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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
This page is part of release 4.04 of the Linux man-pages project. A description of the project,
information about reporting bugs, and the latest version of this page, can be found at
http://www.kernel.org/doc/man-pages/.
Linux 2015-12-05 CAPABILITIES(7)