Linux Container Configuration

This document describes the schema for the Linux-specific section of the container configuration. The Linux container specification uses various kernel features like namespaces, cgroups, capabilities, LSM, and filesystem jails to fulfill the spec.

Default Filesystems

The Linux ABI includes both syscalls and several special file paths. Applications expecting a Linux environment will very likely expect these file paths to be set up correctly.

The following filesystems SHOULD be made available in each container's filesystem:

Path	Type
/proc	proc
/sys	sysfs
/dev/pts	devpts
/dev/shm	tmpfs

Namespaces

A namespace wraps a global system resource in an abstraction that makes it appear to the processes within the namespace that they have their own isolated instance of the global resource. Changes to the global resource are visible to other processes that are members of the namespace, but are invisible to other processes. For more information, see the namespaces(7) man page.

Namespaces are specified as an array of entries inside the namespaces root field. The following parameters can be specified to set up namespaces:

type (string, REQUIRED) - namespace type. The following namespace types are supported:
- pid processes inside the container will only be able to see other processes inside the same container.
- network the container will have its own network stack.
- mount the container will have an isolated mount table.
- ipc processes inside the container will only be able to communicate to other processes inside the same container via system level IPC.
- uts the container will be able to have its own hostname and domain name.
- user the container will be able to remap user and group IDs from the host to local users and groups within the container.
- cgroup the container will have an isolated view of the cgroup hierarchy.
path (string, OPTIONAL) - namespace file. This value MUST be an absolute path in the runtime mount namespace. The runtime MUST place the container process in the namespace associated with that path. The runtime MUST generate an error if path is not associated with a namespace of type type.

If path is not specified, the runtime MUST create a new container namespace of type type.

If a namespace type is not specified in the namespaces array, the container MUST inherit the runtime namespace of that type. If a namespaces field contains duplicated namespaces with same type, the runtime MUST generate an error.

Example

    "namespaces": [
        {
            "type": "pid",
            "path": "/proc/1234/ns/pid"
        },
        {
            "type": "network",
            "path": "/var/run/netns/neta"
        },
        {
            "type": "mount"
        },
        {
            "type": "ipc"
        },
        {
            "type": "uts"
        },
        {
            "type": "user"
        },
        {
            "type": "cgroup"
        }
    ]

User namespace mappings

uidMappings (array of objects, OPTIONAL) describes the user namespace uid mappings from the host to the container. gidMappings (array of objects, OPTIONAL) describes the user namespace gid mappings from the host to the container.

Each entry has the following structure:

hostID (uint32, REQUIRED) - is the starting uid/gid on the host to be mapped to containerID.
containerID (uint32, REQUIRED) - is the starting uid/gid in the container.
size (uint32, REQUIRED) - is the number of ids to be mapped.

The runtime SHOULD NOT modify the ownership of referenced filesystems to realize the mapping. Note that the number of mapping entries MAY be limited by the kernel.

Example

    "uidMappings": [
        {
            "hostID": 1000,
            "containerID": 0,
            "size": 32000
        }
    ],
    "gidMappings": [
        {
            "hostID": 1000,
            "containerID": 0,
            "size": 32000
        }
    ]

Devices

devices (array of objects, OPTIONAL) lists devices that MUST be available in the container. The runtime MAY supply them however it likes (with mknod, by bind mounting from the runtime mount namespace, using symlinks, etc.).

Each entry has the following structure:

type (string, REQUIRED) - type of device: c, b, u or p. More info in mknod(1).
path (string, REQUIRED) - full path to device inside container. If a file already exists at path that does not match the requested device, the runtime MUST generate an error.
major, minor (int64, REQUIRED unless type is p) - major, minor numbers for the device.
fileMode (uint32, OPTIONAL) - file mode for the device. You can also control access to devices with cgroups.
uid (uint32, OPTIONAL) - id of device owner.
gid (uint32, OPTIONAL) - id of device group.

The same type, major and minor SHOULD NOT be used for multiple devices.

