USER_NAMESPACES(7)         Linux Programmer's Manual        USER_NAMESPACES(7)

       user_namespaces - overview of Linux user namespaces

       For an overview of namespaces, see namespaces(7).

       User namespaces isolate security-related identifiers and attributes, in
       particular, user IDs and group IDs (see credentials(7)), the  root  di-
       rectory,  keys  (see  keyrings(7)),  and  capabilities  (see  capabili-
       ties(7)).  A process's user and group IDs can be different  inside  and
       outside  a  user namespace.  In particular, a process can have a normal
       unprivileged user ID outside a user namespace while at  the  same  time
       having a user ID of 0 inside the namespace; in other words, the process
       has full privileges for operations inside the user  namespace,  but  is
       unprivileged for operations outside the namespace.

   Nested namespaces, namespace membership
       User namespaces can be nested; that is, each user namespace--except the
       initial ("root") namespace--has a parent user namespace, and  can  have
       zero  or  more child user namespaces.  The parent user namespace is the
       user namespace of the process that creates the  user  namespace  via  a
       call to unshare(2) or clone(2) with the CLONE_NEWUSER flag.

       The  kernel imposes (since version 3.11) a limit of 32 nested levels of
       user namespaces.  Calls to unshare(2) or clone(2) that would cause this
       limit to be exceeded fail with the error EUSERS.

       Each process is a member of exactly one user namespace.  A process cre-
       ated via fork(2) or clone(2) without the CLONE_NEWUSER flag is a member
       of  the  same  user namespace as its parent.  A single-threaded process
       can join another user namespace with setns(2) if it has the CAP_SYS_AD-
       MIN  in that namespace; upon doing so, it gains a full set of capabili-
       ties in that namespace.

       A call to clone(2) or unshare(2) with the CLONE_NEWUSER flag makes  the
       new  child process (for clone(2)) or the caller (for unshare(2)) a mem-
       ber of the new user namespace created by the call.

       The NS_GET_PARENT ioctl(2)  operation  can  be  used  to  discover  the
       parental relationship between user namespaces; see ioctl_ns(2).

       The  child  process  created  by  clone(2)  with the CLONE_NEWUSER flag
       starts out with a complete set of capabilities in the  new  user  name-
       space.  Likewise, a process that creates a new user namespace using un-
       share(2) or joins an existing user namespace  using  setns(2)  gains  a
       full  set  of  capabilities in that namespace.  On the other hand, that
       process has no capabilities in the parent (in the case of clone(2))  or
       previous  (in the case of unshare(2) and setns(2)) user namespace, even
       if the new namespace is created or joined by the  root  user  (i.e.,  a
       process with user ID 0 in the root namespace).

       Note that a call to execve(2) will cause a process's capabilities to be
       recalculated in the usual way (see capabilities(7)).  Consequently, un-
       less  the  process has a user ID of 0 within the namespace, or the exe-
       cutable file has a nonempty inheritable capabilities mask, the  process
       will  lose  all  capabilities.  See the discussion of user and group ID
       mappings, below.

       A call to clone(2), unshare(2), or  setns(2)  using  the  CLONE_NEWUSER
       flag sets the "securebits" flags (see capabilities(7)) to their default
       values (all flags disabled) in the child (for clone(2)) or caller  (for
       unshare(2),  or  setns(2)).  Note that because the caller no longer has
       capabilities in its original user namespace after a call  to  setns(2),
       it  is not possible for a process to reset its "securebits" flags while
       retaining its user namespace membership by using  a  pair  of  setns(2)
       calls to move to another user namespace and then return to its original
       user namespace.

       The rules for determining whether or not a process has a capability  in
       a particular user namespace are as follows:

       1. A process has a capability inside a user namespace if it is a member
          of that namespace and it has the capability in its  effective  capa-
          bility  set.  A process can gain capabilities in its effective capa-
          bility set in various ways.  For example, it may execute a set-user-
          ID  program  or an executable with associated file capabilities.  In
          addition,  a  process  may  gain  capabilities  via  the  effect  of
          clone(2), unshare(2), or setns(2), as already described.

       2. If  a process has a capability in a user namespace, then it has that
          capability in all child (and further removed descendant)  namespaces
          as well.

