execve()和环境变量 [英] execve() and environment variables

问题描述

我对Linux如何处理传递给execve()的环境变量有疑问:

execve()的简介: int execve(const char *文件名，char * const argv []，char * const envp []);

在调用execve()之前，我们从当前进程的内存映射中分配用于容纳envs/args的内存.但是在execve()之后，调用程序的所有文本/数据/bss/堆栈都会被新程序覆盖，并且旧进程的所有内存映射都不会保留(包括传递的envs/args的内存). /p>

对于新程序，在哪里读取envs/args?内核是否对传递的envs/args进行复制，并将其放置到新的内存映射中，或者其他一些技巧?

解决方案

是.

当进程调用exec时，内核将复制整个argv和envp数组.然后，将它们复制到新的过程映像中-值得注意的是，当程序开始运行时，其堆栈如下所示:

NULL
...
envp[1]
envp[0]
NULL
argv[argc-1]
...
argv[1]
argv[0]
argc

_start中的Glibc启动代码将其压缩为适当的形式以调用main.

(有关更多详细信息，旧过程的副本在 linux/fs/binfmt_elf.c ，程序启动为用特定于体系结构的代码完成，例如 glibc/sysdeps/i386/start.S ， glibc/ports/sysdeps/arm/start.S ，它的存​​在只是为了开始__libc_start_main? p = glibc.git; a = blob; f = csu/libc-start.c; hb = HEAD"rel =" noreferrer> glibc/csu/libc-start.c 会启动main.)

I've got a question with how Linux process the environment varibales passed to execve():

Synoposis for execve(): int execve(const char *filename, char *const argv[], char *const envp[]);

Before calling execve(), we allocate the memory for holding envs/args from current process's memory mapping. But after execve(), all the text/data/bss/stack of the calling process are overwriten by the new program, and all the memory mappings of the old process are not preserved (including the memory for passed envs/args).

For the new program, where to read the envs/args? Does the kernel make a copy of the passed envs/args and placed it onto the new memory mapping, or some other tricks?

解决方案

Yes.

When a process calls exec, the kernel copies the entire argv and envp arrays. Then, these are copied into the new process image -- notably, when the program starts running, its stack looks like:

NULL
...
envp[1]
envp[0]
NULL
argv[argc-1]
...
argv[1]
argv[0]
argc

The Glibc startup code in _start massages this into the proper form to invoke main.

(For more details, the copy from the old process is done in linux/fs/exec.c, the copy to the new process is done in linux/fs/binfmt_elf.c, and program startup is done in architecture-specific code such as glibc/sysdeps/i386/start.S, glibc/sysdeps/x86_64/start.S, or glibc/ports/sysdeps/arm/start.S, which exist just to kick off into __libc_start_main in glibc/csu/libc-start.c which launches main.)

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