Manpage of UNSHARE

UNSHARE

Section: Linux Programmer's Manual (2)
Updated: 2016-07-17
Index
 

NAME

unshare - disassociate parts of the process execution context  

SYNOPSIS

#define _GNU_SOURCE#include <sched.h>int unshare(int flags);
 

DESCRIPTION

unshare() allows a process (or thread) to disassociate parts of its execution context that are currently being shared with other processes (or threads). 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 or thread 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 flagsargument 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_FILESflag. 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_FSflag. Unshare filesystem attributes, so that the calling process no longer shares its root directory (chroot(2)), current directory (chdir(2)), or umask (umask(2)) attributes with any other process.
CLONE_NEWCGROUP (since Linux 4.6)
This flag has the same effect as the clone(2) CLONE_NEWCGROUPflag. Unshare the cgroup namespace. Use of CLONE_NEWCGROUPrequires the CAP_SYS_ADMINcapability.
CLONE_NEWIPC (since Linux 2.6.19)
This flag has the same effect as the clone(2) CLONE_NEWIPCflag. Unshare the IPC namespace, so that the calling process has a private copy of the IPC namespace which is not shared with any other process. Specifying this flag automatically implies CLONE_SYSVSEMas well. Use of CLONE_NEWIPCrequires the CAP_SYS_ADMINcapability.
CLONE_NEWNET (since Linux 2.6.24)
This flag has the same effect as the clone(2) CLONE_NEWNETflag. Unshare the network namespace, so that the calling process is moved into a new network namespace which is not shared with any previously existing process. Use of CLONE_NEWNETrequires the CAP_SYS_ADMINcapability.
CLONE_NEWNS
This flag has the same effect as the clone(2) CLONE_NEWNSflag. Unshare the mount namespace, so that the calling process has a private copy of its namespace which is not shared with any other process. Specifying this flag automatically implies CLONE_FSas well. Use of CLONE_NEWNSrequires the CAP_SYS_ADMINcapability. For further information, see mount_namespaces(7).
CLONE_NEWPID (since Linux 3.8)
This flag has the same effect as the clone(2) CLONE_NEWPIDflag. Unshare the PID namespace, so that the calling process has a new PID namespace for its children which is not shared with any previously existing process. The calling process is notmoved into the new namespace. The first child created by the calling process will have the process ID 1 and will assume the role of init(1) in the new namespace. CLONE_NEWPIDautomatically implies CLONE_THREADas well. Use of CLONE_NEWPIDrequires the CAP_SYS_ADMINcapability. For further information, see pid_namespaces(7).
CLONE_NEWUSER (since Linux 3.8)
This flag has the same effect as the clone(2) CLONE_NEWUSERflag. Unshare the user namespace, so that the calling process is moved into a new user namespace which is not shared with any previously existing process. As with the child process created by clone(2) with the CLONE_NEWUSERflag, the caller obtains a full set of capabilities in the new namespace.
CLONE_NEWUSERrequires that the calling process is not threaded; specifying CLONE_NEWUSERautomatically implies CLONE_THREAD. Since Linux 3.9, CLONE_NEWUSERalso automatically implies CLONE_FS. CLONE_NEWUSERrequires that the user ID and group ID of the calling process are mapped to user IDs and group IDs in the user namespace of the calling process at the time of the call.

For further information on user namespaces, see user_namespaces(7).

CLONE_NEWUTS (since Linux 2.6.19)
This flag has the same effect as the clone(2) CLONE_NEWUTSflag. Unshare the UTS IPC namespace, so that the calling process has a private copy of the UTS namespace which is not shared with any other process. Use of CLONE_NEWUTSrequires the CAP_SYS_ADMINcapability.
CLONE_SYSVSEM (since Linux 2.6.26)
This flag reverses the effect of the clone(2) CLONE_SYSVSEMflag. Unshare System V semaphore adjustment (semadj) values, so that the calling process has a new empty semadjlist that is not shared with any other process. If this is the last process that has a reference to the process's current semadjlist, then the adjustments in that list are applied to the corresponding semaphores, as described in semop(2).