       3. When  a  user namespace is created, the kernel records the effective
          user ID of the creating process as being the "owner"  of  the  name-
          space.   A  process that resides in the parent of the user namespace
          and whose effective user ID matches the owner of the  namespace  has
          all  capabilities in the namespace.  By virtue of the previous rule,
          this means that the process has all capabilities in all further  re-
          moved  descendant  user  namespaces  as  well.  The NS_GET_OWNER_UID
          ioctl(2) operation can be used to discover the user ID of the  owner
          of the namespace; see ioctl_ns(2).

   Effect of capabilities within a user namespace
       Having  a  capability inside a user namespace permits a process to per-
       form operations (that require privilege) only on resources governed  by
       that  namespace.   In  other words, having a capability in a user name-
       space permits a process to perform privileged operations  on  resources
       that  are  governed  by  (nonuser)  namespaces associated with the user
       namespace (see the next subsection).

       On the other hand, there are many privileged operations that affect re-
       sources  that  are not associated with any namespace type, for example,
       changing the system time (governed by CAP_SYS_TIME), loading  a  kernel
       module (governed by CAP_SYS_MODULE), and creating a device (governed by
       CAP_MKNOD).  Only a process with privileges in the initial  user  name-
       space can perform such operations.

       Holding  CAP_SYS_ADMIN  within  the  user  namespace  associated with a
       process's mount namespace allows that process to create bind mounts and
       mount the following types of filesystems:

           * /proc (since Linux 3.8)
           * /sys (since Linux 3.8)
           * devpts (since Linux 3.9)
           * tmpfs(5) (since Linux 3.9)
           * ramfs (since Linux 3.9)
           * mqueue (since Linux 3.9)
           * bpf (since Linux 4.4)

       Holding  CAP_SYS_ADMIN  within  the  user  namespace  associated with a
       process's cgroup namespace allows (since Linux 4.6) that process to the
       mount  the cgroup version 2 filesystem and cgroup version 1 named hier-
       archies (i.e., cgroup filesystems mounted  with  the  "none,name="  op-

       Holding  CAP_SYS_ADMIN  within  the  user  namespace  associated with a
       process's PID namespace allows (since Linux 3.8) that process to  mount
       /proc filesystems.

       Note however, that mounting block-based filesystems can be done only by
       a process that holds CAP_SYS_ADMIN in the initial user namespace.

   Interaction of user namespaces and other types of namespaces
       Starting in Linux 3.8, unprivileged processes  can  create  user  name-
       spaces,  and  other  the  other types of namespaces can be created with
       just the CAP_SYS_ADMIN capability in the caller's user namespace.

       When a non-user-namespace is created, it is owned by the user namespace
       in  which the creating process was a member at the time of the creation
       of the namespace.  Actions on the non-user-namespace require  capabili-
       ties in the corresponding user namespace.

       If  CLONE_NEWUSER  is  specified along with other CLONE_NEW* flags in a
       single clone(2) or unshare(2) call, the user namespace is guaranteed to
       be  created  first,  giving the child (clone(2)) or caller (unshare(2))
       privileges over the remaining namespaces created by the call.  Thus, it
       is  possible  for an unprivileged caller to specify this combination of

       When a new namespace (other than  a  user  namespace)  is  created  via
       clone(2)  or  unshare(2),  the kernel records the user namespace of the
       creating process against the new namespace.  (This association can't be
       changed.)   When  a  process in the new namespace subsequently performs
       privileged operations that operate on global resources isolated by  the
       namespace,  the  permission  checks  are  performed  according  to  the
       process's capabilities in the user namespace that the kernel associated
       with  the  new namespace.  For example, suppose that a process attempts
       to change the hostname (sethostname(2)), a resource governed by the UTS
       namespace.   In  this  case, the kernel will determine which user name-
       space is associated with the process's UTS namespace, and check whether
       the  process  has  the required capability (CAP_SYS_ADMIN) in that user

       The NS_GET_USERNS ioctl(2) operation can be used to discover  the  user
       namespace   with   which   a  non-user  namespace  is  associated;  see

   User and group ID mappings: uid_map and gid_map
       When a user namespace is created, it starts out without  a  mapping  of
       user   IDs   (group   IDs)   to   the   parent   user  namespace.   The
       /proc/[pid]/uid_map  and  /proc/[pid]/gid_map  files  (available  since
       Linux  3.5)  expose the mappings for user and group IDs inside the user
       namespace for the process pid.  These files can be  read  to  view  the
       mappings  in  a user namespace and written to (once) to define the map-

       The description in the following paragraphs explains  the  details  for
       uid_map; gid_map is exactly the same, but each instance of "user ID" is
       replaced by "group ID".

