A reliable way for userspace to check if a msg is from kernel

- IPC_FLG_MSG_FROM_KERNEL status flag is returned to userspace if the
  receive was satisfied by s message which was sent by the kernel on
  behalf of a process. This perfectly reliale information.

- MF_SENDING_FROM_KERNEL flag added to processes to be able to set
  IPC_FLG_MSG_FROM_KERNEL when finishing receive if the receiver
  wasn't ready to receive immediately.

- PM is changed to use this information to confirm that the scheduling
  messages are indeed from the kernel and not faked by a process.

  PM uses sef_receive_status()

- get_work() is removed from PM to make the changes simpler

include/minix/ipcconst.h

@@ -17,4 +17,12 @@
 #define IPC_STATUS_CALL_TO(call) \
 	(((call) & IPC_STATUS_CALL_MASK) << IPC_STATUS_CALL_SHIFT)
 
+#define IPC_FLG_MSG_FROM_KERNEL	1 /* this message originated in the kernel on
+				     behalf of a process, this is a trusted
+				     message, never reply to the sender
+				 */
+#define IPC_STATUS_FLAGS_SHIFT	16
+#define IPC_STATUS_FLAGS(flgs)	((flgs) << IPC_STATUS_FLAGS_SHIFT)
+#define IPC_STATUS_FLAGS_TEST(status, flgs)	\
+		(((status) >> IPC_STATUS_FLAGS_SHIFT) & (flgs))
 #endif /* IPC_CONST_H */

kernel/proc.c

@@ -540,8 +540,12 @@ const int flags;
 	if (!(flags & FROM_KERNEL)) {
 		if(copy_msg_from_user(caller_ptr, m_ptr, &dst_ptr->p_delivermsg))
 			return EFAULT;
-	} else
+	} else {
 		dst_ptr->p_delivermsg = *m_ptr;
+		IPC_STATUS_ADD(dst_ptr,
+				IPC_STATUS_FLAGS(IPC_FLG_MSG_FROM_KERNEL));
+	}
+
 	dst_ptr->p_delivermsg.m_source = caller_ptr->p_endpoint;
 	dst_ptr->p_misc_flags |= MF_DELIVERMSG;
 
@@ -563,8 +567,15 @@ const int flags;
 	if (!(flags & FROM_KERNEL)) {
 		if(copy_msg_from_user(caller_ptr, m_ptr, &caller_ptr->p_sendmsg))
 			return EFAULT;
-	} else
+	} else {
 		caller_ptr->p_sendmsg = *m_ptr;
+		/*
+		 * we need to remember that this message is from kernel so we
+		 * can set the delivery status flags when the message is
+		 * actually delivered
+		 */
+		caller_ptr->p_misc_flags |= MF_SENDING_FROM_KERNEL;
+	}
 
 	RTS_SET(caller_ptr, RTS_SENDING);
 	caller_ptr->p_sendto_e = dst_e;
@@ -693,6 +704,17 @@ const int flags;
 
 	    call = ((*xpp)->p_misc_flags & MF_REPLY_PEND ? SENDREC : SEND);
 	    IPC_STATUS_ADD(caller_ptr, IPC_STATUS_CALL_TO(call));
+
+	    /*
+	     * if the message is originaly from the kernel on behalf of this
+	     * process, we must send the status flags accordingly
+	     */
+	    if ((*xpp)->p_misc_flags & MF_SENDING_FROM_KERNEL) {
+		IPC_STATUS_ADD(caller_ptr,
+				IPC_STATUS_FLAGS(IPC_FLG_MSG_FROM_KERNEL));
+		/* we can clean the flag now, not need anymore */
+		(*xpp)->p_misc_flags &= ~MF_SENDING_FROM_KERNEL;
+	    }
 	    if ((*xpp)->p_misc_flags & MF_SIG_DELAY)
 		sig_delay_done(*xpp);
 

kernel/proc.h

@@ -221,6 +221,7 @@ struct proc {
 #define MF_USED_FPU	0x800   /* process used fpu during last execution run */
 #define MF_FPU_INITIALIZED	0x1000  /* process already used math, so fpu
 					 * regs are significant (initialized)*/
+#define MF_SENDING_FROM_KERNEL	0x2000 /* message of this process is from kernel */
 
