ioctl_ns - ioctl() operations for Linux namespaces
The following
ioctl(2) operations are provided to allow discovery of
namespace relationships (see
user_namespaces(7) and
pid_namespaces(7)). The form of the calls is:
new_fd = ioctl(fd, request);
In each case,
fd refers to a
/proc/[pid]/ns/* file. Both
operations return a new file descriptor on success.
- NS_GET_USERNS (since Linux 4.9)
- Returns a file descriptor that refers to the owning user namespace for the
namespace referred to by fd.
- NS_GET_PARENT (since Linux 4.9)
- Returns a file descriptor that refers to the parent namespace of the
namespace referred to by fd. This operation is valid only for
hierarchical namespaces (i.e., PID and user namespaces). For user
namespaces, NS_GET_PARENT is synonymous with
NS_GET_USERNS.
The new file descriptor returned by these operations is opened with the
O_RDONLY and
O_CLOEXEC (close-on-exec; see
fcntl(2))
flags.
By applying
fstat(2) to the returned file descriptor, one obtains a
stat structure whose
st_dev (resident device) and
st_ino
(inode number) fields together identify the owning/parent namespace. This
inode number can be matched with the inode number of another
/proc/[pid]/ns/{pid,user} file to determine whether that is the
owning/parent namespace.
Either of these
ioctl(2) operations can fail with the following errors:
- EPERM
- The requested namespace is outside of the caller's namespace scope. This
error can occur if, for example, the owning user namespace is an ancestor
of the caller's current user namespace. It can also occur on attempts to
obtain the parent of the initial user or PID namespace.
- ENOTTY
- The operation is not supported by this kernel version.
Additionally, the
NS_GET_PARENT operation can fail with the following
error:
- EINVAL
- fd refers to a nonhierarchical namespace.
See the EXAMPLE section for an example of the use of these operations.
The
NS_GET_NSTYPE operation (available since Linux 4.11) can be used to
discover the type of namespace referred to by the file descriptor
fd:
nstype = ioctl(fd, NS_GET_NSTYPE);
fd refers to a
/proc/[pid]/ns/* file.
The return value is one of the
CLONE_NEW* values that can be specified to
clone(2) or
unshare(2) in order to create a namespace.
The
NS_GET_OWNER_UID operation (available since Linux 4.11) can be used
to discover the owner user ID of a user namespace (i.e., the effective user ID
of the process that created the user namespace). The form of the call is:
uid_t uid;
ioctl(fd, NS_GET_OWNER_UID, &uid);
fd refers to a
/proc/[pid]/ns/user file.
The owner user ID is returned in the
uid_t pointed to by the third
argument.
This operation can fail with the following error:
- EINVAL
- fd does not refer to a user namespace.
Any of the above
ioctl() operations can return the following errors:
- ENOTTY
- fd does not refer to a /proc/[pid]/ns/* file.
Namespaces and the operations described on this page are a Linux-specific.
The example shown below uses the
ioctl(2) operations described above to
perform simple discovery of namespace relationships. The following shell
sessions show various examples of the use of this program.
Trying to get the parent of the initial user namespace fails, since it has no
parent:
$ ./ns_show /proc/self/ns/user p
The parent namespace is outside your namespace scope
Create a process running
sleep(1) that resides in new user and UTS
namespaces, and show that the new UTS namespace is associated with the new
user namespace:
$ unshare -Uu sleep 1000 &
[1] 23235
$ ./ns_show /proc/23235/ns/uts u
Device/Inode of owning user namespace is: [0,3] / 4026532448
$ readlink /proc/23235/ns/user
user:[4026532448]
Then show that the parent of the new user namespace in the preceding example is
the initial user namespace:
$ readlink /proc/self/ns/user
user:[4026531837]
$ ./ns_show /proc/23235/ns/user p
Device/Inode of parent namespace is: [0,3] / 4026531837
Start a shell in a new user namespace, and show that from within this shell, the
parent user namespace can't be discovered. Similarly, the UTS namespace (which
is associated with the initial user namespace) can't be discovered.
$ PS1="sh2$ " unshare -U bash
sh2$ ./ns_show /proc/self/ns/user p
The parent namespace is outside your namespace scope
sh2$ ./ns_show /proc/self/ns/uts u
The owning user namespace is outside your namespace scope
/* ns_show.c
Licensed under the GNU General Public License v2 or later.
*/
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <fcntl.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/ioctl.h>
#include <errno.h>
#include <sys/sysmacros.h>
#ifndef NS_GET_USERNS
#define NSIO 0xb7
#define NS_GET_USERNS _IO(NSIO, 0x1)
#define NS_GET_PARENT _IO(NSIO, 0x2)
#endif
int
main(int argc, char *argv[])
{
int fd, userns_fd, parent_fd;
struct stat sb;
if (argc < 2) {
fprintf(stderr, "Usage: %s /proc/[pid]/ns/[file] [p|u]\n",
argv[0]);
fprintf(stderr, "\nDisplay the result of one or both "
"of NS_GET_USERNS (u) or NS_GET_PARENT (p)\n"
"for the specified /proc/[pid]/ns/[file]. If neither "
"'p' nor 'u' is specified,\n"
"NS_GET_USERNS is the default.\n");
exit(EXIT_FAILURE);
}
/* Obtain a file descriptor for the 'ns' file specified
in argv[1] */
fd = open(argv[1], O_RDONLY);
if (fd == -1) {
perror("open");
exit(EXIT_FAILURE);
}
/* Obtain a file descriptor for the owning user namespace and
then obtain and display the inode number of that namespace */
if (argc < 3 || strchr(argv[2], 'u')) {
userns_fd = ioctl(fd, NS_GET_USERNS);
if (userns_fd == -1) {
if (errno == EPERM)
printf("The owning user namespace is outside "
"your namespace scope\n");
else
perror("ioctl-NS_GET_USERNS");
exit(EXIT_FAILURE);
}
if (fstat(userns_fd, &sb) == -1) {
perror("fstat-userns");
exit(EXIT_FAILURE);
}
printf("Device/Inode of owning user namespace is: "
"[%lx,%lx] / %ld\n",
(long) major(sb.st_dev), (long) minor(sb.st_dev),
(long) sb.st_ino);
close(userns_fd);
}
/* Obtain a file descriptor for the parent namespace and
then obtain and display the inode number of that namespace */
if (argc > 2 && strchr(argv[2], 'p')) {
parent_fd = ioctl(fd, NS_GET_PARENT);
if (parent_fd == -1) {
if (errno == EINVAL)
printf("Can' get parent namespace of a "
"nonhierarchical namespace\n");
else if (errno == EPERM)
printf("The parent namespace is outside "
"your namespace scope\n");
else
perror("ioctl-NS_GET_PARENT");
exit(EXIT_FAILURE);
}
if (fstat(parent_fd, &sb) == -1) {
perror("fstat-parentns");
exit(EXIT_FAILURE);
}
printf("Device/Inode of parent namespace is: [%lx,%lx] / %ld\n",
(long) major(sb.st_dev), (long) minor(sb.st_dev),
(long) sb.st_ino);
close(parent_fd);
}
exit(EXIT_SUCCESS);
}
fstat(2),
ioctl(2),
proc(5),
namespaces(7)