       The uid_map file exposes the mapping of user IDs from  the  user  name-
       space  of  the  process  pid  to the user namespace of the process that
       opened uid_map (but see a qualification to this point below).  In other
       words, processes that are in different user namespaces will potentially
       see different values when reading from a particular uid_map  file,  de-
       pending  on the user ID mappings for the user namespaces of the reading

       Each line in the uid_map file specifies a 1-to-1 mapping of a range  of
       contiguous  user  IDs  between two user namespaces.  (When a user name-
       space is first created, this file is empty.)  The specification in each
       line  takes  the  form  of three numbers delimited by white space.  The
       first two numbers specify the starting user ID in each of the two  user
       namespaces.  The third number specifies the length of the mapped range.
       In detail, the fields are interpreted as follows:

       (1) The start of the range of user IDs in the  user  namespace  of  the
           process pid.

       (2) The  start of the range of user IDs to which the user IDs specified
           by field one map.  How field two is interpreted depends on  whether
           the process that opened uid_map and the process pid are in the same
           user namespace, as follows:

           a) If the two processes are in different user namespaces: field two
              is the start of a range of user IDs in the user namespace of the
              process that opened uid_map.

           b) If the two processes are in the same user namespace:  field  two
              is  the  start of the range of user IDs in the parent user name-
              space of the process pid.   This  case  enables  the  opener  of
              uid_map  (the common case here is opening /proc/self/uid_map) to
              see the mapping of user IDs  into  the  user  namespace  of  the
              process that created this user namespace.

       (3) The  length of the range of user IDs that is mapped between the two
           user namespaces.

       System calls that return user IDs (group IDs)--for example,  getuid(2),
       getgid(2),  and  the  credential  fields  in  the structure returned by
       stat(2)--return the user ID (group ID) mapped into  the  caller's  user

       When  a process accesses a file, its user and group IDs are mapped into
       the initial user namespace for the purpose of permission  checking  and
       assigning IDs when creating a file.  When a process retrieves file user
       and group IDs via stat(2), the IDs are mapped in  the  opposite  direc-
       tion,  to produce values relative to the process user and group ID map-

       The initial user namespace has no parent namespace,  but,  for  consis-
       tency,  the  kernel  provides dummy user and group ID mapping files for
       this namespace.  Looking at the uid_map file (gid_map is the same) from
       a shell in the initial namespace shows:

           $ cat /proc/$$/uid_map
                    0          0 4294967295

       This  mapping  tells  us  that  the range starting at user ID 0 in this
       namespace maps to a range starting at 0  in  the  (nonexistent)  parent
       namespace,  and  the length of the range is the largest 32-bit unsigned
       integer.  This leaves 4294967295 (the 32-bit signed -1 value) unmapped.
       This is deliberate: (uid_t) -1 is used in several interfaces (e.g., se-
       treuid(2)) as a way to specify "no user ID".   Leaving  (uid_t) -1  un-
       mapped and unusable guarantees that there will be no confusion when us-
       ing these interfaces.

   Defining user and group ID mappings: writing to uid_map and gid_map
       After the creation of a new user namespace, the uid_map file of one  of
       the  processes  in  the  namespace may be written to once to define the
       mapping of user IDs in the new user namespace.   An  attempt  to  write
       more than once to a uid_map file in a user namespace fails with the er-
       ror EPERM.  Similar rules apply for gid_map files.

       The lines written to uid_map (gid_map) must conform  to  the  following

       *  The  three  fields must be valid numbers, and the last field must be
          greater than 0.

       *  Lines are terminated by newline characters.

