Question

我正在进行随机数生成领域的研究，我需要演示＆＃34;启动时间熵空洞＆＃34;来自着名的＆＃34; P＆Q; s＆＃34;论文（here）。我们将同时调整同一个最小Linux虚拟机的两个副本，我们期望它们的/ dev / urandom值在启动过程的某个早期点是相同的。

但是，我无法在启动过程中尽早读取/ dev / urandom来发现问题。我们需要在启动过程中更早。

如何获得/ dev / urandom的最早可能值？我们可能需要修改内核，但我们在那里经验很少，需要一些指针。或者，如果有可用的内核工具可以在不重新编译内核的情况下完成，那也很棒。

提前致谢！

Answer 1

urandom是通过设备驱动程序提供的，内核对驱动程序的第一件事是调用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，此驱动程序的init函数为rand_initialize。但请注意，评论表示setup_arch可能会在此设备初始化之前调用add_device randomness()。但是，调用该函数不会添加任何实际的熵（它会向池中提供类似MAC地址的内容，所以如果你有两个完全相同的虚拟机，那么你就很好）。来自评论：

  * 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，熵池在关闭时存储，并在启动时通过init脚本恢复（在我的Ubuntu 14.04上，它在/etc/init.d/urandom中），因此您可能希望在之前从该脚本调用脚本

 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

或类似的电话。

尽早阅读/ dev / urandom

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