                ipc_kmsg_free(kmsg);
                return mr;
        }

        mr = ipc_kmsg_send_always(kmsg);
        if (mr != MACH_MSG_SUCCESS) {
                ipc_kmsg_destroy(kmsg);
        }

        return mr;
}

#if IKM_SUPPORT_LEGACY

mach_msg_return_t
mach_msg_send_from_kernel_with_options(
        mach_msg_header_t       *msg,
        mach_msg_size_t         send_size,
        mach_msg_option_t       option,
        mach_msg_timeout_t      timeout_val)
{
        ipc_kmsg_t kmsg;
        mach_msg_return_t mr;

        mr = ipc_kmsg_get_from_kernel(msg, send_size, &kmsg);
        if (mr != MACH_MSG_SUCCESS)
                return mr;

        mr = ipc_kmsg_copyin_from_kernel_legacy(kmsg);
        if (mr != MACH_MSG_SUCCESS) {
                ipc_kmsg_free(kmsg);
                return mr;
        }
                
        mr = ipc_kmsg_send(kmsg, option, timeout_val);
        if (mr != MACH_MSG_SUCCESS) {
                ipc_kmsg_destroy(kmsg);
        }
        
        return mr;
}

#endif /* IKM_SUPPORT_LEGACY */

/*
 *      Routine:        mach_msg_rpc_from_kernel
 *      Purpose:
 *              Send a message from the kernel and receive a reply.
 *              Uses ith_rpc_reply for the reply port.
 *
 *              This is used by the client side of KernelUser interfaces
 *              to implement Routines.
 *      Conditions:
 *              Nothing locked.
 *      Returns:
 *              MACH_MSG_SUCCESS        Sent the message.
 *              MACH_RCV_PORT_DIED      The reply port was deallocated.
 */

mach_msg_return_t mach_msg_rpc_from_kernel_body(mach_msg_header_t *msg, 
        mach_msg_size_t send_size, mach_msg_size_t rcv_size, boolean_t legacy);

#if IKM_SUPPORT_LEGACY

#undef mach_msg_rpc_from_kernel
mach_msg_return_t
mach_msg_rpc_from_kernel(
        mach_msg_header_t       *msg,
        mach_msg_size_t         send_size,
        mach_msg_size_t         rcv_size);

mach_msg_return_t
mach_msg_rpc_from_kernel(
        mach_msg_header_t       *msg,
        mach_msg_size_t         send_size,
        mach_msg_size_t         rcv_size)
{
    return mach_msg_rpc_from_kernel_body(msg, send_size, rcv_size, TRUE);
}

#endif /* IKM_SUPPORT_LEGACY */

mach_msg_return_t
mach_msg_rpc_from_kernel_proper(
        mach_msg_header_t       *msg,
        mach_msg_size_t         send_size,
        mach_msg_size_t         rcv_size)
{
    return mach_msg_rpc_from_kernel_body(msg, send_size, rcv_size, FALSE);
}

mach_msg_return_t
mach_msg_rpc_from_kernel_body(
        mach_msg_header_t       *msg,
        mach_msg_size_t         send_size,
        mach_msg_size_t         rcv_size,
#if !IKM_SUPPORT_LEGACY
        __unused
#endif
    boolean_t           legacy)
{
        thread_t self = current_thread();
        ipc_port_t reply;
        ipc_kmsg_t kmsg;
        mach_port_seqno_t seqno;
        mach_msg_return_t mr;

        assert(msg->msgh_local_port == MACH_PORT_NULL);

        mr = ipc_kmsg_get_from_kernel(msg, send_size, &kmsg);
        if (mr != MACH_MSG_SUCCESS)
                return mr;

        reply = self->ith_rpc_reply;
        if (reply == IP_NULL) {
                reply = ipc_port_alloc_reply();
                if ((reply == IP_NULL) ||
                    (self->ith_rpc_reply != IP_NULL))
                        panic("mach_msg_rpc_from_kernel");
                self->ith_rpc_reply = reply;
        }

        /* insert send-once right for the reply port */
        kmsg->ikm_header->msgh_local_port = reply;
        kmsg->ikm_header->msgh_bits |=
                MACH_MSGH_BITS(0, MACH_MSG_TYPE_MAKE_SEND_ONCE);

        ip_reference(reply);

