docker运行容器后agetty进程cpu占用率100%

1、最近在使用docker容器的时候，发现宿主机的agetty进程cpu占用率达到100%

在Google上搜了下，引起这个问题的原因是在使用"docker run"运行容器时使用了"/sbin/init"和"--privileged"参数。

使用/sbin/init启动容器并加上--privileged参数，相当于docker容器获得了宿主机的全权委托权限。这时docker容器内部的init与宿主机的init产生了混淆。

# 引用google到的一段话：

 I've done all my testing on them without using --privileged, especially since that's so dangerous (effectively, you're telling this second init process on your system that it's cool to go ahead and manage your system resources, and then giving it access to them as well). I always think of --privileged as a hammer to be used very sparingly.

出于对安全的考虑，在启动容器时，docker容器里的系统只具有一些普通的linux权限，并不具有真正root用户的所有权限。而--privileged=true参数可以让docker容器具有linux root用户的所有权限。

为了解决这个问题，docker后来的版本中docker run增加了两个选项参数"--cap-add"和"--cap-drop"。

--cap-add : 获取default之外的linux的权限

--cap-drop: 放弃default linux权限

从docker官网的文档中可以查到，docker容器具有的default权限及--cap-add可以获取到的扩展权限如下：

Default 权限：

Capability Key	Capability Description
SETPCAP	Modify process capabilities.
MKNOD	Create special files using mknod(2).
AUDIT_WRITE	Write records to kernel auditing log.
CHOWN	Make arbitrary changes to file UIDs and GIDs (see chown(2)).
NET_RAW	Use RAW and PACKET sockets.
DAC_OVERRIDE	Bypass file read, write, and execute permission checks.
FOWNER	Bypass permission checks on operations that normally require the file system UID of the process to match the UID of the file.
FSETID	Don’t clear set-user-ID and set-group-ID permission bits when a file is modified.
KILL	Bypass permission checks for sending signals.
SETGID	Make arbitrary manipulations of process GIDs and supplementary GID list.
SETUID	Make arbitrary manipulations of process UIDs.
NET_BIND_SERVICE	Bind a socket to internet domain privileged ports (port numbers less than 1024).
SYS_CHROOT	Use chroot(2), change root directory.
SETFCAP	Set file capabilities.

通过--cap-add获取到的权限：

Capability Key	Capability Description
SYS_MODULE	Load and unload kernel modules.
SYS_RAWIO	Perform I/O port operations (iopl(2) and ioperm(2)).
SYS_PACCT	Use acct(2), switch process accounting on or off.
SYS_ADMIN	Perform a range of system administration operations.
SYS_NICE	Raise process nice value (nice(2), setpriority(2)) and change the nice value for arbitrary processes.
SYS_RESOURCE	Override resource Limits.
SYS_TIME	Set system clock (settimeofday(2), stime(2), adjtimex(2)); set real-time (hardware) clock.
SYS_TTY_CONFIG	Use vhangup(2); employ various privileged ioctl(2) operations on virtual terminals.
AUDIT_CONTROL	Enable and disable kernel auditing; change auditing filter rules; retrieve auditing status and filtering rules.
MAC_OVERRIDE	Allow MAC configuration or state changes. Implemented for the Smack LSM.
MAC_ADMIN	Override Mandatory Access Control (MAC). Implemented for  the Smack Linux Security Module (LSM).
NET_ADMIN	Perform various network-related operations.
SYSLOG	Perform privileged syslog(2) operations.
DAC_READ_SEARCH	Bypass file read permission checks and directory read and execute permission checks.
LINUX_IMMUTABLE	Set the FS_APPEND_FL and FS_IMMUTABLE_FL i-node flags.
NET_BROADCAST	Make socket broadcasts, and listen to multicasts.
IPC_LOCK	Lock memory (mlock(2), mlockall(2), mmap(2), shmctl(2)).
IPC_OWNER	Bypass permission checks for operations on System V IPC objects.
SYS_PTRACE	Trace arbitrary processes using ptrace(2).
SYS_BOOT	Use reboot(2) and kexec_load(2), reboot and load a new kernel for later execution.
LEASE	Establish leases on arbitrary files (see fcntl(2)).
WAKE_ALARM	Trigger something that will wake up the system.
BLOCK_SUSPEND	Employ features that can block system suspend.

所以，在运行容器时，可以不用--privileged参数的尽量不用，用--cap-add参数替代。如果必须使用--privileged=true参数的，可以通过在宿主机和容器中执行以下命令将agetty关闭。

shell> systemctl stop getty@tty1.service

shell> systemctl mask getty@tty1.service

参考资料：

https://github.com/docker/docker/issues/4040

https://docs.docker.com/engine/reference/run/