unshare(2) [linux man page]
UNSHARE(2) Linux Programmer's Manual UNSHARE(2) NAME
unshare - disassociate parts of the process execution context SYNOPSIS
define _GNU_SOURCE /* See feature_test_macros(7) */ #include <sched.h> int unshare(int flags); DESCRIPTION
unshare() allows a process to disassociate parts of its execution context that are currently being shared with other processes. Part of the execution context, such as the mount namespace, is shared implicitly when a new process is created using fork(2) or vfork(2), while other parts, such as virtual memory, may be shared by explicit request when creating a process using clone(2). The main use of unshare() is to allow a process to control its shared execution context without creating a new process. The flags argument is a bit mask that specifies which parts of the execution context should be unshared. This argument is specified by ORing together zero or more of the following constants: CLONE_FILES Reverse the effect of the clone(2) CLONE_FILES flag. Unshare the file descriptor table, so that the calling process no longer shares its file descriptors with any other process. CLONE_FS Reverse the effect of the clone(2) CLONE_FS flag. Unshare file system attributes, so that the calling process no longer shares its root directory, current directory, or umask attributes with any other process. chroot(2), chdir(2), or umask(2) CLONE_NEWNS This flag has the same effect as the clone(2) CLONE_NEWNS flag. Unshare the mount namespace, so that the calling process has a pri- vate copy of its namespace which is not shared with any other process. Specifying this flag automatically implies CLONE_FS as well. If flags is specified as zero, then unshare() is a no-op; no changes are made to the calling process's execution context. RETURN VALUE
On success, zero returned. On failure, -1 is returned and errno is set to indicate the error. ERRORS
EINVAL An invalid bit was specified in flags. ENOMEM Cannot allocate sufficient memory to copy parts of caller's context that need to be unshared. EPERM flags specified CLONE_NEWNS but the calling process was not privileged (did not have the CAP_SYS_ADMIN capability). VERSIONS
The unshare() system call was added to Linux in kernel 2.6.16. CONFORMING TO
The unshare() system call is Linux-specific. NOTES
Not all of the process attributes that can be shared when a new process is created using clone(2) can be unshared using unshare(). In par- ticular, as at kernel 2.6.16, unshare() does not implement flags that reverse the effects of CLONE_SIGHAND, CLONE_SYSVSEM, CLONE_THREAD, or CLONE_VM. Such functionality may be added in the future, if required. SEE ALSO
clone(2), fork(2), vfork(2), Documentation/unshare.txt COLOPHON
This page is part of release 3.27 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/. Linux 2010-09-10 UNSHARE(2)
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UNSHARE(1) User Commands UNSHARE(1) NAME
unshare - run program with some namespaces unshared from parent SYNOPSIS
unshare [options] [program [arguments]] DESCRIPTION
Unshares the indicated namespaces from the parent process and then executes the specified program. If program is not given, then ``${SHELL}'' is run (default: /bin/sh). The namespaces can optionally be made persistent by bind mounting /proc/pid/ns/type files to a filesystem path and entered with nsenter(1) even after the program terminates (except PID namespaces where permanently running init process is required). Once a persistent namespace is no longer needed, it can be unpersisted with umount(8). See the EXAMPLES section for more details. The namespaces to be unshared are indicated via options. Unshareable namespaces are: mount namespace Mounting and unmounting filesystems will not affect the rest of the system, except for filesystems which are explicitly marked as shared (with mount --make-shared; see /proc/self/mountinfo or findmnt -o+PROPAGATION for the shared flags). For further details, see mount_namespaces(7) and the discussion of the CLONE_NEWNS flag in clone(2). unshare since util-linux version 2.27 automatically sets propagation to private in a new mount namespace to make sure that the new namespace is really unshared. It's possible to disable this feature with option --propagation unchanged. Note that private is the kernel default. UTS namespace Setting hostname or domainname will not affect the rest of the system. For further details, see namespaces(7) and the discussion of the CLONE_NEWUTS flag in clone(2). IPC namespace The process will have an independent namespace for POSIX message queues as well as System V message queues, semaphore sets and shared memory segments. For further details, see namespaces(7) and the discussion of the CLONE_NEWIPC flag in clone(2). network namespace The process will have independent IPv4 and IPv6 stacks, IP routing tables, firewall rules, the /proc/net and /sys/class/net direc- tory trees, sockets, etc. For further details, see namespaces(7) and the discussion of the CLONE_NEWNET flag in clone(2). PID namespace Children will have a distinct set of PID-to-process mappings from their parent. For further details, see pid_namespaces(7) and the discussion of the CLONE_NEWPID flag in clone(2). cgroup namespace The process will have a virtualized view of /proc/self/cgroup, and new cgroup mounts will be rooted at the namespace cgroup root. For further details, see cgroup_namespaces(7) and the discussion of the CLONE_NEWCGROUP flag in clone(2). user namespace The process will have a distinct set of UIDs, GIDs and capabilities. For further details, see user_namespaces(7) and the discussion of the CLONE_NEWUSER flag in clone(2). OPTIONS
