capabilities - overview of Linux capabilities


   For  the  purpose  of  performing  permission  checks, traditional UNIX
   implementations distinguish 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 (usually: effective UID, effective GID, and
   supplementary group list).

   Starting with kernel 2.2, Linux divides  the  privileges  traditionally
   associated  with  superuser 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).

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

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

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

          * 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
          * set  extended  file  attributes  (see  chattr(1)) on arbitrary
          * 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).

          * 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

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

          Bypass permission checks for operations on System V IPC objects.

          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)).

          Set  the  FS_APPEND_FL  and  FS_IMMUTABLE_FL  inode  flags  (see

   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).

          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_PRIORITY (for  a  priority  outside  the
            range 0 to 6), SO_RCVBUFFORCE, and SO_SNDBUFFORCE.

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

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

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

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

   CAP_SETFCAP (since Linux 2.6.24)
          Set file capabilities.

          If  file  capabilities  are  not  supported: grant or remove any
          capability in the caller'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
          capabilities, 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.

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

          * 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
          * 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
          * call fanotify_init(2);
          * call bpf(2);
          * perform privileged KEYCTL_CHOWN and  KEYCTL_SETPERM  keyctl(2)
          * use  ptrace(2)  PTRACE_SECCOMP_GET_FILTER  to  dump  a tracees
            seccomp filters;
          * perform madvise(2) MADV_HWPOISON operation;
          * employ the TIOCSTI ioctl(2)  to  insert  characters  into  the
            input  queue of a terminal other than the caller's controlling
          * 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 privileged  ioctl(2)  operations  on  the  /dev/random
            device (see random(4));
          * perform administrative operations on many device drivers.

          Use reboot(2) and kexec_load(2).

          Use chroot(2).

          * Load   and  unload  kernel  modules  (see  init_module(2)  and
          * in kernels before 2.6.25: drop capabilities from  the  system-
            wide capability bounding set.

          * 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  policies  and  priorities  for arbitrary processes
            (sched_setscheduler(2), sched_setparam(2), shed_setattr(2));
          * set     CPU     affinity     for      arbitrary      processes
          * set  I/O scheduling class and priority for arbitrary processes
          * 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).

          Use acct(2).

          * 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).

          * 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

          * 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 the 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.

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

          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/kernel/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

   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

   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:

          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).

          This  is  a  set  of capabilities preserved across an execve(2).
          Inheritable capabilities 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 elevated capabilities
          should consider using ambient capabilities, described below.

          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  capabilities  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  ambient  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

   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).

          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    capabilities    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.

   File capability sets are ignored if the executable file  resides  on  a
   filesystem mounted with the nosuid option (see mount(2) and mount(8)).

   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]


       P         denotes the value of a thread capability set  before  the

       P'        denotes  the  value  of a thread capability set after the

       F         denotes a file capability set

       cap_bset  is the value of the capability  bounding  set  (described

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

   Safety checking for capability-dumb binaries
   A capability-dumb binary is an application that has been marked to have
   file  capabilities, but has not been converted to use the libcap(3) API
   to manipulate its capabilities.  (In other words, this is a traditional
   set-user-ID-root   program   that   has   been  switched  to  use  file
   capabilities, but whose  code  has  not  been  modified  to  understand
   capabilities.)   For such applications, the effective capability bit is
   set  on  the  file,  so  that  the  file  permitted  capabilities   are
   automatically  enabled  in the process effective set when executing the
   file.  The kernel recognizes a file which has the effective  capability
   bit set as capability-dumb for the purpose of the check described here.

   When  executing  a  capability-dumb  binary,  the  kernel checks if the
   process obtained all permitted capabilities that were specified in  the
   file  permitted  set,  after  the  capability transformations described
   above have been performed.  (The typical  reason  why  this  might  not
   occur  is  that  the  capability  bounding  set  masked out some of the
   capabilities in the file permitted set.)  If the process did not obtain
   the  full set of file permitted capabilities, then execve(2) fails with
   the error EPERM.  This prevents  possible  security  risks  that  could
   arise   when  a  capability-dumb  application  is  executed  with  less
   privilege that it needs.  Note that,  by  definition,  the  application
   could  not  itself recognize this problem, since it does not employ the
   libcap(3) API.

   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 permitted 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  capabilities  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  capability  set, and the result of this operation is
     assigned to the thread's permitted capability  set.   The  capability
     bounding  set  thus places a limit on the permitted capabilities 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

   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  capability.   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

   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

   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), setresuid(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 following capabilities are cleared  from  the
      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  permitted 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:

          Setting this flag allows a thread that has one or more 0 UIDs to
          retain its capabilities 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.)

          Setting this flag stops the  kernel  from  adjusting  capability
          sets  when  the  thread's  effective  and  filesystem  UIDs  are
          switched between zero and nonzero values.  (See  the  subsection
          Effect of user ID changes on capabilities.)

          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.)

          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  corresponding  "base" flag.  The
   locked          flags           are:           SECBIT_KEEP_CAPS_LOCKED,

   The securebits flags can be modified and retrieved using  the  prctl(2)
   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

            /* SECBIT_KEEP_CAPS off */
               SECBIT_KEEP_CAPS_LOCKED |
               SECBIT_NO_SETUID_FIXUP |
               SECBIT_NOROOT |
               /* Setting/locking SECURE_NO_CAP_AMBIENT_RAISE
                  is not required */

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


   No  standards   govern   capabilities,   but   the   Linux   capability
   implementation  is  based on the withdrawn POSIX.1e draft standard; see


   From kernel 2.5.27 to kernel  2.6.26,  capabilities  were  an  optional
   kernel    component,    and   could   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

   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/kernel/cap-bound,  always  masks out this capability,
     and this can not be changed without modifying the kernel  source  and

   * 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.


   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),     proc(5),
   credentials(7), pthreads(7), user_namespaces(7), getcap(8), setcap(8)

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


   This  page  is  part of release 4.09 of the Linux man-pages project.  A
   description of the project, information about reporting bugs,  and  the
   latest     version     of     this    page,    can    be    found    at


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