Example

    "devices": [
        {
            "path": "/dev/fuse",
            "type": "c",
            "major": 10,
            "minor": 229,
            "fileMode": 438,
            "uid": 0,
            "gid": 0
        },
        {
            "path": "/dev/sda",
            "type": "b",
            "major": 8,
            "minor": 0,
            "fileMode": 432,
            "uid": 0,
            "gid": 0
        }
    ]

Default Devices

In addition to any devices configured with this setting, the runtime MUST also supply:

/dev/null
/dev/zero
/dev/full
/dev/random
/dev/urandom
/dev/tty
/dev/console is set up if terminal is enabled in the config by bind mounting the pseudoterminal slave to /dev/console.
/dev/ptmx. A bind-mount or symlink of the container's /dev/pts/ptmx.

Control groups

Also known as cgroups, they are used to restrict resource usage for a container and handle device access. cgroups provide controls (through controllers) to restrict cpu, memory, IO, pids and network for the container. For more information, see the kernel cgroups documentation.

Cgroups Path

cgroupsPath (string, OPTIONAL) path to the cgroups. It can be used to either control the cgroups hierarchy for containers or to run a new process in an existing container.

The value of cgroupsPath MUST be either an absolute path or a relative path.

In the case of an absolute path (starting with /), the runtime MUST take the path to be relative to the cgroups mount point.
In the case of a relative path (not starting with /), the runtime MAY interpret the path relative to a runtime-determined location in the cgroups hierarchy.

If the value is specified, the runtime MUST consistently attach to the same place in the cgroups hierarchy given the same value of cgroupsPath. If the value is not specified, the runtime MAY define the default cgroups path. Runtimes MAY consider certain cgroupsPath values to be invalid, and MUST generate an error if this is the case.

Implementations of the Spec can choose to name cgroups in any manner. The Spec does not include naming schema for cgroups. The Spec does not support per-controller paths for the reasons discussed in the cgroupv2 documentation. The cgroups will be created if they don't exist.

You can configure a container's cgroups via the resources field of the Linux configuration. Do not specify resources unless limits have to be updated. For example, to run a new process in an existing container without updating limits, resources need not be specified.

Runtimes MAY attach the container process to additional cgroup controllers beyond those necessary to fulfill the resources settings.

Example

    "cgroupsPath": "/myRuntime/myContainer",
    "resources": {
        "memory": {
        "limit": 100000,
        "reservation": 200000
        },
        "devices": [
            {
                "allow": false,
                "access": "rwm"
            }
        ]
   }

Device whitelist

devices (array of objects, OPTIONAL) configures the device whitelist. The runtime MUST apply entries in the listed order.

Each entry has the following structure:

allow (boolean, REQUIRED) - whether the entry is allowed or denied.
type (string, OPTIONAL) - type of device: a (all), c (char), or b (block). Unset values mean "all", mapping to a.
major, minor (int64, OPTIONAL) - major, minor numbers for the device. Unset values mean "all", mapping to * in the filesystem API.
access (string, OPTIONAL) - cgroup permissions for device. A composition of r (read), w (write), and m (mknod).

Example

    "devices": [
        {
            "allow": false,
            "access": "rwm"
        },
        {
            "allow": true,
            "type": "c",
            "major": 10,
            "minor": 229,
            "access": "rw"
        },
        {
            "allow": true,
            "type": "b",
            "major": 8,
            "minor": 0,
            "access": "r"
        }
    ]

Memory

memory (object, OPTIONAL) represents the cgroup subsystem memory and it's used to set limits on the container's memory usage. For more information, see the kernel cgroups documentation about memory.

