noble (7) capabilities.7.gz

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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  Linux  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_BPF (since Linux 5.8)
              Employ privileged BPF operations; see bpf(2) and bpf-helpers(7).

              This  capability  was  added  in  Linux  5.8 to separate out BPF functionality from the overloaded
              CAP_SYS_ADMIN capability.

       CAP_CHECKPOINT_RESTORE (since Linux 5.9)
              •  Update /proc/sys/kernel/ns_last_pid (see pid_namespaces(7));
              •  employ the set_tid feature of clone3(2);
              •  read the contents of the symbolic links in /proc/pid/map_files for other processes.

              This capability was added in Linux 5.9 to separate out checkpoint/restore functionality  from  the
              overloaded CAP_SYS_ADMIN capability.

       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);
              •  use the linkat(2) AT_EMPTY_PATH flag to create  a  link  to  a  file  referred  to  by  a  file
                 descriptor.

       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 inode flags (see ioctl_iflags(2)) on arbitrary files;
              •  set Access Control Lists (ACLs) on arbitrary files;
              •  ignore directory sticky bit on file deletion;
              •  modify user extended attributes on sticky directory owned by any user;
              •  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));
              •  Allocate memory using huge pages (memfd_create(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 ioctl_iflags(2)).

       CAP_MAC_ADMIN (since Linux 2.6.25)
              Allow MAC configuration or state changes.  Implemented for the Smack Linux Security Module (LSM).

       CAP_MAC_OVERRIDE (since Linux 2.6.25)
              Override Mandatory Access Control (MAC).  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_PERFMON (since Linux 5.8)
              Employ various performance-monitoring mechanisms, including:

              •  call perf_event_open(2);
              •  employ various BPF operations that have performance implications.

              This capability was added in Linux 5.8 to separate out performance monitoring  functionality  from
              the    overloaded    CAP_SYS_ADMIN    capability.     See    also    the    kernel   source   file
              Documentation/admin-guide/perf-security.rst.

       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 arbitrary capabilities on a file.

              Since Linux 5.12, this capability is also needed to map user ID 0 in a  new  user  namespace;  see
              user_namespaces(7) for details.

       CAP_SETPCAP
              If file capabilities are supported (i.e., since Linux 2.6.24): 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.

              If file capabilities are not supported (i.e., before Linux 2.6.24): 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.)

       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
              Note: this capability is overloaded; see Notes to kernel developers below.

              •  Perform   a  range  of  system  administration  operations  including:  quotactl(2),  mount(2),
                 umount(2), pivot_root(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;
              •  access  the  same  checkpoint/restore  functionality that is governed by CAP_CHECKPOINT_RESTORE
                 (but the latter, weaker capability is preferred for accessing that functionality).
              •  perform the same BPF operations as are governed by CAP_BPF (but the latter,  weaker  capability
                 is preferred for accessing that functionality).
              •  employ  the  same  performance  monitoring  mechanisms  as are governed by CAP_PERFMON (but the
                 latter, weaker capability is preferred for accessing that functionality).
              •  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);
              •  access privileged perf event information;
              •  call setns(2) (requires CAP_SYS_ADMIN in the target namespace);
              •  call fanotify_init(2);
              •  perform privileged 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 privileged ioctl(2) operations on the /dev/random device (see random(4));
              •  install a seccomp(2) filter without first having to set the no_new_privs thread attribute;
              •  modify allow/deny rules for device control groups;
              •  employ the ptrace(2) PTRACE_SECCOMP_GET_FILTER operation to dump tracee's seccomp filters;
              •  employ the ptrace(2) PTRACE_SETOPTIONS operation to suspend the  tracee's  seccomp  protections
                 (i.e., the PTRACE_O_SUSPEND_SECCOMP flag);
              •  perform administrative operations on many device drivers;
              •  modify autogroup nice values by writing to /proc/pid/autogroup (see sched(7)).

       CAP_SYS_BOOT
              Use reboot(2) and kexec_load(2).