       *  There is a limit on the number of lines in the file.  In Linux  4.14
          and  earlier,  this  limit  was (arbitrarily) set at 5 lines.  Since
          Linux 4.15, the limit is 340 lines.   In  addition,  the  number  of
          bytes  written  to  the file must be less than the system page size,
          and the write must be performed at the  start  of  the  file  (i.e.,
          lseek(2)  and pwrite(2) can't be used to write to nonzero offsets in
          the file).

       *  The range of user IDs (group IDs)  specified  in  each  line  cannot
          overlap  with  the ranges in any other lines.  In the initial imple-
          mentation (Linux 3.8), this requirement was satisfied by a  simplis-
          tic  implementation  that  imposed  the further requirement that the
          values in both field 1 and field 2 of successive lines  must  be  in
          ascending numerical order, which prevented some otherwise valid maps
          from being created.  Linux 3.9 and later fix this limitation, allow-
          ing any valid set of nonoverlapping maps.

       *  At least one line must be written to the file.

       Writes that violate the above rules fail with the error EINVAL.

       In   order   for   a   process  to  write  to  the  /proc/[pid]/uid_map
       (/proc/[pid]/gid_map) file, all of the following requirements  must  be

       1. The writing process must have the CAP_SETUID (CAP_SETGID) capability
          in the user namespace of the process pid.

       2. The writing process must either be in  the  user  namespace  of  the
          process pid or be in the parent user namespace of the process pid.

       3. The  mapped  user IDs (group IDs) must in turn have a mapping in the
          parent user namespace.

       4. One of the following two cases applies:

          *  Either the writing process has the CAP_SETUID (CAP_SETGID)  capa-
             bility in the parent user namespace.

             +  No  further  restrictions apply: the process can make mappings
                to arbitrary user IDs (group IDs) in  the  parent  user  name-

          *  Or otherwise all of the following restrictions apply:

             +  The data written to uid_map (gid_map) must consist of a single
                line that maps the writing process's effective user ID  (group
                ID)  in  the  parent user namespace to a user ID (group ID) in
                the user namespace.

             +  The writing process must have the same effective  user  ID  as
                the process that created the user namespace.

             +  In  the  case  of gid_map, use of the setgroups(2) system call
                must first be denied by writing "deny" to the /proc/[pid]/set-
                groups file (see below) before writing to gid_map.

       Writes that violate the above rules fail with the error EPERM.

   Interaction with system calls that change process UIDs or GIDs
       In  a  user  namespace where the uid_map file has not been written, the
       system calls that change user IDs will fail.  Similarly, if the gid_map
       file  has not been written, the system calls that change group IDs will
       fail.  After the uid_map and gid_map files have been written, only  the
       mapped  values  may  be used in system calls that change user and group

       For user IDs, the relevant system calls include setuid(2), setfsuid(2),
       setreuid(2),  and  setresuid(2).   For  group  IDs, the relevant system
       calls include setgid(2), setfsgid(2),  setregid(2),  setresgid(2),  and

       Writing  "deny"  to  the  /proc/[pid]/setgroups  file before writing to
       /proc/[pid]/gid_map will permanently disable  setgroups(2)  in  a  user
       namespace  and  allow writing to /proc/[pid]/gid_map without having the
       CAP_SETGID capability in the parent user namespace.

   The /proc/[pid]/setgroups file
       The /proc/[pid]/setgroups file displays the string "allow" if processes
       in  the  user  namespace that contains the process pid are permitted to
       employ the setgroups(2) system call; it displays "deny" if setgroups(2)
       is  not  permitted in that user namespace.  Note that regardless of the
       value  in  the  /proc/[pid]/setgroups  file  (and  regardless  of   the
       process's  capabilities),  calls to setgroups(2) are also not permitted
       if /proc/[pid]/gid_map has not yet been set.

       A privileged process (one with  the  CAP_SYS_ADMIN  capability  in  the
       namespace)  may  write  either of the strings "allow" or "deny" to this
       file before writing a group ID mapping for this user namespace  to  the
       file  /proc/[pid]/gid_map.   Writing  the  string  "deny"  prevents any
       process in the user namespace from employing setgroups(2).

