ID,简称GID),这与文件拥有者和拥有群组两种属性所对应。
文章开头也提到了,Linux系统并不认识我们的用户名和密码,那问题来了,文件是如何判别它的拥有者名称及群组名称的呢?
每个文件都有自己的拥有者ID和群组ID,在显示文件属性时,系统会根据/etc/passwd和/etc/group文件中的内容,分别找到UID和GID所对应的用户名和群组名,最终显示出来。
在/etc/passwd文件中,利用UID可以找到所对应的用户名,在/etc/group文件中,利用GID可以找到所对应的群组名。
可以做个小实验,在Linux *** 作系统中,常用的有两个账户,分别为root超级管理员账户和普通账户,我们可以先使用root账号登录并执行命令vim
/etc/passwd,在该文件中找到普通用户并将其UID随意改一个数字,这是你会发现,当你查看普通账户所拥有的文件时,你会发现所有文件的拥有者并不是普通用户,而是数字。
一般情况下,当登录Linux *** 作系统后,会先寻找/etc/passwd是否有输入账号,如果没有,则跳出,如果有,则读取对应的UID与GID,随后进入/etc/shadow核对密码,一切完成后,则进入shell管控。
由此可见,UID与GID在日常的账户管理中,发挥着非常重要的作用,因为一不小心就可能访问不了自己的文件,所以温馨提示大家,不要随便改动自己的/etc/passwd与/etc/group文件。
linux命令who am i,who,whoami今天要说的不是成龙的电影我是谁,而是linux里的who系列命令,包括who、whoami和who am i。 先看看这三个命令的输出信息:[rocrocket@rocrocket ~]$ whoamirocrocket[rocrocket@rocrocket ~]$ who am irocrocket pts/32008-12-30 13:17 (:0.0)[rocrocket@rocrocket ~]$ whorocrocket :0 2008-12-30 09:54rocrocket pts/02008-12-30 09:55 (:0.0)rocrocket pts/12008-12-30 09:57 (:0.0)rocrocket pts/32008-12-30 13:17 (:0.0)当我用sudo su(或者sudo su -)更换到root用户之后,你再看看: [rocrocket@rocrocket ~]$ sudo su[root@rocrocket rocrocket]# whoamiroot[root@rocrocket rocrocket]# who am irocrocket pts/32008-12-30 13:17 (:0.0)[root@rocrocket rocrocket]# whorocrocket :0 2008-12-30 09:54rocrocket pts/02008-12-30 09:55 (:0.0)rocrocket pts/12008-12-30 09:57 (:0.0)rocrocket pts/32008-12-30 13:17 (:0.0)看出区别来了吧,whoami显示的是当前 *** 作用户的用户名,而who am i显示的是登录用户的用户名。用linux的术语来解释就是:(实际用户=uid,即user id。有效用户=euid,即effective user id)who am i 显示的是实际用户的用户名,即用户登陆的时候的用户ID。此命令相当于who -m。whoami 显示的是有效用户ID. 好了,明白了两者区别之后,我们来说说who这个命令。有人会问,为什么我sudo su到root之后,who里面却没有显示呢?这是因为su过去的用户进程空间是作为一个子空间存在,他并没有得到一个登录的tty。who这个命令重点是用来查看当前有哪些用户登录到了本台机器上。who -m的作用和who am i的作用是一样的。who -q用来显示当前登录用户的个数。当你觉得who的输出信息晦涩难懂时,可以使用who -H来输出,这样可以在每列加上列名称,有助于阅读。为了执行权限检查,传统的 UNIX 实现区分两种类型的进程:特权进程(其有效用户 ID 为0,称为超级用户或 root),和非特权用户(其有效 UID 非0)。特权进程绕过所有的内核权限检查,而非特权进程受基于进程的认证信息(通常是:有效 UID,有效 GID,和补充组列表)的完整权限检查的支配。
自内核 2.2 版本开始,Linux 将传统上与超级用户关联的特权分为几个单元,称为 capabilities (权能),它们可以被独立的启用或禁用。权能是每个线程的属性。
下面的列表展示了 Linux 上实现的权能,以及每种权能允许的 *** 作或行为:
权能的完整实现需要:
在内核 2.6.24 之前,只有前两个要求能够满足;自内核 2.6.24 开始,所有三个要求都能满足。
每个线程具有三个包含零个或多个上面的权能的权能集合:
A child created via fork(2) inherits copies of its parent's capability sets. See below for a discussion of the treatment of capabilities during execve(2).
