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

NAME
       capabilities - overview of Linux capabilities

DESCRIPTION
       For  the purpose of performing permission checks, traditional UNIX implementations distin‐
       guish two categories of processes: privileged processes (whose effective  user  ID  is  0,
       referred  to  as  superuser  or  root), and unprivileged processes (whose effective UID is
       nonzero).  Privileged processes bypass all kernel permission  checks,  while  unprivileged
       processes are subject to full permission checking based on the process's credentials (usu‐
       ally: effective UID, effective GID, and supplementary group list).

       Starting with kernel 2.2, Linux divides the privileges traditionally associated with supe‐
       ruser  into  distinct units, known as capabilities, which can be independently enabled and
       disabled.  Capabilities are a per-thread attribute.

   Capabilities list
       The following list shows the capabilities implemented on  Linux,  and  the  operations  or
       behaviors that each capability permits:

       CAP_AUDIT_CONTROL (since Linux 2.6.11)
              Enable and disable kernel auditing; change auditing filter rules; retrieve auditing
              status and filtering rules.

       CAP_AUDIT_READ (since Linux 3.16)
              Allow reading the audit log via a multicast netlink socket.

       CAP_AUDIT_WRITE (since Linux 2.6.11)
              Write records to kernel auditing log.

       CAP_BLOCK_SUSPEND (since Linux 3.5)
              Employ  features   that   can   block   system   suspend   (epoll(7)   EPOLLWAKEUP,
              /proc/sys/wake_lock).

       CAP_CHOWN
              Make arbitrary changes to file UIDs and GIDs (see chown(2)).

       CAP_DAC_OVERRIDE
              Bypass file read, write, and execute permission checks.  (DAC is an abbreviation of
              "discretionary access control".)

       CAP_DAC_READ_SEARCH
              * Bypass file read permission checks and  directory  read  and  execute  permission
                checks;
              * Invoke open_by_handle_at(2).

       CAP_FOWNER
              * Bypass  permission  checks on operations that normally require the filesystem UID
                of the process to match the UID of the file (e.g., chmod(2), utime(2)), excluding
                those operations covered by CAP_DAC_OVERRIDE and CAP_DAC_READ_SEARCH;
              * set extended file attributes (see chattr(1)) on arbitrary files;
              * set Access Control Lists (ACLs) on arbitrary files;
              * ignore directory sticky bit on file deletion;
              * specify O_NOATIME for arbitrary files in open(2) and fcntl(2).

       CAP_FSETID
              Don't clear set-user-ID and set-group-ID mode bits when a file is modified; set the
              set-group-ID bit for a file whose GID does not match the filesystem or any  of  the
              supplementary GIDs of the calling process.

       CAP_IPC_LOCK
              Lock memory (mlock(2), mlockall(2), mmap(2), shmctl(2)).

       CAP_IPC_OWNER
              Bypass permission checks for operations on System V IPC objects.

       CAP_KILL
              Bypass  permission  checks for sending signals (see kill(2)).  This includes use of
              the ioctl(2) KDSIGACCEPT operation.

       CAP_LEASE (since Linux 2.4)
              Establish leases on arbitrary files (see fcntl(2)).

       CAP_LINUX_IMMUTABLE
              Set the FS_APPEND_FL and FS_IMMUTABLE_FL inode flags (see chattr(1)).

       CAP_MAC_ADMIN (since Linux 2.6.25)
              Override Mandatory Access Control (MAC).  Implemented for the Smack Linux  Security
              Module (LSM).

       CAP_MAC_OVERRIDE (since Linux 2.6.25)
              Allow MAC configuration or state changes.  Implemented for the Smack LSM.

       CAP_MKNOD (since Linux 2.4)
              Create special files using mknod(2).

       CAP_NET_ADMIN
              Perform various network-related operations:
              * interface configuration;
              * administration of IP firewall, masquerading, and accounting;
              * modify routing tables;
              * bind to any address for transparent proxying;
              * set type-of-service (TOS)
              * clear driver statistics;
              * set promiscuous mode;
              * enabling multicasting;
              * use setsockopt(2) to set the following socket options: SO_DEBUG, SO_MARK, SO_PRI‐
                ORITY (for a priority outside the range 0 to 6), SO_RCVBUFFORCE,  and  SO_SNDBUF‐
                FORCE.

