Linux内核进程管理子系统有什么第四十三回 —— 进程主结构详解（39）

接前一篇文章：Linux内核进程管理子系统有什么第四十二回 —— 进程主结构详解（38）

本文内容参考：

Linux内核进程管理专题报告_linux rseq-CSDN博客

《趣谈Linux操作系统 核心原理篇：第三部分 进程管理》—— 刘超

《图解Linux内核 基于6.x》 —— 姜亚华 机械工业出版社

setuid系统调用及示例-CSDN博客

setuid函数解析 - HelloMarsMan - 博客园

特此致谢！

进程管理核心结构 —— task_struct

8. 进程权限相关成员

进程权限相关成员包括以下几个：

​​	/* Process credentials: *//* Tracer's credentials at attach: */const struct cred __rcu		*ptracer_cred;/* Objective and real subjective task credentials (COW): */const struct cred __rcu		*real_cred;/* Effective (overridable) subjective task credentials (COW): */const struct cred __rcu		*cred;

这几个字段的描述如下：

上一回继续对于struct cred进行解析，本回仍然继续。为了便于理解和回顾，再次贴出struct cred的定义，在include/linux/cred.h中，如下：

/** The security context of a task** The parts of the context break down into two categories:**  (1) The objective context of a task.  These parts are used when some other*	task is attempting to affect this one.**  (2) The subjective context.  These details are used when the task is acting*	upon another object, be that a file, a task, a key or whatever.** Note that some members of this structure belong to both categories - the* LSM security pointer for instance.** A task has two security pointers.  task->real_cred points to the objective* context that defines that task's actual details.  The objective part of this* context is used whenever that task is acted upon.** task->cred points to the subjective context that defines the details of how* that task is going to act upon another object.  This may be overridden* temporarily to point to another security context, but normally points to the* same context as task->real_cred.*/
struct cred {atomic_t	usage;
#ifdef CONFIG_DEBUG_CREDENTIALSatomic_t	subscribers;	/* number of processes subscribed */void		*put_addr;unsigned	magic;
#define CRED_MAGIC	0x43736564
#define CRED_MAGIC_DEAD	0x44656144
#endifkuid_t		uid;		/* real UID of the task */kgid_t		gid;		/* real GID of the task */kuid_t		suid;		/* saved UID of the task */kgid_t		sgid;		/* saved GID of the task */kuid_t		euid;		/* effective UID of the task */kgid_t		egid;		/* effective GID of the task */kuid_t		fsuid;		/* UID for VFS ops */kgid_t		fsgid;		/* GID for VFS ops */unsigned	securebits;	/* SUID-less security management */kernel_cap_t	cap_inheritable; /* caps our children can inherit */kernel_cap_t	cap_permitted;	/* caps we're permitted */kernel_cap_t	cap_effective;	/* caps we can actually use */kernel_cap_t	cap_bset;	/* capability bounding set */kernel_cap_t	cap_ambient;	/* Ambient capability set */
#ifdef CONFIG_KEYSunsigned char	jit_keyring;	/* default keyring to attach requested* keys to */struct key	*session_keyring; /* keyring inherited over fork */struct key	*process_keyring; /* keyring private to this process */struct key	*thread_keyring; /* keyring private to this thread */struct key	*request_key_auth; /* assumed request_key authority */
#endif
#ifdef CONFIG_SECURITYvoid		*security;	/* LSM security */
#endifstruct user_struct *user;	/* real user ID subscription */struct user_namespace *user_ns; /* user_ns the caps and keyrings are relative to. */struct ucounts *ucounts;struct group_info *group_info;	/* supplementary groups for euid/fsgid *//* RCU deletion */union {int non_rcu;			/* Can we skip RCU deletion? */struct rcu_head	rcu;		/* RCU deletion hook */};
} __randomize_layout;