#if IKM_SUPPORT_LEGACY
    if(legacy)
        mr = ipc_kmsg_copyin_from_kernel_legacy(kmsg);
    else
        mr = ipc_kmsg_copyin_from_kernel(kmsg);
#else
    mr = ipc_kmsg_copyin_from_kernel(kmsg);
#endif
    if (mr != MACH_MSG_SUCCESS) {
            ipc_kmsg_free(kmsg);
            return mr;
    }
        mr = ipc_kmsg_send_always(kmsg);
        if (mr != MACH_MSG_SUCCESS) {
                ipc_kmsg_destroy(kmsg);
                return mr;
        }

        for (;;) {
                ipc_mqueue_t mqueue;

                ip_lock(reply);
                if ( !ip_active(reply)) {
                        ip_unlock(reply);
                        ip_release(reply);
                        return MACH_RCV_PORT_DIED;
                }
                if (!self->active) {
                        ip_unlock(reply);
                        ip_release(reply);
                        return MACH_RCV_INTERRUPTED;
                }

                assert(reply->ip_pset_count == 0);
                mqueue = &reply->ip_messages;
                ip_unlock(reply);

                self->ith_continuation = (void (*)(mach_msg_return_t))0;

                ipc_mqueue_receive(mqueue,
                                   MACH_MSG_OPTION_NONE,
                                   MACH_MSG_SIZE_MAX,
                                   MACH_MSG_TIMEOUT_NONE,
                                   THREAD_INTERRUPTIBLE);

                mr = self->ith_state;
                kmsg = self->ith_kmsg;
                seqno = self->ith_seqno;

                if (mr == MACH_MSG_SUCCESS)
                  {
                        break;
                  }

                assert(mr == MACH_RCV_INTERRUPTED);

                if (self->handlers) {
                        ip_release(reply);
                        return(mr);
                }
        }
        ip_release(reply);

        /* 
         * Check to see how much of the message/trailer can be received.
         * We chose the maximum trailer that will fit, since we don't
         * have options telling us which trailer elements the caller needed.
         */
        if (rcv_size >= kmsg->ikm_header->msgh_size) {
                mach_msg_format_0_trailer_t *trailer =  (mach_msg_format_0_trailer_t *)
                        ((vm_offset_t)kmsg->ikm_header + kmsg->ikm_header->msgh_size);

                if (rcv_size >= kmsg->ikm_header->msgh_size + MAX_TRAILER_SIZE) {
                        /* Enough room for a maximum trailer */
                        trailer->msgh_trailer_size = MAX_TRAILER_SIZE;
                } 
                else if (rcv_size < kmsg->ikm_header->msgh_size + 
                           trailer->msgh_trailer_size) {
                        /* no room for even the basic (default) trailer */
                        trailer->msgh_trailer_size = 0;
                }
                assert(trailer->msgh_trailer_type == MACH_MSG_TRAILER_FORMAT_0);
                rcv_size = kmsg->ikm_header->msgh_size + trailer->msgh_trailer_size;
                mr = MACH_MSG_SUCCESS;
        } else {
                mr = MACH_RCV_TOO_LARGE;
        }


        /*
         *      We want to preserve rights and memory in reply!
         *      We don't have to put them anywhere; just leave them
         *      as they are.
         */
#if IKM_SUPPORT_LEGACY
    if(legacy)
        ipc_kmsg_copyout_to_kernel_legacy(kmsg, ipc_space_reply);
    else
        ipc_kmsg_copyout_to_kernel(kmsg, ipc_space_reply);
#else
    ipc_kmsg_copyout_to_kernel(kmsg, ipc_space_reply);
#endif
        ipc_kmsg_put_to_kernel(msg, kmsg, rcv_size);
        return mr;
}


/************** These Calls are set up for kernel-loaded tasks/threads **************/

/*
 *      Routine:        mach_msg_overwrite
 *      Purpose:
 *              Like mach_msg_overwrite_trap except that message buffers
 *              live in kernel space.  Doesn't handle any options.
 *
 *              This is used by in-kernel server threads to make
 *              kernel calls, to receive request messages, and
 *              to send reply messages.
 *      Conditions:
 *              Nothing locked.
 *      Returns:
 */

mach_msg_return_t
mach_msg_overwrite(
        mach_msg_header_t               *msg,
        mach_msg_option_t               option,
        mach_msg_size_t         send_size,
        mach_msg_size_t         rcv_size,
        mach_port_name_t                rcv_name,
        __unused mach_msg_timeout_t     msg_timeout,
        __unused mach_port_name_t       notify,
        __unused mach_msg_header_t      *rcv_msg,
       __unused mach_msg_size_t rcv_msg_size)
{
        ipc_space_t space = current_space();
        vm_map_t map = current_map();
        ipc_kmsg_t kmsg;
        mach_port_seqno_t seqno;
        mach_msg_return_t mr;
        mach_msg_trailer_size_t trailer_size;