       CAP_NET_BIND_SERVICE
              Bind a socket to Internet domain privileged ports (port numbers less than 1024).

       CAP_NET_BROADCAST
              (Unused)  Make socket broadcasts, and listen to multicasts.

       CAP_NET_RAW
              * use RAW and PACKET sockets;
              * bind to any address for transparent proxying.

       CAP_SETGID
              Make  arbitrary manipulations of process GIDs and supplementary GID list; forge GID
              when passing socket credentials via UNIX domain sockets; write a group  ID  mapping
              in a user namespace (see user_namespaces(7)).

       CAP_SETFCAP (since Linux 2.6.24)
              Set file capabilities.

       CAP_SETPCAP
              If file capabilities are not supported: grant or remove any capability in the call‐
              er's permitted capability set to or from any  other  process.   (This  property  of
              CAP_SETPCAP is not available when the kernel is configured to support file capabil‐
              ities, since CAP_SETPCAP has entirely different semantics for such kernels.)

              If file capabilities are supported: add any capability from  the  calling  thread's
              bounding  set  to its inheritable set; drop capabilities from the bounding set (via
              prctl(2) PR_CAPBSET_DROP); make changes to the securebits flags.

       CAP_SETUID
              Make arbitrary manipulations of process UIDs (setuid(2), setreuid(2), setresuid(2),
              setfsuid(2));  forge  UID  when passing socket credentials via UNIX domain sockets;
              write a user ID mapping in a user namespace (see user_namespaces(7)).

       CAP_SYS_ADMIN
              * Perform a range  of  system  administration  operations  including:  quotactl(2),
                mount(2), umount(2), swapon(2), swapoff(2), sethostname(2), and setdomainname(2);
              * perform privileged syslog(2) operations (since Linux 2.6.37, CAP_SYSLOG should be
                used to permit such operations);
              * perform VM86_REQUEST_IRQ vm86(2) command;
              * perform IPC_SET and IPC_RMID operations on arbitrary System V IPC objects;
              * override RLIMIT_NPROC resource limit;
              * perform operations on trusted and security Extended Attributes (see xattr(7));
              * use lookup_dcookie(2);
              * use  ioprio_set(2)  to  assign  IOPRIO_CLASS_RT   and   (before   Linux   2.6.25)
                IOPRIO_CLASS_IDLE I/O scheduling classes;
              * forge PID when passing socket credentials via UNIX domain sockets;
              * exceed  /proc/sys/fs/file-max, the system-wide limit on the number of open files,
                in system calls that open files (e.g., accept(2), execve(2), open(2), pipe(2));
              * employ CLONE_* flags that create new  namespaces  with  clone(2)  and  unshare(2)
                (but, since Linux 3.8, creating user namespaces does not require any capability);
              * call perf_event_open(2);
              * access privileged perf event information;
              * call setns(2) (requires CAP_SYS_ADMIN in the target namespace);
              * call fanotify_init(2);
              * call bpf(2);
              * perform KEYCTL_CHOWN and KEYCTL_SETPERM keyctl(2) operations;
              * perform madvise(2) MADV_HWPOISON operation;
              * employ the TIOCSTI ioctl(2) to insert characters into the input queue of a termi‐
                nal other than the caller's controlling terminal;
              * employ the obsolete nfsservctl(2) system call;
              * employ the obsolete bdflush(2) system call;
              * perform various privileged block-device ioctl(2) operations;
              * perform various privileged filesystem ioctl(2) operations;
              * perform administrative operations on many device drivers.

       CAP_SYS_BOOT
              Use reboot(2) and kexec_load(2).

       CAP_SYS_CHROOT
              Use chroot(2).

       CAP_SYS_MODULE
              Load and unload kernel modules (see init_module(2) and delete_module(2));  in  ker‐
              nels before 2.6.25: drop capabilities from the system-wide capability bounding set.

