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NAME

       user_namespaces - overview of Linux user namespaces

DESCRIPTION

       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  keyrings(7)),  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 NS_GET_PARENT ioctl(2) operation can be used to  discover  the  parental  relationship
       between user namespaces; see ioctl_ns(2).

   Capabilities
       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)).  Consequently, unless the process has a user ID of  0
       within the namespace, or the executable 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  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.   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 perform operations  (that
       require privilege) only on resources governed by that namespace.  In other words, having a
       capability in a user namespace 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 resources 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 namespace 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 hierarchies (i.e., cgroup filesystems  mounted  with
       the "none,name=" option).

       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 namespaces, 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 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
       flags.

       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  namespace  is  associated
       with  the  process's  UTS  namespace,  and  check  whether  the  process  has the required
       capability (CAP_SYS_ADMIN) in that user namespace.

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

   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 mappings.

       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 namespace.

       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 mappings.

       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 rules:

       *  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 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 met:

       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 namespace.

          *  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
                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 IDs.

       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 setgroups(2).

       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 "allow".

       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 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(2) 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 proc(5).

       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
       integer).

   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).)

   Miscellaneous
       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
       mappings.

CONFORMING TO

       Namespaces are a Linux-specific feature.

NOTES

       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.

   Availability
       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 3.12 added support the last of the unsupported major filesystems, XFS.

EXAMPLE

       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
           1000
           $ id -g
           1000

       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 namespace:

           bash$ echo $$
           1
       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

       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("\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("\n");
           fpe("Map strings for -M and -G consist of records of the form:\n");
           fpe("\n");
           fpe("    ID-inside-ns   ID-outside-ns   len\n");
           fpe("\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");

           exit(EXIT_FAILURE);
       }

       /* 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,
                       strerror(errno));
               exit(EXIT_FAILURE);
           }

           if (write(fd, mapping, map_len) != map_len) {
               fprintf(stderr, "ERROR: write %s: %s\n", map_file,
                       strerror(errno));
               exit(EXIT_FAILURE);
           }

           close(fd);
       }

       /* 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,
                       strerror(errno));
               return;
           }

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

           close(fd);
       }

       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) {
               fprintf(stderr,
                       "Failure in child: read from pipe returned != 0\n");
               exit(EXIT_FAILURE);
           }

           close(args->pipe_fd[0]);

           /* Execute a shell command */

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

       #define STACK_SIZE (1024 * 1024)

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

       int
       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)))
               usage(argv[0]);

           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)
               errExit("pipe");

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

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

           /* 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 */

           close(args.pipe_fd[1]);

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

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

           exit(EXIT_SUCCESS);
       }

SEE ALSO

       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.

COLOPHON

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       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at https://www.kernel.org/doc/man-pages/.