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

       user_namespaces - overview of Linux user namespaces

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

       User namespaces isolate security-related identifiers and attributes, in
       particular, user IDs and group IDs (see credentials(7)), the root
       directory, keys (see 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 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

       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)
           * overlayfs (since Linux 5.11)

       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=" option).

       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 namespace.

       If CLONE_NEWUSER is specified along with other CLONE_NEW* flags in a
       single clone(2) or unshare(2) call, the user namespace is guaranteed to
       be created first, giving the child (clone(2)) or caller (unshare(2))
       privileges over the remaining namespaces created by the call.  Thus, it
       is possible for an unprivileged caller to specify this combination of

       When a new 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 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

       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

       *  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

       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

       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).

   Accessing files
       In order to determine permissions when an unprivileged process 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 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-enforced
       restrictions when performing operations on files owned by other users or
       groups.  These capabilities are: CAP_CHOWN, CAP_DAC_OVERRIDE,

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

       *  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

       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 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

       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 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.

       The program below is designed to allow experimenting with user
       namespaces, as well as other types of namespaces.  It creates namespaces
       as specified by command-line options and then executes a command inside
       those namespaces.  The comments and usage() function inside the program
       provide a full explanation of the program.  The following shell session
       demonstrates its use.

       First, we look at the run-time environment:

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

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

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

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

           bash$ echo $$

       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 <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]);


       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.

       This page is part of release 5.11 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                              2021-03-22                 USER_NAMESPACES(7)