Using capset(2), a thread may manipulate its own capability sets (see below).
Since Linux 3.2, the file /proc/sys/kernel/cap_last_cap exposes the numerical value of the highest capability supported by the running kernelthis can be used to determine the highest bit that may be set in a capability set.
Since kernel 2.6.24, the kernel supports associating capability sets with an executable file using setcap(8). The file capability sets are stored in an extended attribute (see setxattr(2)) named security.capability. Writing to this extended attribute requires the CAP_SETFCAP capability. The file capability sets, in conjunction with the capability sets of the thread, determine the capabilities of a thread after an execve(2).
The three file capability sets are:
During an execve(2), the kernel calculates the new capabilities of the process using the following algorithm:
其中:
A privileged file is one that has capabilities or has the set-user-ID or set-group-ID bit set.
In order to provide an all-powerful root using capability sets, during an execve(2):
The upshot of the above rules, combined with the capabilities transformations described above, is that when a process execve(2)s a set-user-ID-root program, or when a process with an effective UID of 0 execve(2)s a program, it gains all capabilities in its permitted and effective capability sets, except those masked out by the capability bounding set. This provides semantics that are the same as those provided by traditional UNIX systems.
The capability bounding set is a security mechanism that can be used to limit the capabilities that can be gained during an execve(2). The bounding set is used in the following ways:
Note that the bounding set masks the file permitted capabilities, but not the inherited capabilities. If a thread maintains a capability in its inherited set that is not in its bounding set, then it can still gain that capability in its permitted set by executing a file that has the capability in its inherited set.
Depending on the kernel version, the capability bounding set is either a system-wide attribute, or a per-process attribute.
In kernels before 2.6.25, the capability bounding set is a system-wide attribute that affects all threads on the system. The bounding set is accessible via the file /proc/sys/kernel/cap-bound. (Confusingly, this bit mask parameter is expressed as a signed decimal number in /proc/sys/kernel/capbound.)
Only the init process may set capabilities in the capability bounding setother than that, the superuser (more precisely: programs with the CAP_SYS_MODULE capability) may only clear capabilities from this set.
On a standard system the capability bounding set always masks out the CAP_SETPCAP capability. To remove this restriction (dangerous!), modify the definition of CAP_INIT_EFF_SET in include/linux/capability.h and rebuild the kernel.
The system-wide capability bounding set feature was added to Linux starting with kernel version 2.2.11.
From Linux 2.6.25, the capability bounding set is a per-thread attribute. (There is no longer a systemwide capability bounding set.)
The bounding set is inherited at fork(2) from the thread's parent, and is preserved across an execve(2).
A thread may remove capabilities from its capability bounding set using the prctl(2) PR_CAPBSET_DROP operation, provided it has the CAP_SETPCAP capability. Once a capability has been dropped from the bounding set, it cannot be restored to that set. A thread can determine if a capability is in its bounding set using the prctl(2) PR_CAPBSET_READ operation.
Removing capabilities from the bounding set is supported only if file capabilities are compiled into the kernel. In kernels before Linux 2.6.33, file capabilities were an optional feature configurable via the CONFIG_SECURITY_FILE_CAPABILITIES option. Since Linux 2.6.33, the configuration option has been removed and file capabilities are always part of the kernel. When file capabilities are compiled into the kernel, the init process (the ancestor of all processes) begins with a full bounding set. If file capabilities are not compiled into the kernel, then init begins with a full bounding set minus CAP_SETPCAP, because this capability has a different meaning when there are no file capabilities.