上一回在讲 struct cred中的第4组字段kuid_t suid和kgid sgid时，

对于setuid系统调用进行知识补强。没有讲完，本回继续讲解这一知识点，力求讲清楚、讲透彻。

知识补强 —— setuid系统调用

再来回顾一下setuid系统调用的功能细节。

前几回讲的都是理论知识，不是很好理解。下边通过一个示例代码以及其运行结果来加深对于setuid系统调用的理解。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
//#define _GNU_SOURCE
#include <unistd.h>
#include <sys/types.h>
#include <errno.h>void show_curr_uids(void)
{int ret;uid_t real_uid;uid_t effective_uid;uid_t saved_uid;ret = getresuid(&real_uid, &effective_uid, &saved_uid);printf("Real uid: %d, Effective uid: %d\n", real_uid, effective_uid);printf("Current UIDs:\n");printf("\tReal uid: %d\n\tEffective uid: %d\n\tSaved uid: %d\n", getuid(), geteuid(), (getresuid(&real_uid, &effective_uid, &saved_uid) == 0) ? saved_uid : -1);
}int main(int argc, char *argv[])
{int ret;uid_t original_ruid, original_euid, original_suid;uid_t target_uid;//获取并打印初始 UIDif (getresuid(&original_ruid, &original_euid, &original_suid) != 0){perror("getresuid");exit(EXIT_FAILURE);}//检查是否以root权限运行if (geteuid() == 0){printf("\nRunning as ROOT (Privileged User)\n");printf("Before setuid.\n");show_curr_uids();//作为root，可以设置为任意有效的UID//这里我们尝试设置为'nobody' 用户 (通常UID 65534, 但请检查你的系统)target_uid = 65534;printf("\nAttempting to set UID to %d (usually 'nobody' user)...\n", target_uid);ret = setuid(target_uid);if (ret == 0){printf("setuid(%d) succeeded.\n", target_uid);printf("\nAfter setuid.\n");show_curr_uids();printf("Note: All UIDs (Real, Effective, Saved) are now %d.\n", target_uid);printf("The process is now running with 'nobody' privileges.\n");}else{perror("setuid");printf("Failed to set UID to %d.\n", target_uid);}}else{printf("\nRunning as a REGULAR USER (UID: %d)\n", getuid());printf("Before setuid.\n");show_curr_uids();//作为普通用户，只能设置为自己的ruid或suidtarget_uid = original_ruid; //选择设置为自己的真实UID (这不会改变任何东西)printf("\nAttempting to set UID to my Real UID (%d)...\n", target_uid);ret = setuid(target_uid);if (ret == 0){printf("setuid(%d) succeeded (as expected).\n", target_uid);printf("\nAfter setuid.\n");show_curr_uids();}else{perror("setuid");printf("Failed to set UID to %d.\n", target_uid);}
#if 0//尝试设置为一个无效的UID(比如一个不存在的或不属于我的UID)//这通常会失败target_uid = 9999; //假设这是一个无效的或不属于当前用户的UIDprintf("\nAttempting to set UID to an invalid/different UID (%d)...\n", target_uid);ret = setuid(target_uid);if (ret == -1){if (errno == EPERM){printf("setuid(%d) failed with EPERM (Operation not permitted) - as expected for a regular user.\n", target_uid);printf("This is because %d is not my Real UID (%d) or Saved Set-UID (%d).\n", target_uid, original_ruid, original_suid);}else{perror("setuid");}printf("After failed setuid.\n");show_curr_uids();}else{printf("setuid(%d) unexpectedly succeeded.\n", target_uid);printf("After unexpected setuid.\n");show_curr_uids();}
#endif}return 0;
}

首先说一下，系统中原本有两个用户：root（UID为0）和ph（UID为1000）。为了做本次setuid测试，笔者专门通过adduser命令创建了用户shiyan（UID为1001）。

接下来看setuid在不同权限下的行为：

（1）以超级用户（root）执行程序

实际命令及结果如下：

如果进程具有超级用户特权，那么这三个UID都被设置成了uid（setuid函数的参数）的值。这里，uid的值65534，是'nobody'用户的UID。

对应代码片段为：

    //检查是否以root权限运行if (geteuid() == 0){printf("\nRunning as ROOT (Privileged User)\n");printf("Before setuid.\n");show_curr_uids();//作为root，可以设置为任意有效的UID//这里我们尝试设置为'nobody' 用户 (通常UID 65534, 但请检查你的系统)target_uid = 65534;printf("\nAttempting to set UID to %d (usually 'nobody' user)...\n", target_uid);ret = setuid(target_uid);if (ret == 0){printf("setuid(%d) succeeded.\n", target_uid);printf("\nAfter setuid.\n");show_curr_uids();printf("Note: All UIDs (Real, Effective, Saved) are now %d.\n", target_uid);printf("The process is now running with 'nobody' privileges.\n");}else{perror("setuid");printf("Failed to set UID to %d.\n", target_uid);}}

这就对应的是setuid系统调用的具体行为（功能细节）中的：

setuid系统调用的其余功能细节，下一回进行解析。