Values for memory specify the limit in bytes, or -1 for unlimited memory.

limit (int64, OPTIONAL) - sets limit of memory usage
reservation (int64, OPTIONAL) - sets soft limit of memory usage
swap (int64, OPTIONAL) - sets limit of memory+Swap usage
kernel (int64, OPTIONAL) - sets hard limit for kernel memory
kernelTCP (int64, OPTIONAL) - sets hard limit for kernel TCP buffer memory

The following properties do not specify memory limits, but are covered by the memory controller:

swappiness (uint64, OPTIONAL) - sets swappiness parameter of vmscan (See sysctl's vm.swappiness) The values are from 0 to 100. Higher means more swappy.
disableOOMKiller (bool, OPTIONAL) - enables or disables the OOM killer. If enabled (false), tasks that attempt to consume more memory than they are allowed are immediately killed by the OOM killer. The OOM killer is enabled by default in every cgroup using the memory subsystem. To disable it, specify a value of true.

Example

    "memory": {
        "limit": 536870912,
        "reservation": 536870912,
        "swap": 536870912,
        "kernel": -1,
        "kernelTCP": -1,
        "swappiness": 0,
        "disableOOMKiller": false
    }

CPU

cpu (object, OPTIONAL) represents the cgroup subsystems cpu and cpusets. For more information, see the kernel cgroups documentation about cpusets.

The following parameters can be specified to set up the controller:

shares (uint64, OPTIONAL) - specifies a relative share of CPU time available to the tasks in a cgroup
quota (int64, OPTIONAL) - specifies the total amount of time in microseconds for which all tasks in a cgroup can run during one period (as defined by period below)
period (uint64, OPTIONAL) - specifies a period of time in microseconds for how regularly a cgroup's access to CPU resources should be reallocated (CFS scheduler only)
realtimeRuntime (int64, OPTIONAL) - specifies a period of time in microseconds for the longest continuous period in which the tasks in a cgroup have access to CPU resources
realtimePeriod (uint64, OPTIONAL) - same as period but applies to realtime scheduler only
cpus (string, OPTIONAL) - list of CPUs the container will run in
mems (string, OPTIONAL) - list of Memory Nodes the container will run in

Example

    "cpu": {
        "shares": 1024,
        "quota": 1000000,
        "period": 500000,
        "realtimeRuntime": 950000,
        "realtimePeriod": 1000000,
        "cpus": "2-3",
        "mems": "0-7"
    }

Block IO

blockIO (object, OPTIONAL) represents the cgroup subsystem blkio which implements the block IO controller. For more information, see the kernel cgroups documentation about blkio.

The following parameters can be specified to set up the controller:

weight (uint16, OPTIONAL) - specifies per-cgroup weight. This is default weight of the group on all devices until and unless overridden by per-device rules.
leafWeight (uint16, OPTIONAL) - equivalents of weight for the purpose of deciding how much weight tasks in the given cgroup has while competing with the cgroup's child cgroups.
weightDevice (array of objects, OPTIONAL) - an array of per-device bandwidth weights. Each entry has the following structure:
- major, minor (int64, REQUIRED) - major, minor numbers for device. For more information, see the mknod(1) man page.
- weight (uint16, OPTIONAL) - bandwidth weight for the device.
- leafWeight (uint16, OPTIONAL) - bandwidth weight for the device while competing with the cgroup's child cgroups, CFQ scheduler only
You MUST specify at least one of weight or leafWeight in a given entry, and MAY specify both.
throttleReadBpsDevice, throttleWriteBpsDevice (array of objects, OPTIONAL) - an array of per-device bandwidth rate limits. Each entry has the following structure:
- major, minor (int64, REQUIRED) - major, minor numbers for device. For more information, see the mknod(1) man page.
- rate (uint64, REQUIRED) - bandwidth rate limit in bytes per second for the device
throttleReadIOPSDevice, throttleWriteIOPSDevice (array of objects, OPTIONAL) - an array of per-device IO rate limits. Each entry has the following structure:
- major, minor (int64, REQUIRED) - major, minor numbers for device. For more information, see the mknod(1) man page.
- rate (uint64, REQUIRED) - IO rate limit for the device