       CAP_SYS_CHROOT
              •  Use chroot(2);
              •  change mount namespaces using setns(2).

       CAP_SYS_MODULE
              •  Load and unload kernel modules (see init_module(2) and delete_module(2));
              •  before Linux 2.6.25: drop capabilities from the system-wide capability bounding set.

       CAP_SYS_NICE
              •  Lower  the 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),
                 sched_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));
              •  allow  the  RLIMIT_NOFILE  resource  limit  on the number of "in-flight" file descriptors to be
                 bypassed when passing file descriptors to  another  process  via  a  UNIX  domain  socket  (see
                 unix(7));
              •  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,        /proc/sys/fs/mqueue/msg_max,        and
                 /proc/sys/fs/mqueue/msgsize_max limits when creating POSIX message queues (see mq_overview(7));
              •  employ the prctl(2) PR_SET_MM operation;
              •  set /proc/pid/oom_score_adj to a value lower  than  the  value  last  set  by  a  process  with
                 CAP_SYS_RESOURCE.

       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:

       •  For all privileged operations, the kernel must check whether the thread has the required capability in
          its effective set.

       •  The kernel must provide system calls allowing a thread's capability sets to be changed and retrieved.

       •  The filesystem must support attaching capabilities to an executable file,  so  that  a  process  gains
          those capabilities when the file is executed.

       Before  Linux  2.6.24,  only  the  first two of these requirements are met; since Linux 2.6.24, all three
       requirements are met.

   Notes to kernel developers
       When adding a new kernel feature that should be governed by a capability, consider the following points.

       •  The goal of capabilities is divide the power of superuser into pieces, such that if a program that has
          one  or  more capabilities is compromised, its power to do damage to the system would be less than the
          same program running with root privilege.

       •  You have the choice of either creating a new capability for  your  new  feature,  or  associating  the
          feature  with  one  of  the  existing  capabilities.   In  order  to keep the set of capabilities to a
          manageable size, the latter option is preferable, unless there are  compelling  reasons  to  take  the
          former  option.  (There is also a technical limit: the size of capability sets is currently limited to
          64 bits.)

       •  To determine which existing capability might best be associated with your new feature, review the list
          of  capabilities  above in order to find a "silo" into which your new feature best fits.  One approach
          to take is to determine if there are other features requiring capabilities that will  always  be  used
          along  with  the  new feature.  If the new feature is useless without these other features, you should
          use the same capability as the other features.

       •  Don't choose CAP_SYS_ADMIN if you can possibly avoid it!  A vast  proportion  of  existing  capability
          checks  are  associated with this capability (see the partial list above).  It can plausibly be called
          "the new root", since on the one hand, it confers a wide range of powers, and on the other  hand,  its
          broad  scope  means  that  this is the capability that is required by many privileged programs.  Don't
          make the problem worse.  The only new features that should be associated with CAP_SYS_ADMIN  are  ones
          that closely match existing uses in that silo.

       •  If  you have determined that it really is necessary to create a new capability for your feature, don't
          make or name it as a "single-use" capability.  Thus, for example, the addition of the highly  specific
          CAP_SYS_PACCT  was  probably  a  mistake.   Instead, try to identify and name your new capability as a
          broader silo into which other related future use cases might fit.

   Thread capability sets
       Each thread has the following 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.

       Bounding (per-thread since Linux 2.6.25)
              The capability bounding set is a mechanism that can be used to limit  the  capabilities  that  are
              gained during execve(2).

              Since  Linux  2.6.25,  this  is  a  per-thread  capability  set.  In older kernels, the capability
              bounding set was a system wide attribute shared by all threads on the system.

              For more details, see Capability bounding set below.

       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.   If  ambient  capabilities  cause  a  process's  permitted  and effective capabilities to
              increase during an execve(2), this  does  not  trigger  the  secure-execution  mode  described  in
              ld.so(8).

       A  child  created  via  fork(2)  inherits  copies  of  its  parent's capability sets.  For details on how
       execve(2) affects capabilities, see Transformation of capabilities during execve() below.