       The essence of the restrictions described in the preceding paragraph is
       that  it is permitted to write to /proc/[pid]/setgroups only so long as
       calling setgroups(2) is disallowed because /proc/[pid]gid_map  has  not
       been  set.   This ensures that a process cannot transition from a state
       where setgroups(2) is allowed to a state where setgroups(2) is  denied;
       a  process  can  transition  only from setgroups(2) being disallowed to
       setgroups(2) being allowed.

       The default value of this file in the initial user  namespace  is  "al-

       Once  /proc/[pid]/gid_map  has been written to (which has the effect of
       enabling setgroups(2) in the user namespace), it is no longer  possible
       to  disallow  setgroups(2)  by  writing "deny" to /proc/[pid]/setgroups
       (the write fails with the error EPERM).

       A child user namespace inherits the /proc/[pid]/setgroups setting  from
       its parent.

       If  the setgroups file has the value "deny", then the setgroups(2) sys-
       tem call can't subsequently be reenabled (by  writing  "allow"  to  the
       file)  in  this user namespace.  (Attempts to do so fail with the error
       EPERM.)  This restriction also propagates down to all child user  name-
       spaces of this user namespace.

       The  /proc/[pid]/setgroups  file was added in Linux 3.19, but was back-
       ported to many earlier stable kernel series, because it addresses a se-
       curity  issue.   The  issue  concerned  files  with permissions such as
       "rwx---rwx".  Such files give fewer permissions to "group" than they do
       to  "other".   This means that dropping groups using setgroups(2) might
       allow a process file access that it did not formerly have.  Before  the
       existence of user namespaces this was not a concern, since only a priv-
       ileged process (one with the CAP_SETGID  capability)  could  call  set-
       groups(2).   However,  with the introduction of user namespaces, it be-
       came possible for an unprivileged process to create a new namespace  in
       which the user had all privileges.  This then allowed formerly unprivi-
       leged users to drop groups and thus gain file access that they did  not
       previously  have.   The /proc/[pid]/setgroups file was added to address
       this security issue, by denying any pathway for an unprivileged process
       to drop groups with setgroups(2).

   Unmapped user and group IDs
       There  are  various  places where an unmapped user ID (group ID) may be
       exposed to user space.  For example, the first process in  a  new  user
       namespace  may call getuid(2) before a user ID mapping has been defined
       for the namespace.  In most such cases, an unmapped  user  ID  is  con-
       verted  to  the  overflow user ID (group ID); the default value for the
       overflow user  ID  (group  ID)  is  65534.   See  the  descriptions  of
       /proc/sys/kernel/overflowuid    and   /proc/sys/kernel/overflowgid   in

       The cases where unmapped IDs are mapped in this fashion include  system
       calls that return user IDs (getuid(2), getgid(2), and similar), creden-
       tials passed  over  a  UNIX  domain  socket,  credentials  returned  by
       stat(2),  waitid(2),  and  the  System V IPC "ctl" IPC_STAT operations,
       credentials  exposed   by   /proc/[pid]/status   and   the   files   in
       /proc/sysvipc/*,  credentials returned via the si_uid field in the sig-
       info_t received with a signal (see sigaction(2)),  credentials  written
       to  the process accounting file (see acct(5)), and credentials returned
       with POSIX message queue notifications (see mq_notify(3)).

       There is one notable case where unmapped user and  group  IDs  are  not
       converted  to  the  corresponding  overflow  ID  value.  When viewing a
       uid_map or gid_map file in which there is no  mapping  for  the  second
       field,  that  field is displayed as 4294967295 (-1 as an unsigned inte-

   Set-user-ID and set-group-ID programs
       When a process inside a user namespace  executes  a  set-user-ID  (set-
       group-ID)  program,  the process's effective user (group) ID inside the
       namespace is changed to whatever value is mapped for the  user  (group)
       ID  of  the  file.   However, if either the user or the group ID of the
       file has no mapping inside the namespace, the  set-user-ID  (set-group-
       ID)  bit  is  silently  ignored:  the  new program is executed, but the
       process's effective user (group) ID is left unchanged.   (This  mirrors
       the  semantics  of executing a set-user-ID or set-group-ID program that
       resides on a filesystem that was mounted with the  MS_NOSUID  flag,  as
       described in mount(2).)

