linux - 尽早阅读/dev/urandom

我正在进行随机数生成领域的研究，我需要演示著名的“P和Q”论文（here）中的“启动时间熵洞”。我们将同时备份同一个最小的Linux虚拟机的两个副本，并且我们希望它们的/dev/urandom值在启动过程的某个早期点是相同的。
但是，在引导过程中，我还不能足够早地阅读/dev/urandom来发现问题。我们需要在启动过程的早期。
如何获取/dev/urandom的最早可能值？我们可能需要修改内核，但我们在这方面的经验很少，需要一些指针。或者，如果有一个内核检测工具可以在不重新编译内核的情况下进行测试，那也太好了。
提前谢谢！

最佳答案

urandom是通过设备驱动程序提供的，内核对该驱动程序所做的第一件事就是调用init调用。
如果你看看这里：http://lxr.free-electrons.com/source/drivers/char/random.c#L1401

  * Note that setup_arch() may call add_device_randomness()
  * long before we get here. This allows seeding of the pools
  * with some platform dependent data very early in the boot
  * process. But it limits our options here. We must use
  * statically allocated structures that already have all
  * initializations complete at compile time. We should also
  * take care not to overwrite the precious per platform data
  * we were given.
  */
 static int rand_initialize(void)
 {
         init_std_data(&input_pool);
         init_std_data(&blocking_pool);
         init_std_data(&nonblocking_pool);
         return 0;
 }
 early_initcall(rand_initialize);

因此，这个驱动程序的init函数是rand_initialize。但是请注意，注释指出setup_arch可能在设备初始化之前调用add_device randomness()。然而，调用该函数并不会增加任何实际的熵（它会向池中提供诸如MAC地址之类的信息，因此如果有两个完全相同的vm，那么就很好了）。从评论中：

  * add_device_randomness() is for adding data to the random pool that
  * is likely to differ between two devices (or possibly even per boot).
  * This would be things like MAC addresses or serial numbers, or the
  * read-out of the RTC. This does *not* add any actual entropy to the
  * pool, but it initializes the pool to different values for devices
  * that might otherwise be identical and have very little entropy
  * available to them (particularly common in the embedded world).

另外，请注意，熵池存储在关闭时，并在启动时通过in it脚本（在我的Ubuntu 14.04上，它位于/etc/init.d/urandom中）恢复，因此您可能希望在调用该脚本之前

 53     (
 54       date +%s.%N
 55
 56       # Load and then save $POOLBYTES bytes,
 57       # which is the size of the entropy pool
 58       if [ -f "$SAVEDFILE" ]
 59       then
 60           cat "$SAVEDFILE"
 61       fi
 62     # Redirect output of subshell (not individual commands)
 63     # to cope with a misfeature in the FreeBSD (not Linux)
 64     # /dev/random, where every superuser write/close causes
 65     # an explicit reseed of the yarrow.
 66     ) >/dev/urandom

或者类似的电话。