       Using capset(2), a thread  may  manipulate  its  own  capability  sets;  see  Programmatically  adjusting
       capability sets 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  Linux  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)  and  xattr(7))
       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 Transformation of capabilities  during  execve()  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 flag is enabled.

   File capability extended attribute versioning
       To  allow  extensibility,  the  kernel  supports  a  scheme  to  encode  a  version  number  inside   the
       security.capability  extended  attribute  that  is  used  to  implement file capabilities.  These version
       numbers are internal to the implementation, and not directly  visible  to  user-space  applications.   To
       date, the following versions are supported:

       VFS_CAP_REVISION_1
              This  was  the  original  file  capability  implementation,  which supported 32-bit masks for file
              capabilities.

       VFS_CAP_REVISION_2 (since Linux 2.6.25)
              This version allows for file capability masks that are 64 bits in size, and was necessary  as  the
              number  of  supported  capabilities grew beyond 32.  The kernel transparently continues to support
              the execution of files that have 32-bit version 1 capability masks, but when  adding  capabilities
              to  files  that  did  not  previously have capabilities, or modifying the capabilities of existing
              files, it automatically uses the version 2 scheme (or possibly the version 3 scheme, as  described
              below).

       VFS_CAP_REVISION_3 (since Linux 4.14)
              Version  3  file  capabilities  are  provided  to  support namespaced file capabilities (described
              below).

              As with version 2 file capabilities, version 3 capability masks are  64  bits  in  size.   But  in
              addition,  the root user ID of namespace is encoded in the security.capability extended attribute.
              (A namespace's root user ID is the value that user ID 0 inside  that  namespace  maps  to  in  the
              initial user namespace.)

              Version  3  file  capabilities  are designed to coexist with version 2 capabilities; that is, on a
              modern Linux system, there may be some files with version 2 capabilities while others have version
              3 capabilities.

       Before  Linux  4.14, the only kind of file capability extended attribute that could be attached to a file
       was a VFS_CAP_REVISION_2 attribute.  Since Linux 4.14, the version of  the  security.capability  extended
       attribute that is attached to a file depends on the circumstances in which the attribute was created.

       Starting  with  Linux  4.14,  a  security.capability  extended  attribute is automatically created as (or
       converted to) a version 3 (VFS_CAP_REVISION_3) attribute if both of the following are true:

       •  The thread writing the attribute resides in a noninitial user namespace.  (More precisely: the  thread
          resides in a user namespace other than the one from which the underlying filesystem was mounted.)

       •  The  thread  has  the  CAP_SETFCAP capability over the file inode, meaning that (a) the thread has the
          CAP_SETFCAP capability in its own user namespace; and (b) the UID and  GID  of  the  file  inode  have
          mappings in the writer's user namespace.

       When  a  VFS_CAP_REVISION_3  security.capability  extended  attribute is created, the root user ID of the
       creating thread's user namespace is saved in the extended attribute.

       By  contrast,  creating  or  modifying  a  security.capability  extended  attribute  from  a   privileged
       (CAP_SETFCAP)  thread  that  resides  in  the namespace where the underlying filesystem was mounted (this
       normally means the initial user  namespace)  automatically  results  in  the  creation  of  a  version  2
       (VFS_CAP_REVISION_2) attribute.

       Note  that  the  creation of a version 3 security.capability extended attribute is automatic.  That is to
       say, when a user-space application writes (setxattr(2)) a security.capability attribute in the version  2
       format,  the  kernel  will  automatically create a version 3 attribute if the attribute is created in the
       circumstances described above.  Correspondingly,  when  a  version  3  security.capability  attribute  is
       retrieved  (getxattr(2))  by  a process that resides inside a user namespace that was created by the root
       user ID (or a descendant of that user namespace), the returned attribute is (automatically) simplified to
       appear  as  a version 2 attribute (i.e., the returned value is the size of a version 2 attribute and does
       not include the root user ID).  These automatic translations mean that no changes are required  to  user-
       space  tools  (e.g.,  setcap(1) and getcap(1)) in order for those tools to be used to create and retrieve
       version 3 security.capability attributes.

