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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 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 unpriv‐
       ileged processes are subject to full permission checking based  on  the
       process's  credentials (usually: effective UID, effective GID, and sup‐
       plementary 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_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) EPOLL‐
	      WAKEUP, /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 exe‐
		cute 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 permission 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  i-node  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.

       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	avail‐
	      able when the kernel is configured to support file capabilities,
	      since CAP_SETPCAP has entirely different semantics for such ker‐
	      nels.)

	      If  file capabilities are supported: add any capability from the
	      calling thread's bounding set to its inheritable set; drop capa‐
	      bilities	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));  make forged UID when
	      passing socket credentials via UNIX domain sockets.

       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;
	      * perform operations on trusted and security Extended Attributes
		(see attr(5));
	      * 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 UID when passing socket credentials;
	      * 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);
	      * call perf_event_open(2);
	      * access privileged perf event information;
	      * call setns(2);
	      * call fanotify_init(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));
	      * set CPU	 affinity  for	arbitrary  processes  (sched_setaffin‐
		ity(2));
	      * set  I/O scheduling class and priority for arbitrary processes
		(ioprio_set(2));
	      * apply migrate_pages(2) to arbitrary processes and  allow  pro‐
		cesses 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;  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 informa‐
	  tion 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_REAL‐
	  TIME_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).  It
	      provides a mechanism for a process to assign capabilities to the
	      permitted set of the new program during an execve(2).

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

       A  child created via fork(2) inherits copies of its parent's capability
       sets.  See below for a discussion of the treatment of capabilities dur‐
       ing 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 ker‐
       nel; 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.capa‐
       bility.	Writing to this extended attribute  requires  the  CAP_SETFCAP
       capability.  The file capability sets, in conjunction with the capabil‐
       ity 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 capabili‐
	      ties 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  capabili‐
	      ties  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'(permitted) = (P(inheritable) & F(inheritable)) |
			   (F(permitted) & cap_bset)

	   P'(effective) = F(effective) ? P'(permitted) : 0

	   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).

   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 transfor‐
       mations 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  pro‐
       vided 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  inherita‐
	 ble  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!),  mod‐
       ify  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	 capa‐
       bility 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 con‐
       figuration option has been removed and  file  capabilities  are	always
       part  of the kernel.  When file capabilities are compiled into the ker‐
       nel, the init process (the ancestor of all  processes)  begins  with  a
       full bounding set.  If file capabilities are not compiled into the ker‐
       nel, 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 pre‐
	  viously 0, and as a result of the UID changes all of these IDs  have
	  a  nonzero value, then all capabilities are cleared from the permit‐
	  ted 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 per‐
	  mitted set is copied to the effective set.

       4. If the filesystem user ID is changed from 0 to  nonzero  (see	 setf‐
	  suid(2)) then the following capabilities are cleared from the effec‐
	  tive	 set:	CAP_CHOWN,   CAP_DAC_OVERRIDE,	  CAP_DAC_READ_SEARCH,
	  CAP_FOWNER,  CAP_FSETID,  CAP_LINUX_IMMUTABLE	 (since Linux 2.2.30),
	  CAP_MAC_OVERRIDE,  and  CAP_MKNOD  (since  Linux  2.2.30).   If  the
	  filesystem UID is changed from nonzero to 0, then any of these capa‐
	  bilities 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.

   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	 pack‐
       age, 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  capabili‐
       ties 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 function‐
	      ality 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.)

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

       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  capabili‐
       ties:

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

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

NOTES
       Since kernel 2.5.27, capabilities are 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
       non-existent  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 since kernel 2.6.24 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  pro‐
	 cesses created on the system.

SEE ALSO
       capget(2),   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), pthreads(7), get‐
       cap(8), setcap(8)

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

COLOPHON
       This page is part of release 3.54 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2013-07-21		       CAPABILITIES(7)
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