Linux reboot() 系统调用的幻数
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Magic numbers of the Linux reboot() system call
提问by Wei Hu
The Linux Programming Interfacehas an exercise in Chapter 3 that goes like this:
Linux 编程接口在第 3 章中有一个练习，如下所示：
When using the Linux-specific reboot() system call to reboot the system, the second argument, magic2, must be specified as one of a set of magic numbers (e.g., LINUX_REBOOT_MAGIC2). What is the significance of these numbers? (Converting them to hexadecimal provides a clue.)
当使用 Linux 特定的 reboot() 系统调用重新启动系统时，必须将第二个参数 magic2 指定为一组幻数之一（例如，LINUX_REBOOT_MAGIC2）。这些数字有什么意义？（将它们转换为十六进制提供了一个线索。）
The man page tells us magic2can be one of LINUX_REBOOT_MAGIC2 (672274793), LINUX_REBOOT_MAGIC2A (85072278), LINUX_REBOOT_MAGIC2B (369367448), or LINUX_REBOOT_MAGIC2C (537993216). I failed to decipher their meaning in hex. I also looked at /usr/include/linux/reboot.h, which didn't give any helpful comment either.
手册页告诉我们magic2可以是 LINUX_REBOOT_MAGIC2 (672274793)、LINUX_REBOOT_MAGIC2A (85072278)、LINUX_REBOOT_MAGIC2B (369367448) 或 LINUX_REBOOT53799C2 之一。我没能破译它们的十六进制含义。我也看了/usr/include/linux/reboot.h，也没有给出任何有用的评论。
I then searched in the kernel's source code for sys_reboot's definition. All I found was a declaration in a header file.
然后我在内核的源代码中搜索了sys_reboot的定义。我发现的只是头文件中的声明。
Therefore, my first question is, what is the significance of these numbers? My second question is, where's sys_reboot's definition, and how did you find it?
因此，我的第一个问题是，这些数字的意义是什么？我的第二个问题是，sys_reboot定义在哪里，你是怎么找到的？
EDIT: I found the definition in kernel/sys.c. I only grepped for sys_reboot, and forgot to grep for the MAGIC numbers. I figured the definition must be hidden behind some macro trick, so I looked at the System.mapfile under /boot, and found it next to ctrl_alt_del. I then grepped for that symbol, which led me to the correct file. If I had compiled the kernel from source code, I could try to find which object file defined the symbol, and go from there.
编辑：我在kernel/sys.c. 我只搜索了sys_reboot，忘记了搜索 MAGIC 数字。我想这个定义一定隐藏在一些宏技巧后面，所以我查看了System.map下的文件/boot，并在ctrl_alt_del. 然后我搜索了那个符号，这让我找到了正确的文件。如果我从源代码编译内核，我可以尝试找到定义符号的目标文件，然后从那里开始。
采纳答案by araqnid
Just a guess, but those numbers look more interesting in hex:
只是猜测，但这些数字在十六进制中看起来更有趣：
672274793 = 0x28121969
85072278 = 0x05121996
369367448 = 0x16041998
537993216 = 0x20112000
Developers' or developers' children's birthdays?
开发者或开发者孩子的生日？
Regarding finding the syscall implementation, I did a git grep -n LINUX_REBOOT_MAGIC2and found the definition in kernel/sys.c. The symbol sys_rebootis generated by the SYSCALL_DEFINE4(reboot, ...gubbins, I suspect.
关于查找系统调用的实现，我做了一个git grep -n LINUX_REBOOT_MAGIC2并在kernel/sys.c中找到了定义。我怀疑这个符号sys_reboot是由SYSCALL_DEFINE4(reboot, ...gubbins生成的。
回答by Ritwik Dey
It's the birthday of Linus Torvalds (The developer of the Linux kernel and the Git version control) and his 3 daughters. works as magic numbers to reboot the system.
这是 Linus Torvalds（Linux 内核和 Git 版本控制的开发者）和他的 3 个女儿的生日。用作重新启动系统的幻数。
