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LXC 4.0 LTS has been released

Mar 25, 2020


The LXC team is pleased to announce the release of LXC 4.0.0!

This is the result of two years of work since the LXC 3.0.0 release and is the fourth LTS release for the LXC project. This release will be supported until June 2025.

Major changes

cgroups: Full cgroup2 support

LXC 4.0 now fully supports the unified cgroup hierarchy. For this to work the whole cgroup driver had to be rewritten. A consequence of this work is that the cgroup layout for LXC containers had to be changed. Older versions of LXC used the layout:


For example, in the legacy cgroup hierarchy the cpuset hierarchy would place the container 's init process into


The supervising monitor process would stay in


LXC 4.0 uses the layout:


For the cpuset controller in the legacy cgroup hierarchy for the container f2 the cgroup would be:


The monitor process now moves into a separate cgroup as well:


For our example this would be:


The monitor's and the container's cgroup will be placed on the same level in the corresponding cgroup hierarchy.
These changes apply to the legacy and the unified hierarchy alike and are not arbitrary. The new, unified cgroup hierarchy imposes specific restrictions where and how a process can be migrated in the cgroup hierarchy. The most important restriction is the leaf-node restriction. This means that only leaf nodes can contain live processes, i.e. if you have the following cgroup tree


then only f2-container can contain live processes whereas non-leaf nodes a and f2-monitor do not. This has the consequence that the old cgroup layout LXC used where the monitor process would have lived in f2-monitor and the container's init process would have lived in f2-container is not possible anymore. The kernel disallows this layout. Instead, the monitor process and the container's init process need to be moved into two leaf-node cgroups on the same level in the cgroup hierarchy. This would mean for a container f2 the layout will be:




The restrictions enforced by the unified cgroup hierarchy also mean, that in order to start fully unprivileged containers cooperation is needed on distributions that make use of an init system which manages cgroups. This applies to all distributions that use systemd as their init system. When a container is started from the shell via lxc-start or other means one either needs to be root to allow LXC to escape to the root cgroup or the init system needs to be instructed to delegate an empty cgroup. In such scenarios it is wise to set the configuration key lxc.cgroup.relative to 1 to prevent LXC from escaping to the root cgroup.

cgroups: Freezer support in CGroup2

As part of the cgroup2 support work for LXC 4.0 we also added support for cgroup2's implementation of the freezer controller which allows to poll until the cgroup is frozen or unfrozen making freezing and unfreezing container's way more reliable than before.

cgroups: eBPF device controller support in CGroup2

LXC 4.0 can now make proper use of the cgroup2 device controller. It will automatically create, load, and attach a eBPF program to the container's cgroup and supports dynamic additional and removal of rules. The configuration format is the same as for the legacy cgroup controller. Only the lxc.cgroup2.devices. prefix instead of the legacy lxc.cgroup.devices prefix needs to be used. LXC continues to support both black- and whitelists.

AppArmor: Deny access to /proc/acpi/**

The default AppArmor profile now denies access to /proc/acpi/ improving safety.

config: Add lxc.autodev.tmpfs.size configuration key

LXC supports creating a usable minimal /dev directory for the container by setting lxc.autodev = 1 in the container's config file. To do this LXC sets up a tmpfs mount on /dev. This tmpfs mount could not be restricted in prior releases. Now it is possible to set a limit on the size of the tmpfs mount by setting lxc.autodev.tmpfs.size to the number of bytes that the tmpfs should be restricted to use.

config: Add lxc.selinux.context.keyring key

This allows to specify the selinux context to be used for the keyring the container uses.

config: Add lxc.keyring.session

Setting this to 1 (default) will cause LXC to create a new session keyring.

file utils: Add fopen_cached() and fdopen_cached

These helpers first read a whole file and then make it available as a stream to be read via regular file-based libc apis. This makes LXC's handling of various files more robust where the underlying file can change while it is read.

api: Add new init_pidfd() member

LXC 4.0 fully supports the new pidfd kernel api the LXC team has merged in the upstream Linux kernel. The pidfd of the container's init process can be requested via c->init_pidfd(c).

memory utils: Add new cleanup api

LXC 4.0 expands the usage of the compiler's cleanup attribute by introducing new internal apis to define and call cleanup macros for complex resource allocations. We have had extremely positive results decreasing bugs around file descriptor and memory leaks significantly by switching to this new way of cleaning up resources.