        if (option & MACH_SEND_MSG) {
                mach_msg_size_t msg_and_trailer_size;
                mach_msg_max_trailer_t  *max_trailer;

                if ((send_size < sizeof(mach_msg_header_t)) || (send_size & 3))
                        return MACH_SEND_MSG_TOO_SMALL;

                if (send_size > MACH_MSG_SIZE_MAX - MAX_TRAILER_SIZE)
                        return MACH_SEND_TOO_LARGE;

                msg_and_trailer_size = send_size + MAX_TRAILER_SIZE;
                kmsg = ipc_kmsg_alloc(msg_and_trailer_size);

                if (kmsg == IKM_NULL)
                        return MACH_SEND_NO_BUFFER;

                (void) memcpy((void *) kmsg->ikm_header, (const void *) msg, send_size);

                kmsg->ikm_header->msgh_size = send_size;

                /* 
                 * Reserve for the trailer the largest space (MAX_TRAILER_SIZE)
                 * However, the internal size field of the trailer (msgh_trailer_size)
                 * is initialized to the minimum (sizeof(mach_msg_trailer_t)), to optimize
                 * the cases where no implicit data is requested.
                 */
                max_trailer = (mach_msg_max_trailer_t *) ((vm_offset_t)kmsg->ikm_header + send_size);
                max_trailer->msgh_sender = current_thread()->task->sec_token;
                max_trailer->msgh_audit = current_thread()->task->audit_token;
                max_trailer->msgh_trailer_type = MACH_MSG_TRAILER_FORMAT_0;
                max_trailer->msgh_trailer_size = MACH_MSG_TRAILER_MINIMUM_SIZE;
        
                mr = ipc_kmsg_copyin(kmsg, space, map, FALSE);
                if (mr != MACH_MSG_SUCCESS) {
                        ipc_kmsg_free(kmsg);
                        return mr;
                }

                do
                        mr = ipc_kmsg_send(kmsg, MACH_MSG_OPTION_NONE,
                                             MACH_MSG_TIMEOUT_NONE);
                while (mr == MACH_SEND_INTERRUPTED);
                assert(mr == MACH_MSG_SUCCESS);
        }

        if (option & MACH_RCV_MSG) {
                thread_t self = current_thread();

                do {
                        ipc_object_t object;
                        ipc_mqueue_t mqueue;

                        mr = ipc_mqueue_copyin(space, rcv_name,
                                               &mqueue, &object);
                        if (mr != MACH_MSG_SUCCESS)
                                return mr;
                        /* hold ref for object */

                        self->ith_continuation = (void (*)(mach_msg_return_t))0;
                        ipc_mqueue_receive(mqueue,
                                           MACH_MSG_OPTION_NONE,
                                           MACH_MSG_SIZE_MAX,
                                           MACH_MSG_TIMEOUT_NONE,
                                           THREAD_ABORTSAFE);
                        mr = self->ith_state;
                        kmsg = self->ith_kmsg;
                        seqno = self->ith_seqno;

                        io_release(object);

                } while (mr == MACH_RCV_INTERRUPTED);
                if (mr != MACH_MSG_SUCCESS)
                        return mr;


                trailer_size = ipc_kmsg_add_trailer(kmsg, space, option, current_thread(), seqno, TRUE,
                                kmsg->ikm_header->msgh_remote_port->ip_context);

                if (rcv_size < (kmsg->ikm_header->msgh_size + trailer_size)) {
                        ipc_kmsg_copyout_dest(kmsg, space);
                        (void) memcpy((void *) msg, (const void *) kmsg->ikm_header, sizeof *msg);
                        ipc_kmsg_free(kmsg);
                        return MACH_RCV_TOO_LARGE;
                }

                mr = ipc_kmsg_copyout(kmsg, space, map, MACH_MSG_BODY_NULL);
                if (mr != MACH_MSG_SUCCESS) {
                        if ((mr &~ MACH_MSG_MASK) == MACH_RCV_BODY_ERROR) {
                                ipc_kmsg_put_to_kernel(msg, kmsg,
                                                kmsg->ikm_header->msgh_size + trailer_size);
                        } else {
                                ipc_kmsg_copyout_dest(kmsg, space);
                                (void) memcpy((void *) msg, (const void *) kmsg->ikm_header, sizeof *msg);
                                ipc_kmsg_free(kmsg);
                        }

