VFS: add dupfrom(2) call

This call copies a file descriptor from a remote process into the
calling process. The call is for the VND driver only, and in the
future, ACLs will prevent any other process from using this call.

Change-Id: Ib16fdd1f1a12cb38a70d7e441dad91bc86898f6d

lib/libminlib/Makefile

@@ -30,9 +30,10 @@ SRCS+= servxcheck.c
 # queryparam
 SRCS+= paramvalue.c
 
-# Minix servers/drivers syscall.
-SRCS+= 	getngid.c getnpid.c getnprocnr.c getnucred.c getnuid.c getprocnr.c \
-	mapdriver.c vm_memctl.c vm_set_priv.c vm_query_exit.c vm_update.c
+# Minix servers/drivers syscall. FIXME: these should be moved into libsys.
+SRCS+= 	dupfrom.c getngid.c getnpid.c getnprocnr.c getnucred.c getnuid.c \
+	getprocnr.c mapdriver.c vm_memctl.c vm_set_priv.c vm_query_exit.c \
+	vm_update.c
 
 SRCS+= oneC_sum.c
 

lib/libminlib/dupfrom.c

@@ -0,0 +1,14 @@
+#include <lib.h>
+#include <string.h>
+
+int
+dupfrom(endpoint_t endpt, int fd)
+{
+	message m;
+
+	memset(&m, 0, sizeof(m));
+	m.VFS_DUPFROM_ENDPT = endpt;
+	m.VFS_DUPFROM_FD = fd;
+
+	return _syscall(VFS_PROC_NR, DUPFROM, &m);
+}