Removing a capability from the bounding set does not remove it from the thread's inherited set. However it does prevent the capability from being added back into the thread's inherited set in the future.
To preserve the traditional semantics for transitions between 0 and nonzero user IDs, the kernel makes the following changes to a thread's capability sets on changes to the thread's real, effective, saved set, and filesystem user IDs (using setuid(2), setresuid(2), or similar):
If a thread that has a 0 value for one or more of its user IDs wants to prevent its permitted capability set being cleared when it resets all of its user IDs to nonzero values, it can do so using the prctl(2) PR_SET_KEEPCAPS operation or the SECBIT_KEEP_CAPS securebits flag described below.
A thread can retrieve and change its capability sets using the capget(2) and capset(2) system calls. However, the use of cap_get_proc(3) and cap_set_proc(3), both provided in the libcap package, is preferred for this purpose. The following rules govern changes to the thread capability sets:
Starting with kernel 2.6.26, and with a kernel in which file capabilities are enabled, Linux implements a set of per-thread securebits flags that can be used to disable special handling of capabilities for UID 0 (root). These flags are as follows:
Each of the above "base" flags has a companion "locked" flag. Setting any of the "locked" flags is irreversible, and has the effect of preventing further changes to the corresponding "base" flag. The locked flags are: SECBIT_KEEP_CAPS_LOCKED, SECBIT_NO_SETUID_FIXUP_LOCKED, SECBIT_NOROOT_LOCKED, and SECBIT_NO_CAP_AMBIENT_RAISE.
The securebits flags can be modified and retrieved using the prctl(2) PR_SET_SECUREBITS and PR_GET_SECUREBITS operations. The CAP_SETPCAP capability is required to modify the flags.
The securebits flags are inherited by child processes. During an execve(2), all of the flags are preserved, except SECBIT_KEEP_CAPS which is always cleared.
An application can use the following call to lock itself, and all of its descendants, into an environment where the only way of gaining capabilities is by executing a program with associated file capabilities:
For a discussion of the interaction of capabilities and user namespaces, see user_namespaces(7).
No standards govern capabilities, but the Linux capability implementation is based on the withdrawn POSIX.1e draft standardsee ⟨ http://wt.tuxomania.net/publications/posix.1e/ ⟩.
From kernel 2.5.27 to kernel 2.6.26, capabilities were an optional kernel component, and can be enabled/disabled via the CONFIG_SECURITY_CAPABILITIES kernel configuration option.
The /proc/PID/task/TID/status file can be used to view the capability sets of a thread. The /proc/PID/status file shows the capability sets of a process's main thread. Before Linux 3.8, nonexistent capabilities were shown as being enabled (1) in these sets. Since Linux 3.8, all nonexistent capabilities (above CAP_LAST_CAP) are shown as disabled (0).
The libcap package provides a suite of routines for setting and getting capabilities that is more comfortable and less likely to change than the interface provided by capset(2) and capget(2). This package also provides the setcap(8) and getcap(8) programs. It can be found at ⟨ http://www.kernel.org/pub/linux/libs/security/linux-privs ⟩.
Before kernel 2.6.24, and from kernel 2.6.24 to kernel 2.6.32 if file capabilities are not enabled, a thread with the CAP_SETPCAP capability can manipulate the capabilities of threads other than itself. However, this is only theoretically possible, since no thread ever has CAP_SETPCAP in either of these cases:
capsh(1), setpriv(1), prctl(2), setfsuid(2), cap_clear(3), cap_copy_ext(3), cap_from_text(3), cap_get_file(3), cap_get_proc(3), cap_init(3), capgetp(3), capsetp(3), libcap(3), credentials(7), user_namespaces(7), pthreads(7), getcap(8), setcap(8)
include/linux/capability.h in the Linux kernel source tree
This page is part of release 4.04 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at http://www.kernel.org/doc/man-pages/ .
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