       Note that a file can have either a version 2  or  a  version  3  security.capability  extended  attribute
       associated  with it, but not both: creation or modification of the security.capability extended attribute
       will automatically modify the version according to the circumstances in which the extended  attribute  is
       created or modified.

   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) & P(bounding)) | P'(ambient)

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

           P'(inheritable) = P(inheritable)    [i.e., unchanged]

           P'(bounding)    = P(bounding)       [i.e., unchanged]

       where:

           P()    denotes the value of a thread capability set before the execve(2)

           P'()   denotes the value of a thread capability set after the execve(2)

           F()    denotes a file capability set

       Note the following details relating to the above capability transformation rules:

       •  The ambient capability set is present only since Linux 4.3.  When determining  the  transformation  of
          the  ambient set during execve(2), a privileged file is one that has capabilities or has the set-user-
          ID or set-group-ID bit set.

       •  Prior to Linux 2.6.25, the bounding set was a system-wide  attribute  shared  by  all  threads.   That
          system-wide  value was employed to calculate the new permitted set during execve(2) in the same manner
          as shown above for P(bounding).

       Note: during the capability transitions described above, file capabilities may  be  ignored  (treated  as
       empty)  for the same reasons that the set-user-ID and set-group-ID bits are ignored; see execve(2).  File
       capabilities are similarly ignored if the kernel was booted with the no_file_caps option.

       Note: according to the rules above, if a process with nonzero user IDs performs  an  execve(2)  then  any
       capabilities  that are present in its permitted and effective sets will be cleared.  For the treatment of
       capabilities when a process with a user ID of zero performs an execve(2), see Capabilities and  execution
       of programs by root below.

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

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

   Capabilities and execution of programs by root
       In order to mirror traditional UNIX semantics, the kernel performs special treatment of file capabilities
       when a process with UID 0 (root) executes a program and when a set-user-ID-root program is executed.

       After  having  performed  any  changes to the process effective ID that were triggered by the set-user-ID
       mode bit of the binary—e.g., switching the effective user ID  to  0  (root)  because  a  set-user-ID-root
       program was executed—the kernel calculates the file capability sets as follows:

       (1)  If the real or effective user ID of the process is 0 (root), then the file inheritable and permitted
            sets are ignored; instead they are notionally considered to be  all  ones  (i.e.,  all  capabilities
            enabled).   (There  is  one  exception to this behavior, described in Set-user-ID-root programs that
            have file capabilities below.)

       (2)  If the effective user ID of the process is 0 (root) or the file effective bit is  in  fact  enabled,
            then the file effective bit is notionally defined to be one (enabled).

       These  notional  values  for the file's capability sets are then used as described above to calculate the
       transformation of the process's capabilities during execve(2).

       Thus, when a process with  nonzero  UIDs  execve(2)s  a  set-user-ID-root  program  that  does  not  have
       capabilities attached, or when a process whose real and effective UIDs are zero execve(2)s a program, the
       calculation of the process's new permitted capabilities simplifies to:

           P'(permitted)   = P(inheritable) | P(bounding)

           P'(effective)   = P'(permitted)

       Consequently, the process gains all capabilities in its permitted and effective capability  sets,  except
       those  masked  out by the capability bounding set.  (In the calculation of P'(permitted), the P'(ambient)
       term can be simplified away because it is by definition a proper subset of P(inheritable).)

       The special treatments of user ID 0 (root) described  in  this  subsection  can  be  disabled  using  the
       securebits mechanism described below.