       When  a  process's  user  and  group  IDs are passed over a UNIX domain
       socket to a process in a different user namespace (see the  description
       of  SCM_CREDENTIALS  in  unix(7)),  they are translated into the corre-
       sponding values as per the receiving process's user and group  ID  map-

       Namespaces are a Linux-specific feature.

       Over  the years, there have been a lot of features that have been added
       to the Linux kernel that have been made available  only  to  privileged
       users  because  of their potential to confuse set-user-ID-root applica-
       tions.  In general, it becomes safe to allow the root user  in  a  user
       namespace  to  use  those features because it is impossible, while in a
       user namespace, to gain more privilege than the root  user  of  a  user
       namespace has.

       Use  of  user  namespaces requires a kernel that is configured with the
       CONFIG_USER_NS option.  User namespaces require support in a  range  of
       subsystems across the kernel.  When an unsupported subsystem is config-
       ured into the kernel, it is not possible to configure  user  namespaces

       As  at  Linux  3.8, most relevant subsystems supported user namespaces,
       but a number of filesystems did not have the infrastructure  needed  to
       map  user  and  group IDs between user namespaces.  Linux 3.9 added the
       required infrastructure support for many of the  remaining  unsupported
       filesystems  (Plan  9 (9P), Andrew File System (AFS), Ceph, CIFS, CODA,
       NFS, and OCFS2).  Linux 3.12 added support the last of the  unsupported
       major filesystems, XFS.

       The  program  below  is designed to allow experimenting with user name-
       spaces, as well as other types of namespaces.  It creates namespaces as
       specified  by  command-line  options and then executes a command inside
       those namespaces.  The comments and usage() function inside the program
       provide a full explanation of the program.  The following shell session
       demonstrates its use.

       First, we look at the run-time environment:

           $ uname -rs     # Need Linux 3.8 or later
           Linux 3.8.0
           $ id -u         # Running as unprivileged user
           $ id -g

       Now start a new shell in new user (-U), mount (-m), and PID (-p)  name-
       spaces, with user ID (-M) and group ID (-G) 1000 mapped to 0 inside the
       user namespace:

           $ ./userns_child_exec -p -m -U -M '0 1000 1' -G '0 1000 1' bash

       The shell has PID 1, because it is the first process  in  the  new  PID

           bash$ echo $$
       Mounting  a new /proc filesystem and listing all of the processes visi-
       ble in the new PID namespace shows that the shell can't  see  any  pro-
       cesses outside the PID namespace:

           bash$ mount -t proc proc /proc
           bash$ ps ax
             PID TTY      STAT   TIME COMMAND
               1 pts/3    S      0:00 bash
              22 pts/3    R+     0:00 ps ax

       Inside  the  user  namespace,  the shell has user and group ID 0, and a
       full set of permitted and effective capabilities:

           bash$ cat /proc/$$/status | egrep '^[UG]id'
           Uid: 0    0    0    0
           Gid: 0    0    0    0
           bash$ cat /proc/$$/status | egrep '^Cap(Prm|Inh|Eff)'
           CapInh:   0000000000000000
           CapPrm:   0000001fffffffff
           CapEff:   0000001fffffffff

   Program source

       /* userns_child_exec.c

          Licensed under GNU General Public License v2 or later

          Create a child process that executes a shell command in new
          namespace(s); allow UID and GID mappings to be specified when
          creating a user namespace.
       #define _GNU_SOURCE
       #include <sched.h>
       #include <unistd.h>
       #include <stdlib.h>
       #include <sys/wait.h>
       #include <signal.h>
       #include <fcntl.h>
       #include <stdio.h>
       #include <string.h>
       #include <limits.h>
       #include <errno.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)

       struct child_args {
           char **argv;        /* Command to be executed by child, with args */
           int    pipe_fd[2];  /* Pipe used to synchronize parent and child */

       static int verbose;

