FILEMON_SET_FD: Disallow changing the fd.

MFC after:	1 week
Suggested by:	mjg
Sponsored by:	EMC / Isilon Storage Division

share/man/man4/filemon.4

@@ -31,7 +31,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd January 28, 2016
+.Dd March 9, 2016
 .Dt FILEMON 4
 .Os
 .Sh NAME
@@ -125,6 +125,19 @@ function returns the value 0 if successful;
 otherwise the value \-1 is returned and the global variable
 .Va errno
 is set to indicate the error.
+.Sh ERRORS
+The
+.Fn ioctl
+system call
+with
+.Dv FILEMON_SET_FD
+will fail if:
+.Bl -tag -width Er
+.It Bq Er EEXIST
+The
+.Nm
+handle is already associated with a file descriptor.
+.El
 .Sh FILES
 .Bl -tag -width ".Pa /dev/filemon"
 .It Pa /dev/filemon

sys/dev/filemon/filemon.c

@@ -163,8 +163,10 @@ filemon_ioctl(struct cdev *dev, u_long cmd, caddr_t data, int flag __unused,
 	switch (cmd) {
 	/* Set the output file descriptor. */
 	case FILEMON_SET_FD:
-		if (filemon->fp != NULL)
-			fdrop(filemon->fp, td);
+		if (filemon->fp != NULL) {
+			error = EEXIST;
+			break;
+		}
 
 		error = fget_write(td, *(int *)data,
 		    cap_rights_init(&rights, CAP_PWRITE),