   Set-user-ID-root programs that have file capabilities
       There is one exception to the behavior described in Capabilities and execution of programs by root above.
       If (a) the binary that is being executed has capabilities attached and  (b)  the  real  user  ID  of  the
       process  is  not  0  (root)  and  (c)  the  effective  user  ID of the process is 0 (root), then the file
       capability bits are honored (i.e., they are not notionally considered to be all ones).  The usual way  in
       which  this situation can arise is when executing a set-UID-root program that also has file capabilities.
       When such a program is executed, the process gains just the capabilities granted by  the  program  (i.e.,
       not  all  capabilities,  as  would occur when executing a set-user-ID-root program that does not have any
       associated file capabilities).

       Note that one can assign empty capability sets to a program file, and thus it is  possible  to  create  a
       set-user-ID-root  program  that  changes the effective and saved set-user-ID of the process that executes
       the program to 0, but confers no capabilities to that process.

   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  inheritable  capabilities.
       If  a  thread  maintains a capability in its inheritable 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
       inheritable set.

       Depending on the kernel version, the capability bounding set is either a system-wide attribute, or a per-
       process attribute.

       Capability bounding set from Linux 2.6.25 onward

       From Linux 2.6.25, the capability bounding set is a per-thread attribute.   (The  system-wide  capability
       bounding set described below no longer exists.)

       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.   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  inheritable  set.
       However  it  does  prevent  the capability from being added back into the thread's inheritable set in the
       future.

       Capability bounding set prior to Linux 2.6.25

       Before Linux 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: a process 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 2.2.11.

   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):

       •  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, effective, and ambient capability sets.

       •  If  the  effective  user  ID  is changed from 0 to nonzero, then all capabilities are cleared from the
          effective set.

       •  If the effective user ID is changed from nonzero to 0,  then  the  permitted  set  is  copied  to  the
          effective set.

       •  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
       SECBIT_KEEP_CAPS securebits flag described below.

   Programmatically adjusting capability sets
       A  thread  can  retrieve  and  change its permitted, effective, and inheritable 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:

       •  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.

       •  (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.

       •  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).

       •  The new effective set must be a subset of the new permitted set.

   The securebits flags: establishing a capabilities-only environment
       Starting  with Linux 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 capabilities in its
              permitted set when it switches all of its UIDs to nonzero values.  If this flag is not  set,  then
              such  a  UID  switch  causes  the  thread to lose all permitted capabilities.  This flag is always
              cleared on an execve(2).

              Note that even with the SECBIT_KEEP_CAPS flag set, the effective  capabilities  of  a  thread  are
              cleared  when  it  switches  its effective UID to a nonzero value.  However, if the thread has set
              this flag and its effective UID is already nonzero, and the thread subsequently switches all other
              UIDs to nonzero values, then the effective capabilities will not be cleared.

              The  setting  of  the  SECBIT_KEEP_CAPS flag is ignored if the SECBIT_NO_SETUID_FIXUP flag is set.
              (The latter flag provides a superset of the effect of the former flag.)

              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 the process's permitted, effective, and  ambient
              capability  sets  when  the  thread's  effective and filesystem UIDs are switched between zero and
              nonzero values.  See Effect of user ID changes on capabilities above.

       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 Capabilities
              and execution of programs by root above.)

       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_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.  Note that the
       SECBIT_* constants are available only after including the <linux/securebits.h> header file.

       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 off */
                   SECBIT_KEEP_CAPS_LOCKED |
                   SECBIT_NO_SETUID_FIXUP |
                   SECBIT_NO_SETUID_FIXUP_LOCKED |
                   SECBIT_NOROOT |
                   SECBIT_NOROOT_LOCKED);
                   /* Setting/locking SECBIT_NO_CAP_AMBIENT_RAISE
                      is not required */

   Per-user-namespace "set-user-ID-root" programs
       A set-user-ID program whose UID matches the UID that created a user namespace will confer capabilities in
       the  process's  permitted  and  effective  sets when executed by any process inside that namespace or any
       descendant user namespace.

       The rules about the transformation of the process's capabilities during  the  execve(2)  are  exactly  as
       described in Transformation of capabilities during execve() and Capabilities and execution of programs by
       root above, with the difference that, in the latter subsection, "root" is the UID of the creator  of  the
       user namespace.