                        return mr;
                }

                (void) memcpy((void *) msg, (const void *) kmsg->ikm_header,
                              kmsg->ikm_header->msgh_size + trailer_size);
                ipc_kmsg_free(kmsg);
        }

        return MACH_MSG_SUCCESS;
}

/*
 *      Routine:        mig_get_reply_port
 *      Purpose:
 *              Called by client side interfaces living in the kernel
 *              to get a reply port.
 */
mach_port_t
mig_get_reply_port(void)
{
        return (MACH_PORT_NULL);
}

/*
 *      Routine:        mig_dealloc_reply_port
 *      Purpose:
 *              Called by client side interfaces to get rid of a reply port.
 */

void
mig_dealloc_reply_port(
        __unused mach_port_t reply_port)
{
}

/*
 *      Routine:        mig_put_reply_port
 *      Purpose:
 *              Called by client side interfaces after each RPC to 
 *              let the client recycle the reply port if it wishes.
 */
void
mig_put_reply_port(
        __unused mach_port_t reply_port)
{
}

/*
 * mig_strncpy.c - by Joshua Block
 *
 * mig_strncp -- Bounded string copy.  Does what the library routine strncpy
 * OUGHT to do:  Copies the (null terminated) string in src into dest, a 
 * buffer of length len.  Assures that the copy is still null terminated
 * and doesn't overflow the buffer, truncating the copy if necessary.
 *
 * Parameters:
 * 
 *     dest - Pointer to destination buffer.
 * 
 *     src - Pointer to source string.
 * 
 *     len - Length of destination buffer.
 */
int 
mig_strncpy(
        char            *dest,
        const char      *src,
        int             len)
{
    int i = 0;

    if (len > 0)
        if (dest != NULL) {
            if (src != NULL)
                   for (i=1; i<len; i++)
                        if (! (*dest++ = *src++))
                            return i;
                *dest = '\0';
        }
    return i;
}

char *
mig_user_allocate(
        vm_size_t       size)
{
        return (char *)kalloc(size);
}

void
mig_user_deallocate(
        char            *data,
        vm_size_t       size)
{
        kfree(data, size);
}

/*
 *      Routine:        mig_object_init
 *      Purpose:
 *              Initialize the base class portion of a MIG object.  We
 *              will lazy init the port, so just clear it for now.
 */
kern_return_t
mig_object_init(
        mig_object_t            mig_object,
        const IMIGObject        *interface)
{
        if (mig_object == MIG_OBJECT_NULL)
                return KERN_INVALID_ARGUMENT;
        mig_object->pVtbl = (const IMIGObjectVtbl *)interface;
        mig_object->port = MACH_PORT_NULL;
        return KERN_SUCCESS;
}

/*
 *      Routine:        mig_object_destroy
 *      Purpose:
 *              The object is being freed.  This call lets us clean
 *              up any state we have have built up over the object's
 *              lifetime.
 *      Conditions:
 *              Since notifications and the port hold references on
 *              on the object, neither can exist when this is called.
 *              This is a good place to assert() that condition.
 */
void
mig_object_destroy(
        __assert_only mig_object_t      mig_object)
{
        assert(mig_object->port == MACH_PORT_NULL);
        return;
}

/*
 *      Routine:        mig_object_reference
 *      Purpose:
 *              Pure virtual helper to invoke the MIG object's AddRef
 *              method.
 *      Conditions:
 *              MIG object port may be locked.
 */
void
mig_object_reference(
        mig_object_t    mig_object)
{
        assert(mig_object != MIG_OBJECT_NULL);
        mig_object->pVtbl->AddRef((IMIGObject *)mig_object);
}

/*
 *      Routine:        mig_object_deallocate
 *      Purpose:
 *              Pure virtual helper to invoke the MIG object's Release
 *              method.
 *      Conditions:
 *              Nothing locked.
 */
void
mig_object_deallocate(
        mig_object_t    mig_object)
{
        assert(mig_object != MIG_OBJECT_NULL);
        mig_object->pVtbl->Release((IMIGObject *)mig_object);
}

/*
 *      Routine:        convert_mig_object_to_port [interface]
 *      Purpose:
 *              Base implementation of MIG outtrans routine to convert from
 *              a mig object reference to a new send right on the object's
 *              port.  The object reference is consumed.
 *      Returns:
 *              IP_NULL - Null MIG object supplied
 *              Otherwise, a newly made send right for the port
 *      Conditions:
 *              Nothing locked.
 */
ipc_port_t
convert_mig_object_to_port(
        mig_object_t    mig_object)
{
        ipc_port_t      port;
        boolean_t       deallocate = TRUE;

        if (mig_object == MIG_OBJECT_NULL)
                return IP_NULL;

        port = mig_object->port;
        while ((port == IP_NULL) ||
               ((port = ipc_port_make_send(port)) == IP_NULL)) {
                ipc_port_t      previous;

