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Message-ID: <53F5317B.6010808@gmail.com>
Date: Wed, 20 Aug 2014 18:38:35 -0500
From: "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To: "Eric W. Biederman" <ebiederm@...ssion.com>
CC: mtk.manpages@...il.com, lkml <linux-kernel@...r.kernel.org>,
"linux-man@...r.kernel.org" <linux-man@...r.kernel.org>,
containers@...ts.linux-foundation.org,
richard.weinberger@...il.com,
Andy Lutomirski <luto@...capital.net>,
"Serge E. Hallyn" <serge@...lyn.com>
Subject: For review: unshare(2) man page
Hello Eric et al.
With the namespaces changes, a number of additions have been
made to the unshare(2) man page, so I will send out the entire page
for review at the same time as the various namespaces page.
The rendered version is below, and the source is attached.
Review comments/suggestions for improvements / bug fixes welcome.
Cheers,
Michael
==
NAME
unshare - disassociate parts of the process execution context
SYNOPSIS
#include <sched.h>
int unshare(int flags);
Feature Test Macro Requirements for glibc (see fea‐
ture_test_macros(7)):
unshare():
Since glibc 2.14:
_GNU_SOURCE
Before glibc 2.14:
_BSD_SOURCE || _SVID_SOURCE
/* _GNU_SOURCE also suffices */
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 spec‐
ified 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
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_NEWIPC (since Linux 2.6.19)
This flag has the same effect as the clone(2) CLONE_NEWIPC
flag. Unshare the System V IPC namespace, so that the
calling process has a private copy of the System V IPC
namespace which is not shared with any other process.
Specifying this flag automatically implies CLONE_SYSVSEM
as well. Use of CLONE_NEWIPC requires the CAP_SYS_ADMIN
capability.
CLONE_NEWNET (since Linux 2.6.24)
This flag has the same effect as the clone(2) CLONE_NEWNET
flag. 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_NEWNET requires the CAP_SYS_ADMIN capability.
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 private copy of its namespace which is not
shared with any other process. Specifying this flag auto‐
matically implies CLONE_FS as well. Use of CLONE_NEWNS
requires the CAP_SYS_ADMIN capability.
CLONE_NEWPID (since Linux 3.8)
This flag has the same effect as the clone(2) CLONE_NEWPID
flag. 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 not moved 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_NEWPID automatically implies
CLONE_THREAD as well. Use of CLONE_NEWPID requires the
CAP_SYS_ADMIN capability. For further information, see
pid_namespaces(7).
CLONE_NEWUSER (since Linux 3.8)
This flag has the same effect as the clone(2)
CLONE_NEWUSER flag. 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_NEWUSER flag, the caller obtains a full set of capa‐
bilities in the new namespace.
CLONE_NEWUSER requires that the calling process is not
threaded; specifying CLONE_NEWUSER automatically implies
CLONE_THREAD. Since Linux 3.9, CLONE_NEWUSER also auto‐
matically implies CLONE_FS. CLONE_NEWUSER requires 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_NEWUTS
flag. 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_NEWUTS
requires the CAP_SYS_ADMIN capability.
CLONE_SYSVSEM (since Linux 2.6.26)
This flag reverses the effect of the clone(2)
CLONE_SYSVSEM flag. Unshare System V semaphore adjustment
(semadj) values, so that the calling process has a new
empty semadj list that is not shared with any other
process. If this is the last process that has a reference
to the process's current semadj list, 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_VM can be
specified in flags if 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. If the process is mul‐
tithreaded, then the use of these flags results in an error.
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.
EINVAL CLONE_THREAD, CLONE_SIGHAND, or CLONE_VM was specified in
flags, and the caller is multithreaded.
ENOMEM Cannot allocate sufficient memory to copy parts of call‐
er's context that need to be unshared.
EPERM The calling process did not have the required privileges
for this operation.
EPERM CLONE_NEWUSER was 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_NEWUSER was specified in flags and 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_NEWUSER was 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/$$/ns/mnt
mnt:[4026531840]
$ sudo ./unshare -m /bin/bash
[sudo] password for cecilia:
# readlink /proc/$$/ns/mnt
mnt:[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 'errno' 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 'i': flags |= CLONE_NEWIPC; break;
case 'm': flags |= CLONE_NEWNS; break;
case 'n': flags |= CLONE_NEWNET; break;
case 'p': flags |= CLONE_NEWPID; break;
case 'u': flags |= CLONE_NEWUTS; break;
case 'U': 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.txt in the Linux kernel source tree
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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