lxc-usernsexec: Make it easy to map own uid

The lxc-usernsexec binary now finds a default mapping as specified in /etc/subuid and /etc/subgid and writes it via newuidmap and newgidmap.

seccomp: Add s390 support

LXC 4.0's seccomp implementation now supports s390 as architecture.

syscalls: Improve manual syscall implementations

Whenever a given syscall is not supported or exposed by the underlying C library of the system LXC will define syscall stubs for important syscalls or new features it deems extremely valuable. This used to be done by checking for __NR_<syscall-name> being defined. But __NR_<syscall-name> being defined depended on the correct headers for the currently running kernel LXC was compiled on being installed and would be problematic whenever LXC was compiled on a system running an older kernel but used or deployed on systems that use a new kernel. In such scenarios LXC could not make use of new kernel features even though it should. We now introduce definitions for __NR_<syscall-name> whenever the system does not define it already and it is an architecture we support (which is basically any architecture). This way we better handle kernel <-> header version mismatches and compilation <-> deployment kernel mismatches.

network: Improved network device creation and removal

We have reworked how network devices are created, tracked, moved between network namespaces, and are removed making low-level network management way more reliable.

network: Allow moving wireless devices

LXC allowed to move wireless network devices (nl80211) into containers. This was broken for a while. With 4.0 the ability to move wireless network devices is restored and improved.

Complete changelog

Here is a complete list of all changes in this release:

Full commit list
  • cgroups: fix attaching to the unified cgroup
  • dir: improve dir backend
  • dir: use cleanup macro in dir_mount()
  • tree-wide: harden mount option parsing
  • lxc_init: add missing O_CLOEXEC
  • lxc_init: move main() down
  • Reset devel flag post-release
  • make dist: add missing files
  • lxc-download: Pre-release bump of compat
  • conf: fix read-only bind mounts
  • utils: allow removal of immutable files
  • lxc-local: remove -l/--list from help
  • lvm: don't generate uuid for ext4 snapshots
  • lxc-update-config: handle lxc.rootfs.backend correctly
  • lxc_copy: only overmount overlay subdirectory with tmpfs
  • overlay: rewrite and simplify
  • lxc-user-nic: enable uid-marked veth devices for uids with 5 digits
  • network: introduce lxc_ifname_alnum_case_sensitive()
  • log: fix cmd logging
  • cgroups: simplify
  • ringbuf: fix cleanup operations
  • mainloop: cleanup
  • log: add missing variable and fix CMD_SYSINFO()
  • log: cleanup
  • log: add missing \
  • start: move reading seccomp profile after pre-start hook
  • lxc_user_nic: rework device creation
  • nl: improve how we surface errors
  • network: use cleanup macros
  • network: use cleanup attributes
  • network: cleanup galore
  • network: use is_empty_string() everywhere
  • network: fix ovs removal
  • log: use global variable to catch statements in loggers
  • cgroups: don't call statements from loggers
  • conf: flatten logic in mount_entry()
  • conf: don't accidently double-mount
  • network: fix moving network devices with custom name
  • network: introduce and use is_empty_string()
  • Makefile: fix typo
  • lxc-unshare: add syscall_wrappers.h to build requirements
  • tree-wide: introduce and use syscall number header
  • raw_syscalls: define __NR_pidfd_send_signal if missing
  • tools: fix -g -u parameters for lxc-execute and lxc-attach
  • ISSUE_TEMPLATE: fix -l -o order
  • lxc_user_nic: don't depend on MAP_FIXED
  • busybox: Mark mqueue optional
  • Auto-create /dev/shm and /dev/mqueue
  • busybox: Fix bad lxc.mount.entry
  • doc: Fix grammar
  • Trigger the mounting of shm file system
  • tree-wide: s/lxc_fini()/lxc_end()/g
  • tree-wide: remove "name" argument from lxc_{fini,abort}()
  • {_}lxc_start: remove "name" argument
  • start: add missing TRACE() call
  • start: better goto target naming in __lxc_start()
  • start: rework cleanup code in __lxc_start()
  • start: simplify lxc_init()
  • conf: don't wrap strings
  • tree-wide: remove last -1 fd initialization with cleanup macros in favor of -EBADF
  • tree-wide: s/__do_close_prot_errno/__do_close/g
  • memory_utils: adapt to new infrastructure
  • tree-wide: port cgroup cleanup to call_cleaner(cgroup_exit)
  • caps: port to call_cleaner() based cleanup
  • memory_utils: add call_cleaner() helper
  • travis: enable all architectures
  • travis: remove libgnutls-dev
  • utils: cleanup
  • file_utils: cleanup macros and improvements
  • api-extensions: use correct headings
  • api-extensions: document "network_veth_router" api extension
  • api-extensions: reflow "seccomp_allow_nesting" api extension
  • api-extensions: reflow "seccomp_notify" api extension
  • api-extensions: reflow "cgroup2_devices" extensions
  • api-extensions: reflow "cgroup2" api extension
  • api-extensions: add "pidfd" api extension
  • lxccontainer: switch to pidfd polling when shutting down containers
  • lxccontainer: switch to pidfds whenever possible
  • start: add ability to detect whether kernel supports pidfds
  • lxccontainer: add init_pidfd() API extension
  • commands: LXC_CMD_GET_INIT_PIDFD
  • lxccontainer.h: document seccomp_notify_fd()
  • commands: use LXC_CMD_REAP_CLIENT_FD in lxc_cmd_get_cgroup2_fd_callback()
  • commands: add ability to audit fd connection and cleanup path
  • doc: Fix typo
  • doc: Add keyring options to Japanese lxc.containers.conf(5)
  • commands: simplify lxc_cmd_fd_cleanup()
  • commands_utils: fix command socket hashing
  • af_unix: fix return value
  • start: cleanup file descriptor closing
  • commands: make sure to always close the client fd
  • commands: improve state client cleanup
  • commands: switch to pid_t to send around pid
  • share_ns: improve error handling
  • share_ns: improve error handling
  • file_utils: handle libcs without fmemopen()
  • cgroups: cleanup
  • cgfsng: use __do_free_string_list all over
  • file_utils: include stdio.h for fmemopen()
  • tests/share_ns: always call pthread_exit()
  • memory_utils: remove unneeded inclusion of mntent.h
  • cgroups: fix memory leak and simplify code
  • tests/share_ns: bugfixes
  • conf: cleanup
  • commands_utils: cleanup
  • commands: cleanup
  • tree-wide: more cleanup macros
  • lxccontainer: increase cleanup macro usage
