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       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
       directory,    keys    (see    keyctl(2)),    and    capabilities   (see
       capabilities(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
       created 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_ADMIN  in that namespace; upon doing so, it gains a full set of
       capabilities 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
       member of the new user namespace created by the call.

       The child process created  by  clone(2)  with  the  CLONE_NEWUSER  flag
       starts  out  with  a  complete  set  of  capabilities  in  the new user
       namespace.  Likewise, a process that creates a new user namespace using
       unshare(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)), so that usually,
       unless it has a user ID of 0 within the  namespace  or  the  executable
       file  has  a  nonempty  inheritable capabilities mask, it 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.

       Having a capability inside  a  user  namespace  permits  a  process  to
       perform  operations (that require privilege) only on resources governed
       by that 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
          capability  set.   A  process can gain capabilities in its effective
          capability 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
          namespace.   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 removed descendant user namespaces as well.

   Interaction of user namespaces and other types of namespaces
       Starting   in   Linux  3.8,  unprivileged  processes  can  create  user
       namespaces, and mount, PID, IPC, network, and  UTS  namespaces  can  be
       created  with  just  the  CAP_SYS_ADMIN capability in the caller's user

       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
       capabilities 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 IPC, mount, network, PID, or UTS 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.

   Restrictions on mount namespaces
       Note the following points with respect to mount namespaces:

       *  A  mount  namespace  has an owner user namespace.  A mount namespace
          whose  owner  user  namespace  is  different  from  the  owner  user
          namespace  of  its  parent  mount  namespace  is  considered  a less
          privileged mount namespace.

       *  When creating a less privileged mount namespace, shared  mounts  are
          reduced  to  slave  mounts.  This ensures that mappings performed in
          less  privileged  mount  namespaces  will  not  propagate  to   more
          privileged mount namespaces.

       *  Mounts  that  come  as  a single unit from more privileged mount are
          locked together and may not be separated in a less privileged  mount
          namespace.   (The unshare(2) CLONE_NEWNS operation brings across all
          of the mounts from the original mount namespace as  a  single  unit,
          and   recursive  mounts  that  propagate  between  mount  namespaces
          propagate as a single unit.)

       *  The mount(2) flags MS_RDONLY, MS_NOSUID, MS_NOEXEC, and the  "atime"
          flags   (MS_NOATIME,  MS_NODIRATIME,  MS_RELATIME)  settings  become
          locked when propagated from a more privileged to a  less  privileged
          mount namespace, and may not be changed in the less privileged mount

       *  A file or directory that is a mount point in one namespace  that  is
          not a mount point in another namespace, may be renamed, unlinked, or
          removed (rmdir(2)) in the mount namespace in which it is not a mount
          point (subject to the usual permission checks).

          Previously,  attempting  to  unlink,  rename,  or  remove  a file or
          directory that was a mount point in another  mount  namespace  would
          result  in the error EBUSY.  That behavior had technical problems of
          enforcement (e.g., for NFS) and permitted denial-of-service  attacks
          against  more  privileged users.  (i.e., preventing individual files
          from being updated by bind mounting on top of them).

   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

       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
       namespace  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,
       depending  on  the  user  ID  mappings  for  the user namespaces of the
       reading processes.

       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
       namespace 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
              namespace 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
       direction,  to produce values relative to the process user and group ID

       The  initial  user  namespace  has  no  parent  namespace,   but,   for
       consistency,  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.,
       setreuid(2)) as a way to specify  "no  user  ID".   Leaving  (uid_t) -1
       unmapped  and  unusable guarantees that there will be no confusion when
       using 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
       error 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 an (arbitrary) limit on the number of lines  in  the  file.
          As  at Linux 3.18, the limit is five 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
          implementation  (Linux  3.8),  this  requirement  was satisfied by a
          simplistic 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,
          allowing 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)
             capability in the parent user namespace.

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

          *  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]/setgroups  file  (see  below)  before  writing  to

       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  only  transition from setgroups(2) being disallowed to
       setgroups(2) being allowed.

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

       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)
       system call can't subsequently be reenabled (by writing "allow" to  the
       file)  in  this  user namespace.  (Attempts to do so will fail with the
       error EPERM.)  This restriction also propagates down to all child  user
       namespaces of this user namespace.

       The  /proc/[pid]/setgroups  file  was  added  in  Linux  3.19,  but was
       backported to many earlier stable kernel series, because it addresses a
       security  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
       privileged process (one with  the  CAP_SETGID  capability)  could  call
       setgroups(2).   However,  with  the introduction of user namespaces, it
       became possible for an unprivileged process to create a  new  namespace
       in  which  the  user  had  all  privileges.  This then allowed formerly
       unprivileged 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() before a user ID mapping has been defined
       for the namespace.   In  most  such  cases,  an  unmapped  user  ID  is
       converted 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),
       credentials 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
       siginfo_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

   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
       corresponding values as per the receiving process's user and  group  ID


       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
       applications.  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
       configured into the kernel,  it  is  not  possible  to  configure  user
       namespaces support.

       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 3.11 added support the last of the  unsupported
       major filesystems, XFS.


       The  program  below  is  designed  to  allow  experimenting  with  user
       namespaces,  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)
       namespaces, 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 $$

       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

       Mounting a new /proc  filesystem  and  listing  all  of  the  processes
       visible  in  the  new  PID namespace shows that the shell can't see any
       processes 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

   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),  setns(2),  unshare(2),  proc(5),
       subgid(5),  subuid(5),  credentials(7), capabilities(7), namespaces(7),

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


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