mllib: Add a utility function for safely reading from /dev/urandom.

OCaml's buffered 'in_channel' has a 64k buffer, so using it to read a
few bytes from /dev/urandom removes a lot of the system's entropy (for
example /proc/sys/kernel/random/entropy_avail goes from ~3000 to 128).

This patch was originally by Edwin Török for builder.ml.  I
generalized it because there are two other places where we did
over-sized reads from /dev/urandom.

builder/Makefile.am

@@ -65,6 +65,7 @@ OBJECTS = \
 	$(top_builddir)/mllib/libdir.cmx \
 	$(top_builddir)/mllib/common_gettext.cmx \
 	$(top_builddir)/mllib/common_utils.cmx \
+	$(top_builddir)/mllib/urandom.cmx \
 	$(top_builddir)/mllib/random_seed.cmx \
 	$(top_builddir)/mllib/hostname.cmx \
 	$(top_builddir)/mllib/firstboot.cmx \

mllib/Makefile.am

@@ -48,6 +48,8 @@ SOURCES = \
 	tty-c.c \
 	tTY.mli \
 	tTY.ml \
+	urandom.mli \
+	urandom.ml \
 	uri-c.c \
 	uRI.mli \
 	uRI.ml
@@ -70,6 +72,7 @@ OBJECTS = \
 	libdir.cmx \
 	common_gettext.cmx \
 	common_utils.cmx \
+	urandom.cmx \
 	random_seed.cmx \
 	hostname.cmx \
 	firstboot.cmx \

mllib/random_seed.ml

@@ -78,13 +78,8 @@ and make_random_seed_file g file =
       (* Default to 512 bytes of randomness. *)
       512 in
 
-  let entropy =
-    (* Get n bytes of randomness from the host. *)
-    let chan = open_in "/dev/urandom" in
-    let buf = String.create n in
-    really_input chan buf 0 n;
-    close_in chan;
-    buf in
+  (* Get n bytes of randomness from the host. *)
+  let entropy = Urandom.urandom_bytes n in
 
   if file_exists then (
     (* Truncate the original file and append, in order to

mllib/urandom.ml

@@ -0,0 +1,48 @@
+(* Read /dev/urandom.
+ * Copyright (C) 2013 Red Hat Inc.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ *)
+
+(* Read and return N bytes (only) from /dev/urandom.
+ *
+ * As pointed out by Edwin Török, previous versions of this had a big
+ * problem.  They used the OCaml buffered I/O library which would read
+ * a lot more data than requested.  This version uses unbuffered I/O
+ * from the Unix module.
+ *)
+
+open Unix
+
+let open_urandom_fd () = openfile "/dev/urandom" [O_RDONLY] 0
+
+let read_byte fd =
+  let s = String.make 1 ' ' in
+  fun () ->
+    if read fd s 0 1 = 0 then (
+      close fd;
+      raise End_of_file
+    );
+    Char.code s.[0]
+
+let urandom_bytes n =
+  assert (n > 0);
+  let ret = String.make n ' ' in
+  let fd = open_urandom_fd () in
+  for i = 0 to n-1 do
+    ret.[i] <- Char.chr (read_byte fd ())
+  done;
+  close fd;
+  ret

mllib/urandom.mli

@@ -0,0 +1,22 @@
+(* Read /dev/urandom.
+ * Copyright (C) 2013 Red Hat Inc.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ *)
+
+(** Read and return N bytes (only) from /dev/urandom. *)
+
+val urandom_bytes : int -> string
+(** Read N bytes from /dev/urandom and return it as a binary string. *)

po/POTFILES-ml

@@ -21,6 +21,7 @@ mllib/progress.ml
 mllib/random_seed.ml
 mllib/tTY.ml
 mllib/uRI.ml
+mllib/urandom.ml
 resize/resize.ml
 sparsify/sparsify.ml
 sysprep/main.ml

sysprep/Makefile.am

@@ -89,6 +89,7 @@ OBJECTS = \
 	$(top_builddir)/mllib/uRI.cmx \
 	$(top_builddir)/mllib/crypt-c.o \
 	$(top_builddir)/mllib/crypt.cmx \
+	$(top_builddir)/mllib/urandom.cmx \
 	$(top_builddir)/mllib/password.cmx \
 	$(top_builddir)/mllib/random_seed.cmx \
 	$(top_builddir)/mllib/hostname.cmx \