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From: adelman@LBL-CSA2.ARPA (Kenneth Adelman)
Newsgroups: comp.os.vms
Subject: Re: User Supplied System Services
Message-ID: <871007055406.0gf@Lbl-Csa2.Arpa>
Date: Wed, 7-Oct-87 08:54:06 EDT
Article-I.D.: Lbl-Csa2.871007055406.0gf
Posted: Wed Oct  7 08:54:06 1987
Date-Received: Sat, 10-Oct-87 09:41:06 EDT
Sender: daemon@ucbvax.BERKELEY.EDU
Reply-To: Adelman@Lbl-Csa2.Arpa (Kenneth Adelman)
Organization: The ARPA Internet
Lines: 41

> The idea was that once in kernel mode the CMKRNL priv would be given
> (temporarily) since kernel code has SETPRV by default and then the QIO
> would be issued.  However, a crash would result apparantly in ASTDEL
> after the QIO.

> The question is...Is it possible to issue a QIO after having been
> dispatched through a CHMK inst to the user routine (using the negative
> code value).  I'm sure that the dispatching was occurring correctly.
> I seem to vaguely remember that it is not possible to call QIO from within
> kernel mode.  Anybody remember for sure? and if not what is the problem?

    There are all sorts of bad implications for having a kernel mode
system service which in turn calls SYS$QIO with the users arguments.
For one, SYS$QIO will be probing its arguments against access in
KERNEL mode, not against access in USER mode, because SYS$QIO goes
through the CHMK dispatcher a second time.

    The easier way to do it is to call the EXE$QIO entry point that
the VMS CHMK dispatcher calls in kernel mode after YOU call SYS$QIO.

    Below is an example of how to write a user written system service
which is a deprivileged $GETJPI call.  This is expected to be called
via the dispatcher in SYS$EXAMPLES. The argument count must be
verified and the argument list probed.

        .ENTRY  CIT_GETJPI,^M<R4,R11>
;
;       This is the deprivileged $GETJPI.
;
        MOVL    W^PCB$Q_PRIV(R4),R11                    ;Save our privileges.
        BBSS    S^#PRV$V_WORLD,W^PCB$Q_PRIV(R4),10$     ; TURN ON WORLD
10$:    CALLG   (AP),@#EXE$GETJPI       ; CALL INTO $GETJPI (NOTE R4=CURPCB)
        MOVL    R11,W^PCB$Q_PRIV(R4)    ; RESTORE OLD PRIVILEGES
        RET

    Your code will probably look something like this, but will need
additional checks that the caller is doing the particular $QIO to the
particular device that you intend him to.

                                                    Kenneth Adelman
                                                    LBL