What is the capability delivery mechanism in Linux

The inheritance of the capability mechanism in Linux means that when a process creates a child process, the child process can inherit the capability permissions of the parent process. In Linux systems, the capability mechanism is a more fine-grained permission control method that allows processes to only have certain permissions instead of all permissions. The original intention of this mechanism is to enhance the security of the system and reduce the administrator's burden of permission management.

In the traditional Linux permission model, the permissions of a process are determined by the user and user group to which it belongs. If a process needs to perform some operations that require privileges, such as changing the system time, loading kernel modules, etc., then the process must run as the root user. The problem with this approach is that once the process runs as the root user, it will have almost all permissions in the system, which is a potential risk to system security.

In order to solve this problem, Linux introduced the capability mechanism. Specifically, each process is given a capability set, which defines the permissions that the process can execute. By subdividing permissions into different capabilities, processes can only have the minimum permissions required, thereby reducing the risk of system abuse.

In Linux, capability permissions are divided into three categories: permitted, effective and inherited. Among them, permitted is the set of permissions actually granted to the process, effective is the set of permissions currently owned by the process, and inheritable is the set of permissions that can be inherited. When a process creates a child process, the child process inherits the inheritable permissions of the parent process. This means that as long as a capability permission is in the inheritable permission set of the parent process, it will automatically appear in the inheritable permission set of the child process.

The following is a specific code example that shows the inheritance of inheritable permissions when the parent process creates a child process:

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/capability.h>

int main() {
    pid_t pid;
    cap_t inheritable;
    cap_flag_value_t cap_flag;

    // 创建子进程
    pid = fork();
    
    if (pid == 0) {
        // 子进程
        
        // 获取继承的inheritable权限
        inheritable = cap_get_proc();
        
        // 展示子进程的inheritable权限集合
        printf("Child process inheritable capabilities:
");
        cap_print(inheritable, NULL);

        // 释放capability对象
        cap_free(inheritable);
        
        exit(0);
    } else if (pid > 0) {
        // 父进程
        
        // 设置inheritable权限
        inheritable = cap_get_proc();
        cap_get_flag(inheritable, CAP_CHOWN, CAP_INHERITABLE, &cap_flag);
        
        if (cap_flag == CAP_SET) {
            // 如果CAP_CHOWN权限在inheritable权限集合中被设置，则取消该权限的继承
            cap_clear_flag(inheritable, CAP_CHOWN, CAP_INHERITABLE);
        }
        
        // 创建子进程
        pid = fork();
        
        if (pid == 0) {
            // 新的子进程
        
            // 获取继承的inheritable权限
            inheritable = cap_get_proc();
            
            // 展示子进程的inheritable权限集合
            printf("New child process inheritable capabilities:
");
            cap_print(inheritable, NULL);

            // 释放capability对象
            cap_free(inheritable);
            
            exit(0);
        }
        
        // 释放capability对象
        cap_free(inheritable);
    } else {
        // fork失败
        fprintf(stderr, "Fork error
");
        exit(1);
    }
    
    return 0;
}

Through the above code example, we can observe the inheritance of the child process created by the parent process The set of inheritable permissions of the parent process. Of course, permissions can be flexibly set and controlled according to specific needs. In this way, the capability mechanism in Linux implements inter-process permission inheritance and fine-grained permission management.

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