       CAP_SYS_NICE
              * Raise  process nice value (nice(2), setpriority(2)) and change the nice value for
                arbitrary processes;
              * set real-time scheduling policies for calling process, and set  scheduling  poli‐
                cies  and  priorities  for arbitrary processes (sched_setscheduler(2), sched_set‐
                param(2), shed_setattr(2));
              * set CPU affinity for arbitrary processes (sched_setaffinity(2));
              * set I/O scheduling class and priority for arbitrary processes (ioprio_set(2));
              * apply migrate_pages(2) to arbitrary processes and allow processes to be  migrated
                to arbitrary nodes;
              * apply move_pages(2) to arbitrary processes;
              * use the MPOL_MF_MOVE_ALL flag with mbind(2) and move_pages(2).

       CAP_SYS_PACCT
              Use acct(2).

       CAP_SYS_PTRACE
              *  Trace arbitrary processes using ptrace(2);
              *  apply get_robust_list(2) to arbitrary processes;
              *  transfer   data   to   or   from   the   memory  of  arbitrary  processes  using
                 process_vm_readv(2) and process_vm_writev(2).
              *  inspect processes using kcmp(2).

       CAP_SYS_RAWIO
              * Perform I/O port operations (iopl(2) and ioperm(2));
              * access /proc/kcore;
              * employ the FIBMAP ioctl(2) operation;
              * open devices for accessing x86 model-specific registers (MSRs, see msr(4))
              * update /proc/sys/vm/mmap_min_addr;
              * create  memory   mappings   at   addresses   below   the   value   specified   by
                /proc/sys/vm/mmap_min_addr;
              * map files in /proc/bus/pci;
              * open /dev/mem and /dev/kmem;
              * perform various SCSI device commands;
              * perform certain operations on hpsa(4) and cciss(4) devices;
              * perform a range of device-specific operations on other devices.

       CAP_SYS_RESOURCE
              * Use reserved space on ext2 filesystems;
              * make ioctl(2) calls controlling ext3 journaling;
              * override disk quota limits;
              * increase resource limits (see setrlimit(2));
              * override RLIMIT_NPROC resource limit;
              * override maximum number of consoles on console allocation;
              * override maximum number of keymaps;
              * allow more than 64hz interrupts from the real-time clock;
              * raise  msg_qbytes  limit  for  a  System  V  message  queue  above  the  limit in
                /proc/sys/kernel/msgmnb (see msgop(2) and msgctl(2));
              * override the /proc/sys/fs/pipe-size-max limit when setting the capacity of a pipe
                using the F_SETPIPE_SZ fcntl(2) command.
              * use  F_SETPIPE_SZ to increase the capacity of a pipe above the limit specified by
                /proc/sys/fs/pipe-max-size;
              * override /proc/sys/fs/mqueue/queues_max limit when creating POSIX message  queues
                (see mq_overview(7));
              * employ prctl(2) PR_SET_MM operation;
              * set /proc/PID/oom_score_adj to a value lower than the value last set by a process
                with CAP_SYS_RESOURCE.

       CAP_SYS_TIME
              Set system clock (settimeofday(2), stime(2), adjtimex(2)); set real-time (hardware)
              clock.

       CAP_SYS_TTY_CONFIG
              Use vhangup(2); employ various privileged ioctl(2) operations on virtual terminals.

       CAP_SYSLOG (since Linux 2.6.37)
              *  Perform privileged syslog(2) operations.  See syslog(2) for information on which
                 operations require privilege.
              *  View kernel addresses exposed via /proc and other interfaces when /proc/sys/ker‐
                 nel/kptr_restrict  has the value 1.  (See the discussion of the kptr_restrict in
                 proc(5).)

       CAP_WAKE_ALARM (since Linux 3.0)
              Trigger something that will  wake  up  the  system  (set  CLOCK_REALTIME_ALARM  and
              CLOCK_BOOTTIME_ALARM timers).

   Past and current implementation
       A full implementation of capabilities requires that:

       1. For  all  privileged  operations,  the  kernel  must  check  whether the thread has the
          required capability in its effective set.

       2. The kernel must provide system calls allowing a thread's capability sets to be  changed
          and retrieved.

       3. The  filesystem  must  support  attaching capabilities to an executable file, so that a
          process gains those capabilities when the file is executed.

       Before kernel 2.6.24, only the first two of  these  requirements  are  met;  since  kernel
       2.6.24, all three requirements are met.

