For review: setns(2) man page [RESEND]

Michael Kerrisk (man-pages)
2014-08-20T23:42:17Z

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[Messed up Eric's email address on first send. Resending...]

Hello Eric et al.

With the namespaces changes, a number of additions have been
made to the setns(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
setns - reassociate thread with a namespace

SYNOPSIS
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <sched.h>

int setns(int fd, int nstype);

DESCRIPTION
Given a file descriptor referring to a namespace, reassociate the
calling thread with that namespace.

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

The nstype argument specifies which type of namespace the calling
thread may be reassociated with. This argument can have one of
the following values:

0 Allow any type of namespace to be joined.

CLONE_NEWIPC (since Linux 3.0)
fd must refer to an IPC namespace.

CLONE_NEWNET (since Linux 3.0)
fd must refer to a network namespace.

CLONE_NEWNS (since Linux 3.8)
fd must refer to a mount namespace.

CLONE_NEWPID (since Linux 3.8)
fd must refer to a PID namespace.

CLONE_NEWUSER (since Linux 3.8)
fd must refer to a user namespace.

CLONE_NEWUTS (since Linux 3.0)
fd must refer to a UTS namespace.

Specifying nstype as 0 suffices if the caller knows (or does not
care) what type of namespace is referred to by fd. Specifying a
nonzero value for nstype is useful if the caller does not know
what type of namespace is referred to by 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 fd if the file
descriptor was opened by another process and, for example, passed
to the caller via a UNIX domain socket.)

CLONE_NEWPID behaves somewhat differently from the other 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 fd is a descendant
(child, grandchild, etc.) of the PID namespace of the caller.
For further details on PID namespaces, see user_namespaces(7).

A process reassociating itself with a user namespace must have
the CAP_SYS_ADMIN capability in the target user namespace. Upon
successfully joining a user namespace, a process is granted all
capabilities in that namespace, regardless of its user and group
IDs. A multithreaded process may not change user namespace with
setns(). It is not permitted to use setns() to reenter the call‐
er's current user namespace. This prevents a caller that has
dropped capabilities from regaining those capabilities via a call
to setns(). For security reasons, a process can't join a new
user namespace if it is sharing filesystem-related attributes
(the attributes whose sharing is controlled by the clone(2)
CLONE_FS flag) with another process. For further details on user
namespaces, see user_namespaces(7).

A process may not be reassociated with a new mount namespace if
it is multithreaded. Changing the mount namespace requires that
the caller possess both CAP_SYS_CHROOT and CAP_SYS_ADMIN capabil‐
ities in its own user namespace and CAP_SYS_ADMIN in the target
mount namespace.

RETURN VALUE
On success, setns() returns 0. On failure, -1 is returned and
errno is set to indicate the error.

ERRORS
EBADF fd is not a valid file descriptor.

EINVAL fd refers to a namespace whose type does not match that
specified in nstype.

EINVAL There is problem with reassociating the thread with the
specified namespace.

EINVAL The caller attempted to join the user namespace in which
it is already a member.

EINVAL The caller shares filesystem (CLONE_FS) state (in particu‐
lar, the root directory) with other processes and tried to
join a new user namespace.

EINVAL The caller is multithreaded and tried to join a new user
namespace.

ENOMEM Cannot allocate sufficient memory to change the specified
namespace.

EPERM The calling thread did not have the required capability
for this operation.

VERSIONS
The setns() system call first appeared in Linux in kernel 3.0;
library support was added to glibc in version 2.14.

CONFORMING TO
The setns() system call is Linux-specific.

NOTES
Not all of the attributes that can be shared when a new thread is
created using clone(2) can be changed using setns().

EXAMPLE
The program below takes two or more arguments. The first argu‐
ment specifies the pathname of a namespace file in an existing
/proc/[pid]/ns/ directory. The remaining arguments specify a
command and its arguments. The program opens the namespace file,
joins that namespace using setns(), and executes the specified
command inside that namespace.

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

We begin by executing the example program in 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 names‐
paces, so that we can see that they are different.

$ su # Need privilege for namespace operations
Password:
# ./newuts bizarro &
[1] 3549
clone() returned 3550
uts.nodename in child: bizarro
uts.nodename in parent: antero
# uname -n # Verify hostname in the shell
antero

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:

# ./ns_exec /proc/3550/ns/uts /bin/bash
# uname -n # Executed in shell started by ns_exec
bizarro

Program source
#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");
}

SEE ALSO
clone(2), fork(2), unshare(2), vfork(2), namespaces(7), unix(7)

--
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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