   Namespaced file capabilities
       Traditional  (i.e.,  version  2) file capabilities associate only a set of capability masks with a binary
       executable file.  When a process executes a binary  with  such  capabilities,  it  gains  the  associated
       capabilities  (within  its  user  namespace) as per the rules described in Transformation of capabilities
       during execve() above.

       Because version 2 file capabilities confer capabilities to the executing process regardless of which user
       namespace  it  resides in, only privileged processes are permitted to associate capabilities with a file.
       Here, "privileged" means a process that has the CAP_SETFCAP capability in the user  namespace  where  the
       filesystem  was mounted (normally the initial user namespace).  This limitation renders file capabilities
       useless for certain use cases.  For example, in user-namespaced containers, it can  be  desirable  to  be
       able  to  create a binary that confers capabilities only to processes executed inside that container, but
       not to processes that are executed outside the container.

       Linux 4.14 added so-called namespaced file capabilities to  support  such  use  cases.   Namespaced  file
       capabilities   are   recorded  as  version  3  (i.e.,  VFS_CAP_REVISION_3)  security.capability  extended
       attributes.  Such an attribute is automatically created in the circumstances described in File capability
       extended attribute versioning above.  When a version 3 security.capability extended attribute is created,
       the kernel records not just the capability masks in the extended attribute, but also the  namespace  root
       user ID.

       As  with  a  binary  that has VFS_CAP_REVISION_2 file capabilities, a binary with VFS_CAP_REVISION_3 file
       capabilities confers capabilities to a process during execve().  However, capabilities are conferred only
       if the binary is executed by a process that resides in a user namespace whose UID 0 maps to the root user
       ID that is saved in the extended attribute, or when executed by a process that resides in a descendant of
       such a namespace.

   Interaction with user namespaces
       For further information on the interaction of capabilities and user namespaces, see user_namespaces(7).

STANDARDS

       No  standards  govern  capabilities,  but  the  Linux capability implementation is based on the withdrawn
       POSIX.1e draft standard ⟨https://archive.org/details/posix_1003.1e-990310⟩.

NOTES

       When attempting to strace(1) binaries that have capabilities (or set-user-ID-root binaries), you may find
       the -u <username> option useful.  Something like:

           $ sudo strace -o trace.log -u ceci ./myprivprog

       From  Linux  2.5.27  to  Linux  2.6.26,  capabilities  were  an  optional  kernel component, and could be
       enabled/disabled via the CONFIG_SECURITY_CAPABILITIES kernel configuration option.

       The /proc/pid/task/TID/status  file  can  be  used  to  view  the  capability  sets  of  a  thread.   The
       /proc/pid/status  file  shows  the  capability  sets  of  a  process's  main  thread.   Before Linux 3.8,
       nonexistent capabilities were shown as being enabled (1) in these sets.  Since Linux 3.8, all nonexistent
       capabilities (above CAP_LAST_CAP) are shown as disabled (0).

       The  libcap  package  provides  a  suite  of  routines  for setting and getting capabilities that is more
       comfortable and less likely to change than the interface  provided  by  capset(2)  and  capget(2).   This
       package also provides the setcap(8) and getcap(8) programs.  It can be found at
       ⟨https://git.kernel.org/pub/scm/libs/libcap/libcap.git/refs/⟩.

       Before Linux 2.6.24, and from Linux 2.6.24 to Linux 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 the CAP_SETPCAP capability, and this can not be changed without modifying the  kernel
          source and rebuilding the kernel.

       •  If  file  capabilities  are  disabled  (i.e.,  the  kernel CONFIG_SECURITY_FILE_CAPABILITIES option is
          disabled), then init starts out with the CAP_SETPCAP 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),    proc(5),
       credentials(7),   pthreads(7),   user_namespaces(7),   captest(8),  filecap(8),  getcap(8),  getpcaps(8),
       netcap(8), pscap(8), setcap(8)

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