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.
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:
Additionally, the NS_GET_PARENT operation can fail with the following error:
See the EXAMPLES section for an example of the use of these operations.
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.
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:
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
Licensed under the GNU General Public License v2 or later.
*/
#include <stdint.h>
#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: "
"[%jx,%jx] / %ju\n",
(uintmax_t) major(sb.st_dev),
(uintmax_t) minor(sb.st_dev),
(uintmax_t) 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: [%jx,%jx] / %ju\n",
(uintmax_t) major(sb.st_dev),
(uintmax_t) minor(sb.st_dev),
(uintmax_t) sb.st_ino);
close(parent_fd);
}