user_namespaces(7) — Linux manual page


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

NAME         top

       user_namespaces - overview of Linux user namespaces

DESCRIPTION         top

       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).

       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) or unshare(2) using the CLONE_NEWUSER flag or a
       call to setns(2) that moves the caller into another user namespace
       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

       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 owned by
       (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 (i.e., calendar) 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 that owns 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 that owns 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="

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

       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 the other types of namespaces can be created with
       just the CAP_SYS_ADMIN capability in the caller's user namespace.

       When a nonuser 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.  Privileged operations on resources
       governed by the nonuser namespace require that the process has the
       necessary capabilities in the user namespace that owns the nonuser

       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 as the owner of 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 owns 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 that owns a nonuser namespace; 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

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

       *  The range of user IDs (group IDs) specified in each line cannot
          overlap with the ranges in any other lines.  In the initial imple‐
          mentation (Linux 3.8), this requirement was satisfied by a sim‐
          plistic 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) capabil‐
          ity 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) ca‐
             pability in the parent user namespace.

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

          *  Or otherwise all of the following restrictions apply:

             +  The data written to uid_map (gid_map) must consist of a sin‐
                gle 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 writ‐
       ten, 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 rele‐
       vant 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 pro‐
       cesses in the user namespace that contains the process pid are per‐
       mitted to employ the setgroups(2) system call; it displays "deny" if
       setgroups(2) is not permitted in that user namespace.  Note that re‐
       gardless of the value in the /proc/[pid]/setgroups file (and regard‐
       less of the process's capabilities), calls to setgroups(2) are also
       not permitted if /proc/[pid]/gid_map has not yet been set.

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

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

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

       Once /proc/[pid]/gid_map has been written to (which has the effect of
       enabling setgroups(2) in the user namespace), it is no longer possi‐
       ble to disallow setgroups(2) by writing "deny" to /proc/[pid]/set‐
       groups (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 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.  Be‐
       fore 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 name‐
       spaces, it became possible for an unprivileged process to create a
       new namespace in which the user had all privileges.  This then al‐
       lowed 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 de‐
       fined 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 sys‐
       tem 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 inte‐

   Accessing files
       In order to determine permissions when an unprivileged process ac‐
       cesses a file, the process credentials (UID, GID) and the file cre‐
       dentials are in effect mapped back to what they would be in the ini‐
       tial user namespace and then compared to determine the permissions
       that the process has on the file.  The same is also of other objects
       that employ the credentials plus permissions mask accessibility
       model, such as System V IPC objects

   Operation of file-related capabilities
       Certain capabilities allow a process to bypass various kernel-en‐
       forced restrictions when performing operations on files owned by
       other users or groups.  These capabilities are: CAP_CHOWN,

       Within a user namespace, these capabilities allow a process to bypass
       the rules if the process has the relevant capability over the file,
       meaning that:

       *  the process has the relevant effective capability in its user
          namespace; and

       *  the file's user ID and group ID both have valid mappings in the
          user namespace.

       The CAP_FOWNER capability is treated somewhat exceptionally: it al‐
       lows a process to bypass the corresponding rules so long as at least
       the file's user ID has a mapping in the user namespace (i.e., the
       file's group ID does not need to have a valid mapping).

   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 descrip‐
       tion of SCM_CREDENTIALS in unix(7)), they are translated into the
       corresponding values as per the receiving process's user and group ID

CONFORMING TO         top

       Namespaces are a Linux-specific feature.

NOTES         top

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

EXAMPLES         top

       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 in‐
       side the user namespace:

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

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

           bash$ echo $$

       Mounting a new /proc filesystem and listing all of the processes vis‐
       ible in the new PID namespace shows that the shell can't see any pro‐
       cesses outside the PID namespace:

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

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

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

   Program source

       /* userns_child_exec.c

          Licensed under GNU General Public License v2 or later

          Create a child process that executes a shell command in new
          namespace(s); allow UID and GID mappings to be specified when
          creating a user namespace.
       #define _GNU_SOURCE
       #include <sched.h>
       #include <unistd.h>
       #include <stdint.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;
           size_t map_len;     /* Length of 'mapping' */

           /* Replace commas in mapping string with newlines */

           map_len = strlen(mapping);
           for (int 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/%jd/setgroups",
                   (intmax_t) 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 = 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 %jd\n",
                       argv[0], (intmax_t) child_pid);

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

           if (uid_map != NULL || map_zero) {
               snprintf(map_path, PATH_MAX, "/proc/%jd/uid_map",
                       (intmax_t) child_pid);
               if (map_zero) {
                   snprintf(map_buf, MAP_BUF_SIZE, "0 %jd 1",
                           (intmax_t) 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/%jd/gid_map",
                       (intmax_t) child_pid);
               if (map_zero) {
                   snprintf(map_buf, MAP_BUF_SIZE, "0 %ld 1",
                           (intmax_t) 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]);


SEE ALSO         top

       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),

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

COLOPHON         top

       This page is part of release 5.09 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                            2020-11-01               USER_NAMESPACES(7)

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