IO.new will be able to decide an access mode from a given descriptor.

Test Script:
{{{
require 'test/unit/assertions.rb'
include Test::Unit::Assertions

fd = IO.sysopen("/tmp/test.txt", "w", 0666)

f = IO.new(fd)
f.write("foo\nbar\nbaz")
f.close

File.open("/tmp/test.txt") {|f|
  assert_equal("foo\nbar\nbaz", f.read)
}

assert_raise(Errno::EBADF){ IO.new(-1) }
assert_raise(Errno::EBADF){ IO.new(32767) }

puts :ok
}}}

io.c

@@ -3183,7 +3183,15 @@ rb_io_initialize(VALUE io, SEL sel, int argc, VALUE *argv)
     }
 
     if (NIL_P(mode)) {
+#ifdef F_GETFL
+	const int oflags = fcntl(fd, F_GETFL);
+	if (oflags == -1) {
+	    rb_sys_fail("fcntl(2) failed");
+	}
+	mode_flags = convert_oflags_to_fmode(oflags);
+#else
 	mode_flags = FMODE_READABLE;
+#endif
     }
     else if (TYPE(mode) == T_STRING) {
 	mode_flags = convert_mode_string_to_fmode(mode);