  • autotools: fix lxc-init build with clang-10
  • tree-wide: improve logging
  • tree-wide: make files cloexec whenever possible
  • attach: cleanup various helpers
  • attach: use logging helpers when handling no new privileges
  • attach: use cleanup macros and logging helpers when fetching seccomp
  • attach: use LXC_INVALID_{G,U}ID macros
  • attach: use cleanup macros in lxc_attach_getpwshell()
  • attach: fix fd leak
  • attach: cleanup
  • cgroup2_devices: fix logic error
  • commands: remove unused variables
  • commands_utils: fix socket leak when adding state client
  • commands_utils: indicate taking ownership of state_client_fd in
  • lxc_add_state_client()
  • commands_utils: fix socket leak in when adding state client
  • af_unix: cleanup
  • network: Uses netlink for IP neighbour proxy management
  • utils: only move_fd() when fdopen() has been successful
  • api-extensions: document cgroup2_devices and cgroup2 api extensions
  • src/lxc/raw_syscalls.c: fix sparc assembly
  • cgroups: honor lxc.cgroup.pattern if set explicitly II
  • cgroups: honor lxc.cgroup.pattern if set explicitly
  • cgroups: remove unused method and cleanup cgroup_exit()
  • tree-wide: improve setgroups() dropping
  • lxclock: fix a small memory leak
  • container.conf: Document that order is important in config_jump_table
  • container.conf: Fix option ordering in config_jump_table
  • Currently lxc.selinux.context.keyring is placed after
  • container.conf: Fix off by 2 in option parsing
  • doc: Add doc for keyring options
  • container.conf: Add option to disable session keyring creation
  • container.conf: Add option to set keyring SELinux context
  • cgroups: fix default cgroup pattern
  • start: fix container killing logic
  • network: Restore fixed MTU functionality
  • test: increase timeout for api reboot tests
  • cgroup.c: fix memory leak at cgroup init failed
  • network: rework network device creation
  • network: fix network device removal
  • tests: log api reboot test failures
  • network: fix typ and formatting in comment
  • network: improve veth device creation
  • start: handle kernel header and kernel incompatability
  • tests: timeout after 60 seconds
  • mainloop: add missing \n
  • Suppress useless udhcpc directory
  • start: remove procfs pidfd support
  • create_run_template(): Double "will mount" in a comment
  • cmd: fix shebang
  • travis: enable -fsanitize=undefined
  • fd: only add valid fd to mainloop
  • seccomp: support s390 seccomp
  • api_extensions: advertise cgroup2 support
  • cgroups/cgfsng: do not prematurely close file descriptors
  • cgroups/cgfsng: improve cgroup creation and removal
  • cgroups/cgfsng: rework cgroup removal
  • cgroups/cgfsng: rework legacy cpuset handling
  • cgroupfs/cgfsng: pass cgroup to cg_legacy_handle_cpuset_hierarchy() as const char *
  • cgroups: use explicit unsigned type for bitfield
  • cgroups: flatten hierarchy
  • file_utils: use O_NOCTTY | O_NOFOLLOW
  • cgroups/devices: enable devpath semantics for cgroup2 device controller
  • cgroups/cgfsng: replace lxc_write_file()
  • cgroups/cgfsng: cgfsng_devices_activate()
  • cgroups/cgfsng: rework cgfsng_nrtasks()
  • cgroups/cgfsng: rework cgfsng_mount()
  • cgroups/cgfsng: rework cgfsng_chown()
  • cgroups/cgfsng: rework cgfsng_attach()
  • cgroups/cgfsng: rework cgfsng_setup_limits()
  • cgroups/cgfsng: rework cgfsng_setup_limits_legacy()
  • cgroups/cgfsng: rework cgfsng_{get,set}()
  • cgroups/cgfsng: rework cgfsng_unfreeze()
  • cgroups/cgfsng: rework cgfsng_get_hierarchies()
  • cgroups/cgfsng: rework cgfsng_num_hierarchies()
  • cgroups/cgfsng: rework cgfsng_escape()
  • cgroups/cgfsng: rework cgfsng_payload_enter()
  • cgroups/cgfsng: rework cgfsng_payload_create()
  • tree-wide: s/__unused/__lxc_unused/g
  • cgroups/cgfsng: rework cgroup attach
  • cgroups/cgfsng: don't dereference NULL-pointer
  • cgroups/cgfsng: log chown_cgroup_wrapper()
  • cgroups/cgfsng: rework cgroup2 unprivileged delegation
  • cgroups/cgfsng: rework cgfsng_{monitor,payload}_delegate_controllers()
  • cgroups/cgfsng: rework cgfsng_monitor_enter()
  • cgroups/cgfsng: rework cgfsng_monitor_create()
  • cgroups/cgfsng: rework cgfsng_monitor_destroy()
  • cgroups/cgfsng: rework cgfsng_payload_destroy()
  • log: remove unused compiler attribute
  • start: replace compiler attributes
  • log: replace compiler attributes
  • attach: replace closing helpers
  • compiler: add __unused attribute
  • {log, macro}: remove unused logging functions
  • lxccontainer: replace logging functions
  • confile_utils: replace logging functions
  • cgroups: rework return values of some functions
  • cgroups/cgroup2_devices: replace logging functions
  • cgroups/cgroup: replace logging functions
