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       capabilities - overview of Linux capabilities


       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,

       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

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

              Make arbitrary changes to file UIDs and GIDs (see chown(2)).

              Bypass file read, write, and execute permission checks.  (DAC is an abbreviation of
              "discretionary access control".)

              •  Bypass  file  read  permission  checks and directory read and execute permission
              •  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.

              •  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).

              •  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.

              •  Lock memory (mlock(2), mlockall(2), mmap(2), shmctl(2));
              •  Allocate memory using huge pages (memfd_create(2), mmap(2), shmctl(2)).

              Bypass permission checks for operations on System V IPC objects.

              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)).

              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).

              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

              Bind a socket to Internet domain privileged ports (port numbers less than 1024).

              (Unused)  Make socket broadcasts, and listen to multicasts.

              •  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.

              •  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.

              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.)

              •  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)).

              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
              •  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
              •  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
              •  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
              •  install  a seccomp(2) filter without first having to set the no_new_privs thread
              •  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)).

              Use reboot(2) and kexec_load(2).

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

              •  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

              •  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).

              Use acct(2).

              •  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).

              •  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
              •  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.

              •  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
              •  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
              •  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.

              Set system clock (settimeofday(2), stime(2), adjtimex(2)); set real-time (hardware)

              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

              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).

              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.

              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

              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

       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()

       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).

              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:

              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]


           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

       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_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:

              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

              This  flag  provides  the  same functionality as the older prctl(2) PR_SET_KEEPCAPS

              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.

              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.)

              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

       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:

                   /* SECBIT_KEEP_CAPS off */
                   SECBIT_KEEP_CAPS_LOCKED |
                   SECBIT_NO_SETUID_FIXUP |
                   SECBIT_NO_SETUID_FIXUP_LOCKED |
                   SECBIT_NOROOT |
                   /* 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

   Interaction with user namespaces
       For  further  information  on  the  interaction  of  capabilities and user namespaces, see


       No standards govern capabilities, but the Linux capability implementation is based on  the
       withdrawn POSIX.1e draft standard; see ⟨⟩.


       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

       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

       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.


       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