       static void
       usage(char *pname)
           fprintf(stderr, "Usage: %s [options] cmd [arg...]\n\n", pname);
           fprintf(stderr, "Create a child process that executes a shell "
                   "command in a new user namespace,\n"
                   "and possibly also other new namespace(s).\n\n");
           fprintf(stderr, "Options can be:\n\n");
       #define fpe(str) fprintf(stderr, "    %s", str);
           fpe("-i          New IPC namespace\n");
           fpe("-m          New mount namespace\n");
           fpe("-n          New network namespace\n");
           fpe("-p          New PID namespace\n");
           fpe("-u          New UTS namespace\n");
           fpe("-U          New user namespace\n");
           fpe("-M uid_map  Specify UID map for user namespace\n");
           fpe("-G gid_map  Specify GID map for user namespace\n");
           fpe("-z          Map user's UID and GID to 0 in user namespace\n");
           fpe("            (equivalent to: -M '0 <uid> 1' -G '0 <gid> 1')\n");
           fpe("-v          Display verbose messages\n");
           fpe("If -z, -M, or -G is specified, -U is required.\n");
           fpe("It is not permitted to specify both -z and either -M or -G.\n");
           fpe("Map strings for -M and -G consist of records of the form:\n");
           fpe("    ID-inside-ns   ID-outside-ns   len\n");
           fpe("A map string can contain multiple records, separated"
               " by commas;\n");
           fpe("the commas are replaced by newlines before writing"
               " to map files.\n");


       /* Update the mapping file 'map_file', with the value provided in
          'mapping', a string that defines a UID or GID mapping. A UID or
          GID mapping consists of one or more newline-delimited records
          of the form:

              ID_inside-ns    ID-outside-ns   length

          Requiring the user to supply a string that contains newlines is
          of course inconvenient for command-line use. Thus, we permit the
          use of commas to delimit records in this string, and replace them
          with newlines before writing the string to the file. */

       static void
       update_map(char *mapping, char *map_file)
           int fd, j;
           size_t map_len;     /* Length of 'mapping' */

           /* Replace commas in mapping string with newlines */

           map_len = strlen(mapping);
           for (j = 0; j < map_len; j++)
               if (mapping[j] == ',')
                   mapping[j] = '\n';

           fd = open(map_file, O_RDWR);
           if (fd == -1) {
               fprintf(stderr, "ERROR: open %s: %s\n", map_file,

           if (write(fd, mapping, map_len) != map_len) {
               fprintf(stderr, "ERROR: write %s: %s\n", map_file,


       /* Linux 3.19 made a change in the handling of setgroups(2) and the
          'gid_map' file to address a security issue. The issue allowed
          *unprivileged* users to employ user namespaces in order to drop
          The upshot of the 3.19 changes is that in order to update the
          'gid_maps' file, use of the setgroups() system call in this
          user namespace must first be disabled by writing "deny" to one of
          the /proc/PID/setgroups files for this namespace.  That is the
          purpose of the following function. */

       static void
       proc_setgroups_write(pid_t child_pid, char *str)
           char setgroups_path[PATH_MAX];
           int fd;

           snprintf(setgroups_path, PATH_MAX, "/proc/%ld/setgroups",
                   (long) child_pid);

           fd = open(setgroups_path, O_RDWR);
           if (fd == -1) {

               /* We may be on a system that doesn't support
                  /proc/PID/setgroups. In that case, the file won't exist,
                  and the system won't impose the restrictions that Linux 3.19
                  added. That's fine: we don't need to do anything in order
                  to permit 'gid_map' to be updated.

                  However, if the error from open() was something other than
                  the ENOENT error that is expected for that case,  let the
                  user know. */

               if (errno != ENOENT)
                   fprintf(stderr, "ERROR: open %s: %s\n", setgroups_path,

           if (write(fd, str, strlen(str)) == -1)
               fprintf(stderr, "ERROR: write %s: %s\n", setgroups_path,


       static int              /* Start function for cloned child */
       childFunc(void *arg)
           struct child_args *args = (struct child_args *) arg;
           char ch;

           /* Wait until the parent has updated the UID and GID mappings.
              See the comment in main(). We wait for end of file on a
              pipe that will be closed by the parent process once it has
              updated the mappings. */

           close(args->pipe_fd[1]);    /* Close our descriptor for the write
                                          end of the pipe so that we see EOF
                                          when parent closes its descriptor */
           if (read(args->pipe_fd[0], &ch, 1) != 0) {
                       "Failure in child: read from pipe returned != 0\n");


