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          user_namespaces - overview of Linux user namespaces

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

          User namespaces isolate security-related identifiers and
          attributes, in particular, user IDs and group IDs (see
          credentials(7)), the root directory, keys (see 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 opera-
          tions 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 names-
          paces.  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 capabili-
          ties in that namespace.

          A call to clone(2) or unshare(2) with the CLONE_NEWUSER flag
          makes the new child process (for clone(2)) or the caller
          (for unshare(2)) a 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

          The child process created by clone(2) with the CLONE_NEWUSER

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          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 pre-
          vious (in the case of unshare(2) and setns(2)) user names-
          pace, even if the new namespace is created or joined by the
          root user (i.e., a process with user ID 0 in the root names-

          Note that a call to execve(2) will cause a process's capa-
          bilities 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 dis-
          abled) 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 mem-
          bership by using a pair of setns(2) calls to move to another
          user namespace and then return to its original user names-

          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

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             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 fur-
             ther removed descendant user namespaces as well.  The
             NS_GET_OWNER_UID ioctl(2) operation can be used to dis-
             cover the user ID of the owner of the namespace; see

        Effect of capabilities within a user
          Having a capability inside a user namespace permits a pro-
          cess to perform operations (that require privilege) only on
          resources governed by that namespace.  In other words, hav-
          ing 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 (gov-
          erned 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


          Holding CAP_SYS_ADMIN within the user namespace that owns a
          process's PID namespace allows (since Linux 3.8) that pro-
          cess 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

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          initial user namespace.

        Interaction of user namespaces and other
          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 opera-
          tions on resources governed by the nonuser namespace require
          that the process has the necessary capabilities in the user
          namespace that owns the nonuser 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 pos-
          sible for an unprivileged caller to specify this combination
          of flags.

          When a new namespace (other than a user namespace) is cre-
          ated 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 privi-
          leged operations that operate on global resources isolated
          by the namespace, the permission checks are performed
          according to the process's capabilities in the user names-
          pace that the kernel associated with the new namespace.  For
          example, suppose that a process attempts to change the host-
          name (sethostname(2)), a resource governed by the UTS names-
          pace.  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

        User and group ID mappings: uid_map
          When a user namespace is created, it starts out without a
          mapping of user IDs (group IDs) to the parent user names-
          pace.  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.

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          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, depend-
          ing 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 num-
          bers 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 fol-

          (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 inter-
              preted depends on whether the process that opened
              uid_map and the process pid are in the same user names-
              pace, as follows:

              a) If the two processes are in different user names-
                 paces: field two is the start of a range of user IDs
                 in the user namespace of the process that opened

              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 map-
                 ping of user IDs into the user namespace of the pro-
                 cess 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.

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

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

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

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

        Defining user and group ID mappings:
          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.  Simi-
          lar 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).

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          *  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

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

          Writes that violate the above rules fail with the error

          In order for a process to write to the /proc/[pid]/uid_map
          (/proc/[pid]/gid_map) file, all of the following require-
          ments 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 map-
             ping 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

                +  The writing process must have the same effective
                   user ID as the process that created the user names-

                +  In the case of gid_map, use of the setgroups(2)

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                   system call must first be denied by writing dqdenydq
                   to the /proc/[pid]/setgroups file (see below)
                   before writing to gid_map.

          Writes that violate the above rules fail with the error

        Interaction with system calls that change
          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 dqdenydq 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 dqallowdq
          if processes in the user namespace that contains the process
          pid are permitted to employ the setgroups(2) system call; it
          displays dqdenydq 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 dqallowdq or
          dqdenydq to this file before writing a group ID mapping for
          this user namespace to the file /proc/[pid]/gid_map. Writing
          the string dqdenydq 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.

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

          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
          dqdenydq 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 dqdenydq, then the
          setgroups(2) system call can't subsequently be reenabled (by
          writing dqallowdq 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 names-
          paces 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 pro-
          cess 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 capabil-
          ity) could call setgroups(2).  However, with the introduc-
          tion of user namespaces, it became possible for an unprivi-
          leged 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

        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

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          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).

        Accessing files
          In order to determine permissions when an unprivileged pro-
          cess accesses a file, the process credentials (UID, GID) and
          the file credentials are in effect mapped back to what they
          would be in the initial 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 creden-
          tials 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-enforced restrictions when performing operations on
          files owned by other users or groups.  These capabilities
          CAP_FOWNER, and CAP_FSETID.

          Within a user namespace, these capabilities allow a process
          to bypass the rules if the process has the relevant capabil-
          ity 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 allows 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

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

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

          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 con-
          fuse 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 names-
          pace, 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.

          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 com-
          ments and usage() function inside the program provide a full
          explanation of the program.  The following shell session

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          demonstrates its use.

          First, we look at the run-time environment:

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

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

              $ ./userns_child_exec -p -m -U -M aq0 1000 1aq -G aq0 1000 1aq bash

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

              bash$ echo $$

          Mounting a new /proc filesystem and listing all of the pro-
          cesses 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/$$/status | egrep aqha[UG]idaq
              Uid: 0    0    0    0
              Gid: 0    0    0    0
              bash$ cat /proc/$$/status | egrep aqhaCap(Prm|Inh|Eff)aq
              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

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             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 aqerrnoaq 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 useraqs UID and GID to 0 in user namespace\n");
              fpe("            (equivalent to: -M aq0 <uid> 1aq -G aq0 <gid> 1aq)\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");

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     USER_NAMESPACES(7)        (2020-11-01)         USER_NAMESPACES(7)

              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 aqmap_fileaq, with the value provided in
             aqmappingaq, 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 aqmappingaq */

              /* Replace commas in mapping string with newlines */

              map_len = strlen(mapping);
              for (int j = 0; j < map_len; j++)
                  if (mapping[j] == aq,aq)
                      mapping[j] = aq\naq;

              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,


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     USER_NAMESPACES(7)        (2020-11-01)         USER_NAMESPACES(7)

          /* Linux 3.19 made a change in the handling of setgroups(2) and the
             aqgid_mapaq 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
             aqgid_mapsaq 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 doesnaqt support
                     /proc/PID/setgroups. In that case, the file wonaqt exist,
                     and the system wonaqt impose the restrictions that Linux 3.19
                     added. Thataqs fine: we donaqt need to do anything in order
                     to permit aqgid_mapaq 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

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     USER_NAMESPACES(7)        (2020-11-01)         USER_NAMESPACES(7)

                 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 childaqs 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 aq+aq character in
                 the final getopt() argument prevents GNU-style permutation
                 of command-line options. Thataqs useful, since sometimes
                 the aqcommandaq to be executed by this program itself
                 has command-line options. We donaqt 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 aqiaq: flags |= CLONE_NEWIPC;        break;
                  case aqmaq: flags |= CLONE_NEWNS;         break;

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     USER_NAMESPACES(7)        (2020-11-01)         USER_NAMESPACES(7)

                  case aqnaq: flags |= CLONE_NEWNET;        break;
                  case aqpaq: flags |= CLONE_NEWPID;        break;
                  case aquaq: flags |= CLONE_NEWUTS;        break;
                  case aqvaq: verbose = 1;                  break;
                  case aqzaq: map_zero = 1;                 break;
                  case aqMaq: uid_map = optarg;             break;
                  case aqGaq: gid_map = optarg;             break;
                  case aqUaq: 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 childaqs 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 processaqs 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);

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     USER_NAMESPACES(7)        (2020-11-01)         USER_NAMESPACES(7)

                  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]);


          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-

          This page is part of release 5.10 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

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