   Thread capability sets
       Each thread has three capability sets containing zero or more of the above capabilities:

       Permitted:
              This  is  a  limiting  superset  for the effective capabilities that the thread may
              assume.  It is also a limiting superset for the capabilities that may be  added  to
              the  inheritable  set  by a thread that does not have the CAP_SETPCAP capability in
              its effective set.

              If a thread drops a capability from its permitted set, it can never reacquire  that
              capability  (unless  it  execve(2)s either a set-user-ID-root program, or a program
              whose associated file capabilities grant that capability).

       Inheritable:
              This is a set of capabilities preserved across an execve(2).  Inheritable capabili‐
              ties  remain  inheritable  when executing any program, and inheritable capabilities
              are added to the permitted set when executing a program that has the  corresponding
              bits set in the file inheritable set.

              Because  inheritable capabilities are not generally preserved across execve(2) when
              running as a non-root user, applications that wish to run helper programs with ele‐
              vated capabilities should consider using ambient capabilities, described below.

       Effective:
              This is the set of capabilities used by the kernel to perform permission checks for
              the thread.

       Ambient (since Linux 4.3):
              This is a set of capabilities that are preserved across an execve(2) of  a  program
              that  is  not  privileged.   The ambient capability set obeys the invariant that no
              capability can ever be ambient if it is not both permitted and inheritable.

              The ambient capability set can be directly modified using prctl(2).  Ambient  capa‐
              bilities  are  automatically  lowered  if  either of the corresponding permitted or
              inheritable capabilities is lowered.

              Executing a program that changes UID or GID due to the set-user-ID or  set-group-ID
              bits or executing a program that has any file capabilities set will clear the ambi‐
              ent set.  Ambient capabilities are added to the permitted set and assigned  to  the
              effective set when execve(2) is called.

       A  child  created  via fork(2) inherits copies of its parent's capability sets.  See below
       for a discussion of the treatment of capabilities during execve(2).

       Using capset(2), a thread may manipulate its own capability sets (see below).

       Since Linux 3.2, the file /proc/sys/kernel/cap_last_cap exposes the numerical value of the
       highest  capability  supported  by  the  running kernel; this can be used to determine the
       highest bit that may be set in a capability set.

   File capabilities
       Since kernel 2.6.24, the kernel supports associating capability sets  with  an  executable
       file  using  setcap(8).  The file capability sets are stored in an extended attribute (see
       setxattr(2)) named security.capability.  Writing to this extended attribute  requires  the
       CAP_SETFCAP capability.  The file capability sets, in conjunction with the capability sets
       of the thread, determine the capabilities of a thread after an execve(2).

       The three file capability sets are:

       Permitted (formerly known as forced):
              These capabilities are automatically permitted to the  thread,  regardless  of  the
              thread's inheritable capabilities.

       Inheritable (formerly known as allowed):
              This  set is ANDed with the thread's inheritable set to determine which inheritable
              capabilities are enabled in the permitted set of the thread after the execve(2).

       Effective:
              This is not a set, but rather just a single bit.  If this bit is set,  then  during
              an  execve(2)  all of the new permitted capabilities for the thread are also raised
              in the effective set.  If this bit is not set, then after an execve(2), none of the
              new permitted capabilities is in the new effective set.

              Enabling  the  file  effective  capability  bit  implies that any file permitted or
              inheritable capability that causes a thread to acquire the corresponding  permitted
              capability  during an execve(2) (see the transformation rules described below) will
              also acquire that capability in its effective set.  Therefore, when assigning capa‐
              bilities  to  a file (setcap(8), cap_set_file(3), cap_set_fd(3)), if we specify the
              effective flag as being enabled for any capability, then the  effective  flag  must
              also be specified as enabled for all other capabilities for which the corresponding
              permitted or inheritable flags is enabled.

   Transformation of capabilities during execve()
       During an execve(2), the kernel calculates the new capabilities of the process  using  the
       following algorithm:

           P'(ambient) = (file is privileged) ? 0 : P(ambient)

           P'(permitted) = (P(inheritable) & F(inheritable)) |
                           (F(permitted) & cap_bset) | P'(ambient)

           P'(effective) = F(effective) ? P'(permitted) : P'(ambient)

           P'(inheritable) = P(inheritable)    [i.e., unchanged]

       where:

           P         denotes the value of a thread capability set before the execve(2)

           P'        denotes the value of a capability set after the execve(2)

           F         denotes a file capability set

           cap_bset  is the value of the capability bounding set (described below).