  • cgroups/cgfsng: replace logging functions
  • confile: replace logging helpers
  • network: replace logging helpers
  • commands: replace logging helpers
  • attach: s/minus_one_set_errno(/ret_set_errno(-1, /g
  • af_unix: s/minus_one_set_errno(/ret_set_errno(-1, /g
  • macro: add ret_errno()
  • log: rearrange
  • cgroup2: rework controller delegation
  • "busy" field set to -1 instead of 0
  • "busy" field set to 1 instead of 0
  • Init "busy" field to -1 as 0 is valid fd
  • config: Fix parsing of mount options
  • cgroups/devices: correctly verify bpf device useability in cgfsng_devices_activate()
  • cgroups: improve container cgroup attaching
  • lxc: switch to SPDX
  • commands: use logging return helpers
  • cgfsng: rework cgroup2 attach
  • cgroups/devices: do not log error when bpf device feature is not available
  • freezer: cleanup
  • cgroups/freezer: fix and improve cgroup2 freezer implementation
  • cgroups: add DEFAULT_MOUNTPOINT #define
  • cgroups/devices: use dedicated enums
  • cgroups/devices: introduce ebpf device cgroup global rule types
  • cgroups/devices: handle NULL
  • configure: enable -Wunused-but-set-variable
  • cgroups/cgfsng: implement cgroup2 device controller live update
  • conf: record cgroup2 devices in parsed format
  • cgroups/cgfsng: "atomically" replace bpf device programs
  • macro: remove unused macros
  • api_extension: add cgroup2_devices api extension
  • cgroups: add cgroup2 device controller support
  • cgfsng: return attach fail if container stopped
  • conf: fix memory leak for set config rootfs options
  • fix wrong order of bridge/nic in error message
  • Typo in a comment
  • tests: use /dev/loop-control instead of /dev/network_latency
  • fix build on toolchain without SSP
  • Update cgroup.h
  • terminal: prevent returning invalid pointer
  • terminal: make lxc_terminal_signal_fini() static
  • lxc-usernsexec: support easily mapping own uid
  • tests: add tests making sure the exit code is appropriate.
  • terminal: return NULL on error in terminal_signal_init
  • terminal: prevent memory leak for lxc_terminal_state
  • apparmor: Prevent writes to /proc/acpi/**
  • syscall_wrappers: rename internal memfd_create to memfd_create_lxc
  • lxc/tools/lxc/destroy: Restores error message on container destroy
  • Update lxc.containers.conf(5) in Japanese
  • Bad sgml/man translation
  • Add more info about lxc.start.order in Japanese man
  • Add autodev.tmpfs.size to Japanese lxc.container.conf(5)
  • lxc-destroy: send successful output messages to log info instead of error.
  • doc: Add more info about 'lxc.start.order'
  • update obsolete functions
  • Add autodev.tmpfs.size config parameter
  • start: handle setting pdeath signal in new pidns
  • start: pidfds obviously start - like any fd - at 0
  • Fix lxc-update-config in network.address
  • allow users to configure the option --enable-feature or --with-package, if an option is given run shell commands action-if-given
  • Set minimun autoconf version to 2.69 and change obsolete function AC_HELP_STRING for AS_HELP_STRING
  • doc: Add the[i].veth.mode option in Japanese lxc.container.conf(5)
  • doc: Add Japanese pam_cgfs(8) man page
  • doc: add man page for pam_cgfs
  • Ensures OpenSSL compatibility with older versions of EVP API.
  • utils: Copying source filename to avoid missing info.
  • cgroups: unify cgfsng_{un}freeze()
  • cgroups: initialize cgroup root directory - encore
  • cgroups: check for empty cgroups on freeze/unfreeze
  • cgroups: initialize cgroup root directory
  • [aa-profile] Deny access to /proc/acpi/**
  • lxc-attach: make sure exit status of command is returned
  • cgfsng: mount pure unified cgroup layout correctly
  • lxc-create: check absoule path for param '--dir'
  • cgroups: support cgroup2 freezer
  • attach: don't close stdout of getent
  • utils: Fix wrong integer of a function parameter.
  • try to fix search user instead of search substring
  • lxccontainer: do_lxcapi_detach_interface to support detaching wlan devices
  • cgroups: initialize cpuset properly
  • network: restore ability to move nl80211 devices
  • pidfds: don't print a scary warning on ENOSYS
  • tree-wide: initialize all auto-cleanup variables
  • suppress false-negative error in templates and nvidia hook
  • Container's specific file/directory names
  • Use file/directory names from macro.h
  • tree-wide: fix wrong copy-paste for licenses

Support and upgrade

LXC 4.0.0 will be supported until June 2025 and our current LTS release, LXC 3.0 will now switch to a slower maintenance pace, only getting critical bugfixes and security updates.

We strongly recommend all LXC users to plan an upgrade to the 4.0 branch.



The LXC 4.0.0 release was brought to you by a total of 30 contributors.