 /* Scheduling priorities for p_priority. Values must start at zero (highest
  * priority) and increment.  Priorities of the processes in the boot image 

servers/pm/main.c

@@ -40,7 +40,6 @@ EXTERN unsigned long calls_stats[NCALLS];
 #endif
 
 FORWARD _PROTOTYPE( void sendreply, (void)				);
-FORWARD _PROTOTYPE( void get_work, (void)				);
 FORWARD _PROTOTYPE( int get_nice_value, (int queue)			);
 FORWARD _PROTOTYPE( void handle_fs_reply, (void)			);
 
@@ -70,14 +69,32 @@ PUBLIC int main()
 
   /* This is PM's main loop-  get work and do it, forever and forever. */
   while (TRUE) {
-	get_work();		/* wait for an PM system call */
+	  int ipc_status;
+
+	  /* Wait for the next message and extract useful information from it. */
+	  if (sef_receive_status(ANY, &m_in, &ipc_status) != OK)
+		  panic("PM sef_receive error");
+	  who_e = m_in.m_source;	/* who sent the message */
+	  if(pm_isokendpt(who_e, &who_p) != OK)
+		  panic("PM got message from invalid endpoint: %d", who_e);
+	  call_nr = m_in.m_type;	/* system call number */
+
+	  /* Process slot of caller. Misuse PM's own process slot if the kernel is
+	   * calling. This can happen in case of synchronous alarms (CLOCK) or or
+	   * event like pending kernel signals (SYSTEM).
+	   */
+	  mp = &mproc[who_p < 0 ? PM_PROC_NR : who_p];
+	  if(who_p >= 0 && mp->mp_endpoint != who_e) {
+		  panic("PM endpoint number out of sync with source: %d",
+				  			mp->mp_endpoint);
+	  }
 
 	/* Drop delayed calls from exiting processes. */
 	if (mp->mp_flags & EXITING)
 		continue;
 
 	/* Check for system notifications first. Special cases. */
-	if (is_notify(call_nr)) {
+	if (is_ipc_notify(ipc_status)) {
 		switch(who_p) {
 			case CLOCK:
 				pm_expire_timers(m_in.NOTIFY_TIMESTAMP);
@@ -116,7 +133,13 @@ PUBLIC int main()
 		break;
 	case SCHEDULING_NO_QUANTUM:
 		/* This message was sent from the kernel, don't reply */
-		do_noquantum();
+		if (IPC_STATUS_FLAGS_TEST(ipc_status, IPC_FLG_MSG_FROM_KERNEL)) {
+			do_noquantum();
+		} else {
+			printf("PM: process %s/%d faked SCHEDULING_NO_QUANTUM "
+					"message!\n",
+					mp->mp_name, mp->mp_endpoint);
+		}
 		continue;
 	default:
 		/* Else, if the system call number is valid, perform the
@@ -314,29 +337,6 @@ PRIVATE int sef_cb_signal_manager(endpoint_t target, int signo)
   return r;
 }
 
-/*===========================================================================*
- *				get_work				     *
- *===========================================================================*/
-PRIVATE void get_work()
-{
-/* Wait for the next message and extract useful information from it. */
-  if (sef_receive(ANY, &m_in) != OK)
-	panic("PM sef_receive error");
-  who_e = m_in.m_source;	/* who sent the message */
-  if(pm_isokendpt(who_e, &who_p) != OK)
-	panic("PM got message from invalid endpoint: %d", who_e);
-  call_nr = m_in.m_type;	/* system call number */
-
-  /* Process slot of caller. Misuse PM's own process slot if the kernel is
-   * calling. This can happen in case of synchronous alarms (CLOCK) or or 
-   * event like pending kernel signals (SYSTEM).
-   */
-  mp = &mproc[who_p < 0 ? PM_PROC_NR : who_p];
-  if(who_p >= 0 && mp->mp_endpoint != who_e) {
-	panic("PM endpoint number out of sync with source: %d", mp->mp_endpoint);
-  }
-}
-
 /*===========================================================================*
  *				setreply				     *
  *===========================================================================*/