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NAME

       capabilities - overview of Linux capabilities

DESCRIPTION

       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).

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

       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 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.

       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 terminal 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  kernels 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 policies and  priorities  for  arbitrary  processes
                (sched_setscheduler(2), sched_setparam(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/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
              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  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.

       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 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
       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   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.

   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  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
       /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  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
       operation.

       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), 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
          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 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:

       SECBIT_KEEP_CAPS
              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.)

       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 values.  (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  corresponding  "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_namespaces(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/kernel/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

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       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/.