       A  privileged file is one that has capabilities or has the set-user-ID or set-group-ID bit
       set.

   Capabilities and execution of programs by root
       In order to provide an all-powerful root using capability sets, during an execve(2):

       1. If a set-user-ID-root program is being executed, or the real user ID of the process  is
          0 (root) then the file inheritable and permitted sets are defined to be all ones (i.e.,
          all capabilities enabled).

       2. If a set-user-ID-root program is being executed, then the file effective bit is defined
          to be one (enabled).

       The  upshot  of  the above rules, combined with the capabilities transformations described
       above, is that when a process execve(2)s a set-user-ID-root program,  or  when  a  process
       with  an effective UID of 0 execve(2)s a program, it gains all capabilities in its permit‐
       ted and effective capability sets, except those masked out by the capability bounding set.
       This provides semantics that are the same as those provided by traditional UNIX systems.

   Capability bounding set
       The capability bounding set is a security mechanism that can be used to limit the capabil‐
       ities that can be gained during an execve(2).  The bounding set is used in  the  following
       ways:

       * During  an execve(2), the capability bounding set is ANDed with the file permitted capa‐
         bility set, and the result of this operation is assigned to the thread's permitted capa‐
         bility  set.  The capability bounding set thus places a limit on the permitted capabili‐
         ties that may be granted by an executable file.

       * (Since Linux 2.6.25) The capability bounding set acts as a  limiting  superset  for  the
         capabilities  that  a thread can add to its inheritable set using capset(2).  This means
         that if a capability is not in the bounding set, then a thread can't add this capability
         to its inheritable set, even if it was in its permitted capabilities, and thereby cannot
         have this capability preserved in its permitted set when it execve(2)s a file  that  has
         the capability in its inheritable set.

       Note  that  the  bounding set masks the file permitted capabilities, but not the inherited
       capabilities.  If a thread maintains a capability in its inherited set that is not in  its
       bounding  set,  then it can still gain that capability in its permitted set by executing a
       file that has the capability in its inherited set.

       Depending on the kernel version, the capability  bounding  set  is  either  a  system-wide
       attribute, or a per-process attribute.

       Capability bounding set prior to Linux 2.6.25

       In  kernels  before  2.6.25,  the  capability bounding set is a system-wide attribute that
       affects all threads  on  the  system.   The  bounding  set  is  accessible  via  the  file
       /proc/sys/kernel/cap-bound.   (Confusingly,  this  bit  mask  parameter  is expressed as a
       signed decimal number in /proc/sys/kernel/cap-bound.)

       Only the init process may set capabilities in the  capability  bounding  set;  other  than
       that, the superuser (more precisely: programs with the CAP_SYS_MODULE capability) may only
       clear capabilities from this set.

       On a standard system the capability bounding set always masks out the CAP_SETPCAP capabil‐
       ity.   To  remove this restriction (dangerous!), modify the definition of CAP_INIT_EFF_SET
       in include/linux/capability.h and rebuild the kernel.

       The system-wide capability bounding set feature was added to Linux  starting  with  kernel
       version 2.2.11.

       Capability bounding set from Linux 2.6.25 onward

       From  Linux  2.6.25,  the capability bounding set is a per-thread attribute.  (There is no
       longer a system-wide capability bounding set.)

       The bounding set is inherited at fork(2) from the thread's parent, and is preserved across
       an execve(2).

       A  thread  may  remove  capabilities  from  its capability bounding set using the prctl(2)
       PR_CAPBSET_DROP operation, provided it has the CAP_SETPCAP capability.  Once a  capability
       has  been  dropped from the bounding set, it cannot be restored to that set.  A thread can
       determine if a capability is in its bounding set using the prctl(2) PR_CAPBSET_READ opera‐
       tion.

       Removing  capabilities  from  the  bounding set is supported only if file capabilities are
       compiled into the kernel.  In kernels before  Linux  2.6.33,  file  capabilities  were  an
       optional  feature  configurable  via  the CONFIG_SECURITY_FILE_CAPABILITIES option.  Since
       Linux 2.6.33, the configuration option has been removed and file capabilities  are  always
       part of the kernel.  When file capabilities are compiled into the kernel, the init process
       (the ancestor of all processes) begins with a full bounding set.  If file capabilities are
       not compiled into the kernel, then init begins with a full bounding set minus CAP_SETPCAP,
       because this capability has a different meaning when there are no file capabilities.