                /*
                 * Either the port was never set up, or it was just
                 * deallocated out from under us by the no-senders
                 * processing.  In either case, we must:
                 *      Attempt to make one
                 *      Arrange for no senders
                 *      Try to atomically register it with the object
                 *              Destroy it if we are raced.
                 */
                port = ipc_port_alloc_kernel();
                ip_lock(port);
                ipc_kobject_set_atomically(port,
                                           (ipc_kobject_t) mig_object,
                                           IKOT_MIG);

                /* make a sonce right for the notification */
                port->ip_sorights++;
                ip_reference(port);

                ipc_port_nsrequest(port, 1, port, &previous);
                /* port unlocked */

                assert(previous == IP_NULL);

                if (OSCompareAndSwapPtr((void *)IP_NULL, (void *)port,
                                                                                        (void * volatile *)&mig_object->port)) {
                        deallocate = FALSE;
                } else {
                        ipc_port_dealloc_kernel(port);
                        port = mig_object->port;
                }
        }

        if (deallocate)
                mig_object->pVtbl->Release((IMIGObject *)mig_object);

        return (port);
}


/*
 *      Routine:        convert_port_to_mig_object [interface]
 *      Purpose:
 *              Base implementation of MIG intrans routine to convert from
 *              an incoming port reference to a new reference on the
 *              underlying object. A new reference must be created, because
 *              the port's reference could go away asynchronously.
 *      Returns:
 *              NULL - Not an active MIG object port or iid not supported
 *              Otherwise, a reference to the underlying MIG interface
 *      Conditions:
 *              Nothing locked.
 */
mig_object_t
convert_port_to_mig_object(
        ipc_port_t      port,
        const MIGIID    *iid)
{
        mig_object_t    mig_object;
        void            *ppv;

        if (!IP_VALID(port))
                return NULL;

        ip_lock(port);
        if (!ip_active(port) || (ip_kotype(port) != IKOT_MIG)) {
                ip_unlock(port);
                return NULL;
        }

        /*
         * Our port points to some MIG object interface.  Now
         * query it to get a reference to the desired interface.
         */
        ppv = NULL;
        mig_object = (mig_object_t)port->ip_kobject;
        mig_object->pVtbl->QueryInterface((IMIGObject *)mig_object, iid, &ppv);
        ip_unlock(port);
        return (mig_object_t)ppv;
}

/*
 *      Routine:        mig_object_no_senders [interface]
 *      Purpose:
 *              Base implementation of a no-senders notification handler
 *              for MIG objects. If there truly are no more senders, must
 *              destroy the port and drop its reference on the object.
 *      Returns:
 *              TRUE  - port deallocate and reference dropped
 *              FALSE - more senders arrived, re-registered for notification
 *      Conditions:
 *              Nothing locked.
 */

boolean_t
mig_object_no_senders(
        ipc_port_t              port,
        mach_port_mscount_t     mscount)
{
        mig_object_t            mig_object;

        ip_lock(port);
        if (port->ip_mscount > mscount) {
                ipc_port_t      previous;

                /*
                 * Somebody created new send rights while the
                 * notification was in-flight.  Just create a
                 * new send-once right and re-register with 
                 * the new (higher) mscount threshold.
                 */
                /* make a sonce right for the notification */
                port->ip_sorights++;
                ip_reference(port);
                ipc_port_nsrequest(port, mscount, port, &previous);
                /* port unlocked */

                assert(previous == IP_NULL);
                return (FALSE);
        }

        /*
         * Clear the port pointer while we have it locked.
         */
        mig_object = (mig_object_t)port->ip_kobject;
        mig_object->port = IP_NULL;

        /*
         * Bring the sequence number and mscount in
         * line with ipc_port_destroy assertion.
         */
        port->ip_mscount = 0;
        port->ip_messages.imq_seqno = 0;
        ipc_port_destroy(port); /* releases lock */
        
        /*
         * Release the port's reference on the object.
         */
        mig_object->pVtbl->Release((IMIGObject *)mig_object);
        return (TRUE);
}       

/*
 * Kernel implementation of the notification chain for MIG object
 * is kept separate from the actual objects, since there are expected
 * to be much fewer of them than actual objects.
 *
 * The implementation of this part of MIG objects is coming
 * "Real Soon Now"(TM).
 */