In addition, CLONE_THREAD, CLONE_SIGHAND, and CLONE_VMcan be specified in flagsif the caller is single threaded (i.e., it is not sharing its address space with another process or thread). In this case, these flags have no effect. (Note also that specifying CLONE_THREADautomatically implies CLONE_VM, and specifying CLONE_VMautomatically implies CLONE_SIGHAND.) If the process is multithreaded, then the use of these flags results in an error.

If flagsis 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 errnois set to indicate the error.  

ERRORS

EINVAL
An invalid bit was specified in flags.
EINVAL
CLONE_THREAD, CLONE_SIGHAND, or CLONE_VMwas specified in flags, and the caller is multithreaded.
ENOMEM
Cannot allocate sufficient memory to copy parts of caller's context that need to be unshared.
EPERM
The calling process did not have the required privileges for this operation.
EPERM
CLONE_NEWUSERwas specified in flags, but either the effective user ID or the effective group ID of the caller does not have a mapping in the parent namespace (see user_namespaces(7)).
EPERM (since Linux 3.9)
CLONE_NEWUSERwas specified in flagsand the caller is in a chroot environment (i.e., the caller's root directory does not match the root directory of the mount namespace in which it resides).
EUSERS (since Linux 3.11)
CLONE_NEWUSERwas specified in flags, and the call would cause the limit on the number of nested user namespaces to be exceeded. See user_namespaces(7).
 

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 particular, as at kernel 3.8, unshare() does not implement flags that reverse the effects of CLONE_SIGHAND, CLONE_THREAD, or CLONE_VM. Such functionality may be added in the future, if required.  

EXAMPLE

The program below provides a simple implementation of the unshare(1) command, which unshares one or more namespaces and executes the command supplied in its command-line arguments. Here's an example of the use of this program, running a shell in a new mount namespace, and verifying that the original shell and the new shell are in separate mount namespaces:
$ readlink /proc/$$/ns/mntmnt:[4026531840]
$ sudo ./unshare -m /bin/bash[sudo] password for cecilia:
# readlink /proc/$$/ns/mntmnt:[4026532325]

The differing output of the two readlink(1) commands shows that the two shells are in different mount namespaces.  

Program source

/* unshare.c

   A simple implementation of the unshare(1) command: unshare
   namespaces and execute a command.
*/
#define _GNU_SOURCE
#include <sched.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>

/* A simple error-handling function: print an error message based
   on the value in aqerrnoaq and terminate the calling process */

#define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                        } while (0)

static void
usage(char *pname)
{
    fprintf(stderr, "Usage: %s [options] program [arg...]\n", pname);
    fprintf(stderr, "Options can be:\n");
    fprintf(stderr, "    -i   unshare IPC namespace\n");
    fprintf(stderr, "    -m   unshare mount namespace\n");
    fprintf(stderr, "    -n   unshare network namespace\n");
    fprintf(stderr, "    -p   unshare PID namespace\n");
    fprintf(stderr, "    -u   unshare UTS namespace\n");
    fprintf(stderr, "    -U   unshare user namespace\n");
    exit(EXIT_FAILURE);
}

int
main(int argc, char *argv[])
{
    int flags, opt;

    flags = 0;

    while ((opt = getopt(argc, argv, "imnpuU")) != -1) {
        switch (opt) {
        case aqiaq: flags |= CLONE_NEWIPC;        break;
        case aqmaq: flags |= CLONE_NEWNS;         break;
        case aqnaq: flags |= CLONE_NEWNET;        break;
        case aqpaq: flags |= CLONE_NEWPID;        break;
        case aquaq: flags |= CLONE_NEWUTS;        break;
        case aqUaq: flags |= CLONE_NEWUSER;       break;
        default:  usage(argv[0]);
        }
    }

    if (optind >= argc)
        usage(argv[0]);

    if (unshare(flags) == -1)
        errExit("unshare");

    execvp(argv[optind], &argv[optind]);
    errExit("execvp");
}
 

SEE ALSO

unshare(1), clone(2), fork(2), kcmp(2), setns(2), vfork(2), namespaces(7)

Documentation/unshare.txtin the Linux kernel source tree


 

Index

NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
VERSIONS
CONFORMING TO
NOTES
EXAMPLE
Program source
SEE ALSO

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