Example

    "blockIO": {
        "weight": 10,
        "leafWeight": 10,
        "weightDevice": [
            {
                "major": 8,
                "minor": 0,
                "weight": 500,
                "leafWeight": 300
            },
            {
                "major": 8,
                "minor": 16,
                "weight": 500
            }
        ],
        "throttleReadBpsDevice": [
            {
                "major": 8,
                "minor": 0,
                "rate": 600
            }
        ],
        "throttleWriteIOPSDevice": [
            {
                "major": 8,
                "minor": 16,
                "rate": 300
            }
        ]
    }

Huge page limits

hugepageLimits (array of objects, OPTIONAL) represents the hugetlb controller which allows to limit the HugeTLB usage per control group and enforces the controller limit during page fault. For more information, see the kernel cgroups documentation about HugeTLB.

Each entry has the following structure:

pageSize (string, REQUIRED) - hugepage size
limit (uint64, REQUIRED) - limit in bytes of hugepagesize HugeTLB usage

Example

    "hugepageLimits": [
        {
            "pageSize": "2MB",
            "limit": 209715200
        }
   ]

Network

network (object, OPTIONAL) represents the cgroup subsystems net_cls and net_prio. For more information, see the kernel cgroups documentations about net_cls cgroup and net_prio cgroup.

The following parameters can be specified to set up the controller:

classID (uint32, OPTIONAL) - is the network class identifier the cgroup's network packets will be tagged with
priorities (array of objects, OPTIONAL) - specifies a list of objects of the priorities assigned to traffic originating from processes in the group and egressing the system on various interfaces. The following parameters can be specified per-priority:
- name (string, REQUIRED) - interface name in runtime network namespace
- priority (uint32, REQUIRED) - priority applied to the interface

Example

    "network": {
        "classID": 1048577,
        "priorities": [
            {
                "name": "eth0",
                "priority": 500
            },
            {
                "name": "eth1",
                "priority": 1000
            }
        ]
   }

PIDs

pids (object, OPTIONAL) represents the cgroup subsystem pids. For more information, see the kernel cgroups documentation about pids.

The following parameters can be specified to set up the controller:

limit (int64, REQUIRED) - specifies the maximum number of tasks in the cgroup

Example

    "pids": {
        "limit": 32771
   }

IntelRdt

intelRdt (object, OPTIONAL) represents the Intel Resource Director Technology. If intelRdt is set, the runtime MUST write the container process ID to the <container-id>/tasks file in a mounted resctrl pseudo-filesystem, using the container ID from start and creating the <container-id> directory if necessary. If no mounted resctrl pseudo-filesystem is available in the runtime mount namespace, the runtime MUST generate an error.

If `intelRdt` is not set, the runtime MUST NOT manipulate any `resctrl` pseudo-filesystems.

The following parameters can be specified for the container:

l3CacheSchema (string, OPTIONAL) - specifies the schema for L3 cache id and capacity bitmask (CBM). If l3CacheSchema is set, runtimes MUST write the value to the schemata file in the <container-id> directory discussed in intelRdt.

If l3CacheSchema is not set, runtimes MUST NOT write to schemata files in any resctrl pseudo-filesystems.

Example

Consider a two-socket machine with two L3 caches where the default CBM is 0xfffff and the max CBM length is 20 bits. Tasks inside the container only have access to the "upper" 80% of L3 cache id 0 and the "lower" 50% L3 cache id 1:

"linux": {
    "intelRdt": {
        "l3CacheSchema": "L3:0=ffff0;1=3ff"
    }
}

Sysctl

sysctl (object, OPTIONAL) allows kernel parameters to be modified at runtime for the container. For more information, see the sysctl(8) man page.

