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3.5. devices

The devices subsystem allows or denies access to devices by tasks in a cgroup.
specifies devices to which tasks in a cgroup have access. Each entry has four fields: type, major, minor, and access. The values used in the type, major, and minor fields correspond to device types and node numbers specified in Linux Allocated Devices, otherwise known as the Linux Devices List and available from
type can have one of the following three values:
  • a — applies to all devices, both character devices and block devices
  • b — specifies a block device
  • c — specifies a character device
major, minor
major and minor are device node numbers specified by Linux Allocated Devices. The major and minor numbers are separated by a colon. For example, 8 is the major number that specifies SCSI disk drives, and the minor number 1 specifies the first partition on the first SCSI disk drive; therefore 8:1 fully specifies this partition, corresponding to a file system location of /dev/sda1.
* can stand for all major or all minor device nodes, for example 9:* (all RAID devices) or *:* (all devices).
access is a sequence of one or more of the following letters:
  • r — allows tasks to read from the specified device
  • w — allows tasks to write to the specified device
  • m — allows tasks to create device files that do not yet exist
For example, when access is specified as r, tasks can only read from the specified device, but when access is specified as rw, tasks can read from and write to the device.
specifies devices that tasks in a cgroup cannot access. The syntax of entries is identical with devices.allow.
reports the devices for which access controls have been set for tasks in this cgroup.