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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 im-
       plementations distinguish two categories	of processes: privileged  pro-
       cesses  (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_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)	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_DE-
		BUG, 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 do-
	      main 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 ca-
	      pability 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 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), se-
	      treuid(2), setresuid(2), setfsuid(2)); forge  UID	 when  passing
	      socket credentials via UNIX domain sockets; write	a user ID map-
	      ping 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 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) (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	 name-
		space);
	      *	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 in-
		put 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_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;
	      *	 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  de-
		vices.

       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_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 ef-
	      fective 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 ca-
	      pabilities 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 nu-
       merical 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  ac-
	      quire 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 en-
	      abled 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'(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  ef-
	  fective 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 ex-
       ecve(2)s	a program, it gains all	capabilities in	its permitted and  ef-
       fective	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  in-
	 heritable  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	attri-
       bute.  (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,  be-
       cause  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  ca-
	  pabilities 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 ef-
	  fective  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 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  in-
	  heritable  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  ex-
       ecve(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 ca-
       pabilities 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	 name-
       spaces, see user_namespaces(7).

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
       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 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
       capsh(1),     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),
       user_namespaces(7), pthreads(7),	getcap(8), setcap(8)

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

COLOPHON
       This page is part of release 3.74 of the	Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       http://www.kernel.org/doc/man-pages/.

Linux				  2014-09-21		       CAPABILITIES(7)

NAME | DESCRIPTION | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

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