       Removing a capability from the bounding set does not remove it from the thread's inherited
       set.   However  it  does  prevent  the  capability from being added back into the thread's
       inherited set in the future.

   Effect of user ID changes on capabilities
       To preserve the traditional semantics for transitions between 0 and nonzero user IDs,  the
       kernel  makes  the  following  changes  to  a  thread's  capability sets on changes to the
       thread's real, effective, saved set, and filesystem  user  IDs  (using  setuid(2),  setre‐
       suid(2), or similar):

       1. If  one or more of the real, effective or saved set user IDs was previously 0, and as a
          result of the UID changes all of these IDs have a nonzero value, then all  capabilities
          are cleared from the permitted and effective capability sets.

       2. If  the  effective  user  ID  is  changed  from 0 to nonzero, then all capabilities are
          cleared from the effective set.

       3. If the effective user ID is changed from nonzero to 0, then the permitted set is copied
          to the effective set.

       4. If the filesystem user ID is changed from 0 to nonzero (see setfsuid(2)), then the fol‐
          lowing capabilities are cleared from the effective  set:  CAP_CHOWN,  CAP_DAC_OVERRIDE,
          CAP_DAC_READ_SEARCH,  CAP_FOWNER, CAP_FSETID, CAP_LINUX_IMMUTABLE (since Linux 2.6.30),
          CAP_MAC_OVERRIDE, and CAP_MKNOD (since Linux 2.6.30).  If the filesystem UID is changed
          from nonzero to 0, then any of these capabilities that are enabled in the permitted set
          are enabled in the effective set.

       If a thread that has a 0 value for one or more of its user IDs wants to prevent  its  per‐
       mitted  capability set being cleared when it resets all of its user IDs to nonzero values,
       it can do  so  using  the  prctl(2)  PR_SET_KEEPCAPS  operation  or  the  SECBIT_KEEP_CAPS
       securebits flag described below.

   Programmatically adjusting capability sets
       A  thread  can  retrieve  and change its capability sets using the capget(2) and capset(2)
       system calls.  However, the use of cap_get_proc(3) and cap_set_proc(3), both  provided  in
       the  libcap package, is preferred for this purpose.  The following rules govern changes to
       the thread capability sets:

       1. If the caller does not have the CAP_SETPCAP capability, the new inheritable set must be
          a subset of the combination of the existing inheritable and permitted sets.

       2. (Since Linux 2.6.25) The new inheritable set must be a subset of the combination of the
          existing inheritable set and the capability bounding set.

       3. The new permitted set must be a subset of the existing permitted set (i.e., it  is  not
          possible to acquire permitted capabilities that the thread does not currently have).

       4. The new effective set must be a subset of the new permitted set.

   The securebits flags: establishing a capabilities-only environment
       Starting  with  kernel  2.6.26,  and with a kernel in which file capabilities are enabled,
       Linux implements a set of per-thread securebits flags that can be used to disable  special
       handling of capabilities for UID 0 (root).  These flags are as follows:

       SECBIT_KEEP_CAPS
              Setting  this  flag allows a thread that has one or more 0 UIDs to retain its capa‐
              bilities when it switches all of its UIDs to a nonzero value.  If this flag is  not
              set,  then such a UID switch causes the thread to lose all capabilities.  This flag
              is always cleared on an execve(2).  (This flag provides the same  functionality  as
              the older prctl(2) PR_SET_KEEPCAPS operation.)

       SECBIT_NO_SETUID_FIXUP
              Setting  this  flag  stops  the  kernel  from  adjusting  capability  sets when the
              threads's effective and filesystem UIDs are switched between zero and nonzero  val‐
              ues.  (See the subsection Effect of User ID Changes on Capabilities.)

       SECBIT_NOROOT
              If this bit is set, then the kernel does not grant capabilities when a set-user-ID-
              root program is executed, or when a process with an effective  or  real  UID  of  0
              calls  execve(2).   (See  the  subsection Capabilities and execution of programs by
              root.)

