Linux 内核源码分析 -- getuid, geteuid

Posted 2020-12-09 scriptkid

Linux 内核源码分析 -- getuid，geteuid

getuid

获取用户标识号

via：https://man7.org/linux/man-pages/man2/geteuid.2.html

returns the real user ID of the calling process.

SYSCALL_DEFINE0(getuid)
{
	/* Only we change this so SMP safe */
	return from_kuid_munged(current_user_ns(), current_uid());
}

current_user_ns()

一个宏，用来获取当前进程的 cred

#define current_user_ns()	(current_cred_xxx(user_ns))
#define current_cred_xxx(xxx)			({							current_cred()->xxx;			})

/**
 * current_cred - Access the current task‘s subjective credentials
 *
 * Access the subjective credentials of the current task.  RCU-safe,
 * since nobody else can modify it.
 */
#define current_cred() 	rcu_dereference_protected(current->cred, 1)

current_uid()

获取进程的 cred->uid

#define current_uid()		(current_cred_xxx(uid))
#define current_cred_xxx(xxx)			({							current_cred()->xxx;			})

/**
 * current_cred - Access the current task‘s subjective credentials
 *
 * Access the subjective credentials of the current task.  RCU-safe,
 * since nobody else can modify it.
 */
#define current_cred() 	rcu_dereference_protected(current->cred, 1)

其实跟上面的 current_user_ns() 差不多，就是用 current_cred_xxx() 拼接

current_cred() 获取当前进程的 cred

展开宏就是 ：current->cred->uid

from_kuid_munged()

uid_t from_kuid_munged(struct user_namespace *targ, kuid_t kuid)
{
	uid_t uid;
	uid = from_kuid(targ, kuid);

    // 如果 uid 是 -1 的话,把 uid 设置成 65534 (overflow)
	if (uid == (uid_t) -1)
		uid = overflowuid;
	return uid;
}

from_kuid()

static inline uid_t from_kuid(struct user_namespace *to, kuid_t kuid)
{
	return __kuid_val(kuid);
}

__kuid_val()

static inline uid_t __kuid_val(kuid_t uid)
{
    // 获取 kuid_t 结构的 val,这个就是 uid 的值了
	return uid.val;
}

geteuid

获取用户有效标识号

via：https://man7.org/linux/man-pages/man2/geteuid.2.html

returns the effective user ID of the calling process.

SYSCALL_DEFINE0(geteuid)
{
	/* Only we change this so SMP safe */
	return from_kuid_munged(current_user_ns(), current_euid());
}

其实和上面的 getuid 是一样的

总结

获取用户标识号 和 获取用户有效标识号 其实就是获取 current->cred->uid->val, current->cred->euid->val