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

include/lib.h

@@ -44,6 +44,7 @@ int mapdriver(char *label, int major, int style, int flags);
 pid_t getnpid(endpoint_t proc_ep);
 uid_t getnuid(endpoint_t proc_ep);
 gid_t getngid(endpoint_t proc_ep);
+int dupfrom(endpoint_t endpt, int fd);
 ssize_t pread64(int fd, void *buf, size_t count, u64_t where);
 ssize_t pwrite64(int fd, const void *buf, size_t count, u64_t where);
 

include/minix/callnr.h

@@ -47,6 +47,7 @@
 #define FSTAT		  52
 #define LSTAT		  53
 #define IOCTL		  54
+#define DUPFROM		  56
 #define FS_READY	  57
 #define PIPE2		  58
 #define EXEC		  59

include/minix/com.h

@@ -887,6 +887,10 @@
 #define VFS_PFS_FD		m2_i3
 #define VFS_PFS_FILP		m2_p1
 
+/* Field names for the dupfrom(2) call. */
+#define VFS_DUPFROM_ENDPT	m1_i1
+#define VFS_DUPFROM_FD		m1_i2
+
 /* Field names for GETRUSAGE related calls */
 #define RU_ENDPT	m1_i1	/* indicates a process for sys_getrusage */
 #define RU_WHO		m1_i1	/* who argument in getrusage call */