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
       directory,  keys  (see  keyctl(2)),  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_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 mem-
       ber 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 names-
       pace.  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  per-
       form  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  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  names-
          pace.   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 names-
       paces, and mount, PID, IPC, network, and UTS 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  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 names-
          pace 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 privi-
          leged 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  propa-
          gate 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 direc-
          tory  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  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 names-
       pace 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  read-
       ing 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  names-
       pace  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  names-
              pace  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.,
       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 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  names-

          *  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  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) sys-
       tem 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 back-
       ported 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 priv-
       ileged process (one with the CAP_SETGID  capability)  could  call  set-
       groups(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  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 3.11 added support the last of the  unsupported
       major filesystems, XFS.

       The  program  below is designed to allow experimenting with user names-
       paces, 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) names-
       paces,  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  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

   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.

       This page is part of release 4.04 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                             2015-03-29                USER_NAMESPACES(7)
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