Example

    "sysctl": {
        "net.ipv4.ip_forward": "1",
        "net.core.somaxconn": "256"
   }

Seccomp

Seccomp provides application sandboxing mechanism in the Linux kernel. Seccomp configuration allows one to configure actions to take for matched syscalls and furthermore also allows matching on values passed as arguments to syscalls. For more information about Seccomp, see Seccomp kernel documentation. The actions, architectures, and operators are strings that match the definitions in seccomp.h from libseccomp and are translated to corresponding values.

seccomp (object, OPTIONAL)

The following parameters can be specified to set up seccomp:

defaultAction (string, REQUIRED) - the default action for seccomp. Allowed values are the same as syscalls[].action.
architectures (array of strings, OPTIONAL) - the architecture used for system calls. A valid list of constants as of libseccomp v2.3.2 is shown below.
- SCMP_ARCH_X86
- SCMP_ARCH_X86_64
- SCMP_ARCH_X32
- SCMP_ARCH_ARM
- SCMP_ARCH_AARCH64
- SCMP_ARCH_MIPS
- SCMP_ARCH_MIPS64
- SCMP_ARCH_MIPS64N32
- SCMP_ARCH_MIPSEL
- SCMP_ARCH_MIPSEL64
- SCMP_ARCH_MIPSEL64N32
- SCMP_ARCH_PPC
- SCMP_ARCH_PPC64
- SCMP_ARCH_PPC64LE
- SCMP_ARCH_S390
- SCMP_ARCH_S390X
- SCMP_ARCH_PARISC
- SCMP_ARCH_PARISC64
syscalls (array of objects, OPTIONAL) - match a syscall in seccomp.

While this property is OPTIONAL, some values of defaultAction are not useful without syscalls entries. For example, if defaultAction is SCMP_ACT_KILL and syscalls is empty or unset, the kernel will kill the container process on its first syscall.

Each entry has the following structure:
- names (array of strings, REQUIRED) - the names of the syscalls. names MUST contain at least one entry.
- action (string, REQUIRED) - the action for seccomp rules. A valid list of constants as of libseccomp v2.3.2 is shown below.
  - SCMP_ACT_KILL
  - SCMP_ACT_TRAP
  - SCMP_ACT_ERRNO
  - SCMP_ACT_TRACE
  - SCMP_ACT_ALLOW
- args (array of objects, OPTIONAL) - the specific syscall in seccomp.
  
  Each entry has the following structure:
  - index (uint, REQUIRED) - the index for syscall arguments in seccomp.
  - value (uint64, REQUIRED) - the value for syscall arguments in seccomp.
  - valueTwo (uint64, OPTIONAL) - the value for syscall arguments in seccomp.
  - op (string, REQUIRED) - the operator for syscall arguments in seccomp. A valid list of constants as of libseccomp v2.3.2 is shown below.
    - SCMP_CMP_NE
    - SCMP_CMP_LT
    - SCMP_CMP_LE
    - SCMP_CMP_EQ
    - SCMP_CMP_GE
    - SCMP_CMP_GT
    - SCMP_CMP_MASKED_EQ

Example

    "seccomp": {
        "defaultAction": "SCMP_ACT_ALLOW",
        "architectures": [
            "SCMP_ARCH_X86",
            "SCMP_ARCH_X32"
        ],
        "syscalls": [
            {
                "names": [
                    "getcwd",
                    "chmod"
                ],
                "action": "SCMP_ACT_ERRNO"
            }
        ]
    }

Rootfs Mount Propagation

rootfsPropagation (string, OPTIONAL) sets the rootfs's mount propagation. Its value is either slave, private, shared or unbindable. The Shared Subtrees article in the kernel documentation has more information about mount propagation.

Example

    "rootfsPropagation": "slave",

Masked Paths

maskedPaths (array of strings, OPTIONAL) will mask over the provided paths inside the container so that they cannot be read. The values MUST be absolute paths in the container namespace.

Example

    "maskedPaths": [
        "/proc/kcore"
    ]

Readonly Paths

readonlyPaths (array of strings, OPTIONAL) will set the provided paths as readonly inside the container. The values MUST be absolute paths in the container namespace.

Example

    "readonlyPaths": [
        "/proc/sys"
    ]

Mount Label

mountLabel (string, OPTIONAL) will set the Selinux context for the mounts in the container.

Example

    "mountLabel": "system_u:object_r:svirt_sandbox_file_t:s0:c715,c811"

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