setns.2: Clarify capabilities required for reassociating with a mount namespace

[thirdparty/man-pages.git] / man2 / setns.2

.\" Copyright (C) 2011, Eric Biederman <ebiederm@xmission.com>
.\" and Copyright (C) 2011, 2012, Michael Kerrisk <mtk.manpages@gamil.com>
.\"
.\" %%%LICENSE_START(GPLv2_ONELINE)
.\" Licensed under the GPLv2
.\" %%%LICENSE_END
.\"
.TH SETNS 2 2013-01-01 "Linux" "Linux Programmer's Manual"
.SH NAME
setns \- reassociate thread with a namespace
.SH SYNOPSIS
.nf
.BR "#define _GNU_SOURCE" "             /* See feature_test_macros(7) */"
.B #include <sched.h>
.sp
.BI "int setns(int " fd ", int " nstype );
.fi
.SH DESCRIPTION
Given a file descriptor referring to a namespace,
reassociate the calling thread with that namespace.

The
.I fd
argument is a file descriptor referring to one of the namespace entries in a
.I /proc/[pid]/ns/
directory; see
.BR proc (5)
for further information on
.IR /proc/[pid]/ns/ .
The calling thread will be reassociated with the corresponding namespace,
subject to any constraints imposed by the
.I nstype
argument.

The
.I nstype
argument specifies which type of namespace
the calling thread may be reassociated with.
This argument can have one of the following values:
.TP
.BR 0
Allow any type of namespace to be joined.
.TP
.BR CLONE_NEWIPC
.I fd
must refer to an IPC namespace.
.TP
.BR CLONE_NEWNET
.I fd
must refer to a network namespace.
.TP
.BR CLONE_NEWNS
.I fd
must refer to a mount namespace.
.TP
.BR CLONE_NEWPID
.I fd
must refer to a PID namespace.
.TP
.BR CLONE_NEWUSER
.I fd
must refer to a user namespace.
.TP
.BR CLONE_NEWUTS
.I fd
must refer to a UTS namespace.
.PP
Specifying
.I nstype
as 0 suffices if the caller knows (or does not care)
what type of namespace is referred to by
.IR fd .
Specifying a nonzero value for
.I nstype
is useful if the caller does not know what type of namespace is referred to by
.IR fd
and wants to ensure that the namespace is of a particular type.
(The caller might not know the type of the namespace referred to by
.IR fd
if the file descriptor was opened by another process and, for example,
passed to the caller via a UNIX domain socket.)

.B CLONE_NEWPID
behaves somewhat differently from the other
.I nstype
values:
reassociating the calling thread with a PID namespace only changes
the PID namespace that child processes of the caller will be created in;
it does not change the PID namespace of the caller itself.
Reassociating with a PID namespace is only allowed if the
PID namespace specified by
.IR fd
is a descendant (child, grandchild, etc.)
PID namespace of the PID namespace of the caller.

A multi-threaded process may not change user namespace with
.BR setns ().
A process may not reassociate the thread with the caller's user
namespace.
A process reassociating itself with a user namespace must have
.B CAP_SYS_ADMIN
.\" See kernel/user_namespace.c:userns_install() [3.8 source]
privileges in the target user namespace.

A process may not be reassociated with a new mount namespace if it is
multi-threaded.
.\" Above check is in fs/namespace.c:mntns_install() [3.8 source]
Changing the mount namespace requires that the caller possess both
.B CAP_SYS_CHROOT
and
.BR CAP_SYS_ADMIN 
capabilities in its own user namespace and
.BR CAP_SYS_ADMIN 
in the target mount namespace.

.SH RETURN VALUE
On success,
.IR setns ()
returns 0.
On failure, \-1 is returned and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EBADF
.I fd
is not a valid file descriptor.
.TP
.B EINVAL
.I fd
refers to a namespace whose type does not match that specified in
.IR nstype ,
or there is problem with reassociating
the thread with the specified namespace.
.TP
.B ENOMEM
Cannot allocate sufficient memory to change the specified namespace.
.TP
.B EPERM
The calling thread did not have the required privilege
.RB ( CAP_SYS_ADMIN )
for this operation.
.SH VERSIONS
The
.BR setns ()
system call first appeared in Linux in kernel 3.0;
library support was added to glibc in version 2.14;
Support for PID, user, and mount namespaces first appeared in Linux 3.8.
.SH CONFORMING TO
The
.BR setns ()
system call is Linux-specific.
.SH NOTES
Not all of the attributes that can be shared when
a new thread is created using
.BR clone (2)
can be changed using
.BR setns ().
.SH EXAMPLE
The program below takes two or more arguments.
The first argument specifies the pathname of a namespace file in an existing
.I /proc/[pid]/ns/
directory.
The remaining arguments specify a command and its arguments.
The program opens the namespace file, joins that namespace using
.BR setns (),
and executes the specified command inside that namespace.

The following shell session demonstrates the use of this program
(compiled as a binary named
.IR ns_exec )
in conjunction with the
.BR CLONE_NEWUTS
example program in the
.BR clone (2)
man page (complied as a binary named
.IR newuts ).

We begin by executing the example program in
.BR clone (2)
in the background.
That program creates a child in a separate UTS namespace.
The child changes the hostname in its namespace,
and then both processes display the hostnames in their UTS namespaces,
so that we can see that they are different.

.nf
.in +4n
$ \fBsu\fP                   # Need privilege for namespace operations
Password:
# \fB./newuts bizarro &\fP
[1] 3549
clone() returned 3550
uts.nodename in child:  bizarro
uts.nodename in parent: antero
# \fBuname \-n\fP             # Verify hostname in the shell
antero
.in
.fi

We then run the program shown below,
using it to execute a shell.
Inside that shell, we verify that the hostname is the one
set by the child created by the first program:

.nf
.in +4n
# \fB./ns_exec /proc/3550/ns/uts /bin/bash\fP
# \fBuname \-n\fP             # Executed in shell started by ns_exec
bizarro
.in
.fi
.SS Program source
.nf
#define _GNU_SOURCE
#include <fcntl.h>
#include <sched.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>

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

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

    if (argc < 3) {
        fprintf(stderr, "%s /proc/PID/ns/FILE cmd args...\\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    fd = open(argv[1], O_RDONLY);   /* Get descriptor for namespace */
    if (fd == \-1)
        errExit("open");

    if (setns(fd, 0) == \-1)         /* Join that namespace */
        errExit("setns");

    execvp(argv[2], &argv[2]);      /* Execute a command in namespace */
    errExit("execvp");
}
.fi
.SH SEE ALSO
.BR clone (2),
.BR fork (2),
.BR unshare (2),
.BR vfork (2),
.BR proc (5),
.BR unix (7)