           /* Execute a shell command */

           printf("About to exec %s\n", args->argv[0]);
           execvp(args->argv[0], args->argv);

       #define STACK_SIZE (1024 * 1024)

       static char child_stack[STACK_SIZE];    /* Space for child's stack */

       main(int argc, char *argv[])
           int flags, opt, map_zero;
           pid_t child_pid;
           struct child_args args;
           char *uid_map, *gid_map;
           const int MAP_BUF_SIZE = 100;
           char map_buf[MAP_BUF_SIZE];
           char map_path[PATH_MAX];

           /* Parse command-line options. The initial '+' character in
              the final getopt() argument prevents GNU-style permutation
              of command-line options. That's useful, since sometimes
              the 'command' to be executed by this program itself
              has command-line options. We don't want getopt() to treat
              those as options to this program. */

           flags = 0;
           verbose = 0;
           gid_map = NULL;
           uid_map = NULL;
           map_zero = 0;
           while ((opt = getopt(argc, argv, "+imnpuUM:G:zv")) != -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 'v': verbose = 1;                  break;
               case 'z': map_zero = 1;                 break;
               case 'M': uid_map = optarg;             break;
               case 'G': gid_map = optarg;             break;
               case 'U': flags |= CLONE_NEWUSER;       break;
               default:  usage(argv[0]);

           /* -M or -G without -U is nonsensical */

           if (((uid_map != NULL || gid_map != NULL || map_zero) &&
                       !(flags & CLONE_NEWUSER)) ||
                   (map_zero && (uid_map != NULL || gid_map != NULL)))

           args.argv = &argv[optind];

           /* We use a pipe to synchronize the parent and child, in order to
              ensure that the parent sets the UID and GID maps before the child
              calls execve(). This ensures that the child maintains its
              capabilities during the execve() in the common case where we
              want to map the child's effective user ID to 0 in the new user
              namespace. Without this synchronization, the child would lose
              its capabilities if it performed an execve() with nonzero
              user IDs (see the capabilities(7) man page for details of the
              transformation of a process's capabilities during execve()). */

           if (pipe(args.pipe_fd) == -1)

           /* Create the child in new namespace(s) */

           child_pid = clone(childFunc, child_stack + STACK_SIZE,
                             flags | SIGCHLD, &args);
           if (child_pid == -1)

           /* Parent falls through to here */

           if (verbose)
               printf("%s: PID of child created by clone() is %ld\n",
                       argv[0], (long) child_pid);

           /* Update the UID and GID maps in the child */

           if (uid_map != NULL || map_zero) {
               snprintf(map_path, PATH_MAX, "/proc/%ld/uid_map",
                       (long) child_pid);
               if (map_zero) {
                   snprintf(map_buf, MAP_BUF_SIZE, "0 %ld 1", (long) getuid());
                   uid_map = map_buf;
               update_map(uid_map, map_path);

           if (gid_map != NULL || map_zero) {
               proc_setgroups_write(child_pid, "deny");

               snprintf(map_path, PATH_MAX, "/proc/%ld/gid_map",
                       (long) child_pid);
               if (map_zero) {
                   snprintf(map_buf, MAP_BUF_SIZE, "0 %ld 1", (long) getgid());
                   gid_map = map_buf;
               update_map(gid_map, map_path);

           /* Close the write end of the pipe, to signal to the child that we
              have updated the UID and GID maps */


           if (waitpid(child_pid, NULL, 0) == -1)      /* Wait for child */

           if (verbose)
               printf("%s: terminating\n", argv[0]);


       newgidmap(1), newuidmap(1), clone(2), ptrace(2), setns(2),  unshare(2),
       proc(5),  subgid(5),  subuid(5),  capabilities(7), cgroup_namespaces(7)
       credentials(7), namespaces(7), pid_namespaces(7)

       The kernel source file Documentation/namespaces/resource-control.txt.

       This page is part of release 4.16 of the Linux  man-pages  project.   A
       description  of  the project, information about reporting bugs, and the
       latest    version    of    this    page,    can     be     found     at

Linux                             2018-02-02                USER_NAMESPACES(7)

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