Linux reboot() 系统调用的幻数 [英] Magic numbers of the Linux reboot() system call

问题描述

Linux 编程接口在第 3 章中有一个练习是这样的:

The Linux Programming Interface has an exercise in Chapter 3 that goes like this:

当使用 Linux 特定的 reboot()重新启动系统的系统调用，第二个参数，magic2，必须是指定为一组魔法之一数字(例如，LINUX_REBOOT_MAGIC2).这些有什么意义数字?(将它们转换为十六进制提供了线索.)

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.)

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_REBOOT_MAGIC2B (369367448) 之一.我没能破译它们的十六进制含义.我还查看了 /usr/include/linux/reboot.h，它也没有给出任何有用的评论.

The man page tells us magic2 can 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.

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.

I then searched in the kernel's source code for sys_reboot's definition. All I found was a declaration in a header file.

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 的定义在哪里，你是怎么找到的?

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?

EDIT:我在 kernel/sys.c 中找到了定义.我只对 sys_reboot 进行了搜索，而忘了对 MAGIC 数字进行搜索.我想这个定义一定隐藏在一些宏技巧后面，所以我查看了 /boot 下的 System.map 文件，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.

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.map file 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.

推荐答案

只是猜测，但这些数字在十六进制中看起来更有趣:

Just a guess, but those numbers look more interesting in hex:

672274793 = 0x28121969
 85072278 = 0x05121996
369367448 = 0x16041998
537993216 = 0x20112000

开发者或开发者孩子的生日?

Developers' or developers' children's birthdays?

关于找到系统调用的实现，我做了一个 git grep -n LINUX_REBOOT_MAGIC2 并在 kernel/sys.c 中找到了定义.符号 sys_reboot 是由 SYSCALL_DEFINE4(reboot, ... gubbins，我怀疑生成的.

Regarding finding the syscall implementation, I did a git grep -n LINUX_REBOOT_MAGIC2 and found the definition in kernel/sys.c. The symbol sys_reboot is generated by the SYSCALL_DEFINE4(reboot, ... gubbins, I suspect.

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