-i, --ipc[=file] Unshare the IPC namespace. If file is specified, then a persistent namespace is created by a bind mount. -m, --mount[=file] Unshare the mount namespace. If file is specified, then a persistent namespace is created by a bind mount. Note that file has to be located on a filesystem with the propagation flag set to private. Use the command findmnt -o+PROPAGATION when not sure about the current setting. See also the examples below. -n, --net[=file] Unshare the network namespace. If file is specified, then a persistent namespace is created by a bind mount. -p, --pid[=file] Unshare the PID namespace. If file is specified then persistent namespace is created by a bind mount. See also the --fork and --mount-proc options. -u, --uts[=file] Unshare the UTS namespace. If file is specified, then a persistent namespace is created by a bind mount. -U, --user[=file] Unshare the user namespace. If file is specified, then a persistent namespace is created by a bind mount. -C, --cgroup[=file] Unshare the cgroup namespace. If file is specified then persistent namespace is created by bind mount. -f, --fork Fork the specified program as a child process of unshare rather than running it directly. This is useful when creating a new PID namespace. --mount-proc[=mountpoint] Just before running the program, mount the proc filesystem at mountpoint (default is /proc). This is useful when creating a new PID namespace. It also implies creating a new mount namespace since the /proc mount would otherwise mess up existing programs on the system. The new proc filesystem is explicitly mounted as private (with MS_PRIVATE|MS_REC). -r, --map-root-user Run the program only after the current effective user and group IDs have been mapped to the superuser UID and GID in the newly cre- ated user namespace. This makes it possible to conveniently gain capabilities needed to manage various aspects of the newly created namespaces (such as configuring interfaces in the network namespace or mounting filesystems in the mount namespace) even when run unprivileged. As a mere convenience feature, it does not support more sophisticated use cases, such as mapping multiple ranges of UIDs and GIDs. This option implies --setgroups=deny. --propagation private|shared|slave|unchanged Recursively set the mount propagation flag in the new mount namespace. The default is to set the propagation to private. It is possible to disable this feature with the argument unchanged. The option is silently ignored when the mount namespace (--mount) is not requested. --setgroups allow|deny Allow or deny the setgroups(2) system call in a user namespace. To be able to call setgroups(2), the calling process must at least have CAP_SETGID. But since Linux 3.19 a further restriction applies: the kernel gives permission to call setgroups(2) only after the GID map (/proc/pid/gid_map) has been set. The GID map is writable by root when setgroups(2) is enabled (i.e. allow, the default), and the GID map becomes writable by unprivileged processes when setgroups(2) is permanently disabled (with deny). -V, --version Display version information and exit. -h, --help Display help text and exit. NOTES
The proc and sysfs filesystems mounting as root in a user namespace have to be restricted so that a less privileged user can not get more access to sensitive files that a more privileged user made unavailable. In short the rule for proc and sysfs is as close to a bind mount as possible. EXAMPLES
# unshare --fork --pid --mount-proc readlink /proc/self 1 Establish a PID namespace, ensure we're PID 1 in it against a newly mounted procfs instance. $ unshare --map-root-user --user sh -c whoami root Establish a user namespace as an unprivileged user with a root user within it. # touch /root/uts-ns # unshare --uts=/root/uts-ns hostname FOO # nsenter --uts=/root/uts-ns hostname FOO # umount /root/uts-ns Establish a persistent UTS namespace, and modify the hostname. The namespace is then entered with nsenter. The namespace is destroyed by unmounting the bind reference. # mount --bind /root/namespaces /root/namespaces # mount --make-private /root/namespaces # touch /root/namespaces/mnt # unshare --mount=/root/namespaces/mnt Establish a persistent mount namespace referenced by the bind mount /root/namespaces/mnt. This example shows a portable solution, because it makes sure that the bind mount is created on a shared filesystem. SEE ALSO
clone(2), unshare(2), namespaces(7), mount(8) AUTHORS
Mikhail Gusarov <dottedmag@dottedmag.net> Karel Zak <kzak@redhat.com> AVAILABILITY
The unshare command is part of the util-linux package and is available from https://www.kernel.org/pub/linux/utils/util-linux/. util-linux February 2016 UNSHARE(1)