       SECBIT_NO_CAP_AMBIENT_RAISE
              Setting  this  flag  disallows  raising  ambient  capabilities  via  the   prctl(2)
              PR_CAP_AMBIENT_RAISE operation.

       Each of the above "base" flags has a companion "locked" flag.  Setting any of the "locked"
       flags is irreversible, and has the effect of preventing further changes to the correspond‐
       ing     "base"     flag.      The     locked     flags    are:    SECBIT_KEEP_CAPS_LOCKED,
       SECBIT_NO_SETUID_FIXUP_LOCKED, SECBIT_NOROOT_LOCKED, and SECBIT_NO_CAP_AMBIENT_RAISE.

       The securebits flags can be modified and retrieved using  the  prctl(2)  PR_SET_SECUREBITS
       and  PR_GET_SECUREBITS  operations.   The CAP_SETPCAP capability is required to modify the
       flags.

       The securebits flags are inherited by child processes.  During an execve(2),  all  of  the
       flags are preserved, except SECBIT_KEEP_CAPS which is always cleared.

       An application can use the following call to lock itself, and all of its descendants, into
       an environment where the only way of gaining capabilities is by executing a  program  with
       associated file capabilities:

           prctl(PR_SET_SECUREBITS,
                   SECBIT_KEEP_CAPS_LOCKED |
                   SECBIT_NO_SETUID_FIXUP |
                   SECBIT_NO_SETUID_FIXUP_LOCKED |
                   SECBIT_NOROOT |
                   SECBIT_NOROOT_LOCKED);

   Interaction with user namespaces
       For  a  discussion of the interaction of capabilities and user namespaces, see user_names‐
       paces(7).

CONFORMING TO
       No standards govern capabilities, but the Linux capability implementation is based on  the
       withdrawn POSIX.1e draft standard; see ⟨http://wt.tuxomania.net/publications/posix.1e/⟩.

NOTES
       From  kernel  2.5.27 to kernel 2.6.26, capabilities were an optional kernel component, and
       can be enabled/disabled via the CONFIG_SECURITY_CAPABILITIES kernel configuration option.

       The /proc/PID/task/TID/status file can be used to view the capability sets  of  a  thread.
       The  /proc/PID/status  file  shows the capability sets of a process's main thread.  Before
       Linux 3.8, nonexistent capabilities were shown as being enabled (1) in these sets.   Since
       Linux 3.8, all nonexistent capabilities (above CAP_LAST_CAP) are shown as disabled (0).

       The  libcap package provides a suite of routines for setting and getting capabilities that
       is more comfortable and less likely to change than the interface provided by capset(2) and
       capget(2).   This  package  also provides the setcap(8) and getcap(8) programs.  It can be
       found at
       ⟨http://www.kernel.org/pub/linux/libs/security/linux-privs⟩.

       Before kernel 2.6.24, and from kernel 2.6.24 to kernel 2.6.32 if file capabilities are not
       enabled,  a  thread  with  the  CAP_SETPCAP  capability can manipulate the capabilities of
       threads other than itself.  However, this is only theoretically possible, since no  thread
       ever has CAP_SETPCAP in either of these cases:

       * In the pre-2.6.25 implementation the system-wide capability bounding set, /proc/sys/ker‐
         nel/cap-bound, always masks out this capability, and this can  not  be  changed  without
         modifying the kernel source and rebuilding.

       * If  file  capabilities  are disabled in the current implementation, then init starts out
         with this capability removed from its per-process bounding set, and that bounding set is
         inherited by all other processes created on the system.

SEE ALSO
       capsh(1),    setpriv(1),    prctl(2),    setfsuid(2),    cap_clear(3),    cap_copy_ext(3),
       cap_from_text(3), cap_get_file(3), cap_get_proc(3), cap_init(3),  capgetp(3),  capsetp(3),
       libcap(3), credentials(7), user_namespaces(7), pthreads(7), getcap(8), setcap(8)

       include/linux/capability.h in the Linux kernel source tree

COLOPHON
       This  page  is  part of release 4.04 of the Linux man-pages project.  A description of the
       project, information about reporting bugs, and the latest version of  this  page,  can  be
       found at http://www.kernel.org/doc/man-pages/.

Linux                                       2015-12-05                            CAPABILITIES(7)

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