linux capability是啥？
資料來源：
http://man7.org/linux/man-pages/man7/capabilities.7.html
“ For the purpose of performing permission checks, traditional UNIX
implementations distinguish two categories of processes: privileged
processes (whose effective user ID is 0, referred to as superuser or
root), and unprivileged processes (whose effective UID is nonzero).
Privileged processes bypass all kernel permission checks, while
unprivileged processes are subject to full permission checking based
on the process's credentials (usually: effective UID, effective GID,
and supplementary group list).
Starting with kernel 2.2, Linux divides the privileges traditionally
associated with superuser into distinct units, known as capabilities,
which can be independently enabled and disabled. Capabilities are a
per-thread attribute.
”
原來linux系統爲了將系統權限作了分類，雖然是root用戶，如果沒有賦予相關的權限也是白搭。
下面命令列出了系統支持的capability：
[root@centos /]# capsh --print
Current: = cap_chown,cap_dac_override,cap_dac_read_search,cap_fowner,cap_fsetid,cap_kill,cap_setgid,cap_setuid,cap_setpcap,cap_linux_immutable,cap_net_bind_service,cap_net_broadcast,cap_net_admin,cap_net_raw,cap_ipc_lock,cap_ipc_owner,cap_sys_module,cap_sys_rawio,cap_sys_chroot,cap_sys_ptrace,cap_sys_pacct,cap_sys_admin,cap_sys_boot,cap_sys_nice,cap_sys_resource,cap_sys_time,cap_sys_tty_config,cap_mknod,cap_lease,cap_audit_write,cap_audit_control,cap_setfcap,cap_mac_override,cap_mac_admin,cap_syslog,35,36+ep
Bounding set =cap_chown,cap_dac_override,cap_dac_read_search,cap_fowner,cap_fsetid,cap_kill,cap_setgid,cap_setuid,cap_setpcap,cap_linux_immutable,cap_net_bind_service,cap_net_broadcast,cap_net_admin,cap_net_raw,cap_ipc_lock,cap_ipc_owner,cap_sys_module,cap_sys_rawio,cap_sys_chroot,cap_sys_ptrace,cap_sys_pacct,cap_sys_admin,cap_sys_boot,cap_sys_nice,cap_sys_resource,cap_sys_time,cap_sys_tty_config,cap_mknod,cap_lease,cap_audit_write,cap_audit_control,cap_setfcap,cap_mac_override,cap_mac_admin,cap_syslog,35,36
Securebits: 00/0x0/1'b0
secure-noroot: no (unlocked)
secure-no-suid-fixup: no (unlocked)
secure-keep-caps: no (unlocked)
uid=0(root)
gid=0(root)
groups=0(root)
下面來看看docker-containerd這個進程所有的capability：
cat /proc/`pidof docker-containerd`/status | grep Cap
CapInh: 0000000000000000
CapPrm: 0000001fffffffff
CapEff: 0000001fffffffff
CapBnd: 0000001fffffffff
CapAmb: 0000000000000000
capsh --decode=0x1fffffffff // 解碼
0x0000001fffffffff=cap_chown,cap_dac_override,cap_dac_read_search,cap_fowner,cap_fsetid,cap_kill,cap_setgid,cap_setuid,cap_setpcap,cap_linux_immutable,cap_net_bind_service,cap_net_broadcast,cap_net_admin,cap_net_raw,cap_ipc_lock,cap_ipc_owner,cap_sys_module,cap_sys_rawio,cap_sys_chroot,cap_sys_ptrace,cap_sys_pacct,cap_sys_admin,cap_sys_boot,cap_sys_nice,cap_sys_resource,cap_sys_time,cap_sys_tty_config,cap_mknod,cap_lease,cap_audit_write,cap_audit_control,cap_setfcap,cap_mac_override,cap_mac_admin,cap_syslog,35,36
對比發現，與系統支持的相符合。（getpcaps `pidof docker-containerd`可以得到同樣的輸出）
下面來看看docker容器的capability：
[root@centos opt]#docker run -ti centos /bin/bash
[root@f45f03e236ec /]# capsh --print
Current: = cap_chown,cap_dac_override,cap_fowner,cap_fsetid,cap_kill,cap_setgid,cap_setuid,cap_setpcap,cap_net_bind_service,cap_net_raw,cap_sys_chroot,cap_mknod,cap_audit_write,cap_setfcap+eip
Bounding set =cap_chown,cap_dac_override,cap_fowner,cap_fsetid,cap_kill,cap_setgid,cap_setuid,cap_setpcap,cap_net_bind_service,cap_net_raw,cap_sys_chroot,cap_mknod,cap_audit_write,cap_setfcap
Securebits: 00/0x0/1'b0
secure-noroot: no (unlocked)
secure-no-suid-fixup: no (unlocked)
secure-keep-caps: no (unlocked)
uid=0(root)
gid=0(root)
groups=
對比發現，容器少了大致下面的capability：
cap_net_admin,cap_net_broadcast,cap_sys_module,cap_sys_ptrace,cap_sys_pacct,cap_sys_admin,cap_sys_boot,cap_sys_nice,cap_sys_resource,cap_sys_time,cap_syslog
因此，容器用戶不允許執行ip、time這些命令。
vendor/github.com/containerd/containerd/oci/spec_unix.go，這個文件定義了缺省的capability。
func defaultCaps() []string {
return []string{
"CAP_CHOWN",
"CAP_DAC_OVERRIDE",
"CAP_FSETID",
"CAP_FOWNER",
"CAP_MKNOD",
"CAP_NET_RAW",
"CAP_SETGID",
"CAP_SETUID",
"CAP_SETFCAP",
"CAP_SETPCAP",
"CAP_NET_BIND_SERVICE",
"CAP_SYS_CHROOT",
"CAP_KILL",
"CAP_AUDIT_WRITE",
}
}
下面的命令可以動態的改動容器所有的capability：
[root@centos opt]#docker run --cap-drop all --cap-add net_admin -ti centos /bin/bash
[root@1db73e0aaf38 /]# capsh --print
Current: = cap_net_admin+eip // 只具備net_admin
Bounding set =cap_net_admin
Securebits: 00/0x0/1'b0
secure-noroot: no (unlocked)
secure-no-suid-fixup: no (unlocked)
secure-keep-caps: no (unlocked)
uid=0(root)
gid=0(root)
groups=