Provided by: libcap-dev_2.44-1ubuntu0.22.04.1_amd64 bug


       cap_get_proc,   cap_set_proc,  capgetp,  cap_get_bound,  cap_drop_bound,  cap_get_ambient,
       cap_set_ambient,  cap_reset_ambient,   cap_get_secbits,   cap_set_secbits,   cap_get_mode,
       cap_set_mode,   cap_mode_name,   cap_get_pid,   cap_setuid,   cap_setgroups  -  capability
       manipulation on processes


       #include <sys/capability.h>

       cap_t cap_get_proc(void);

       int cap_set_proc(cap_t cap_p);

       int cap_get_bound(cap_value_t cap);

       CAP_IS_SUPPORTED(cap_value_t cap);

       int cap_drop_bound(cap_value_t cap);

       int cap_get_ambient(cap_value_t cap);

       int cap_set_ambient(cap_value_t cap, cap_flag_value_t value);

       int cap_reset_ambient(void);


       unsigned cap_get_secbits(void);

       int cap_set_secbits(unsigned bits);

       cap_mode_t cap_get_mode(void);

       const char *cap_mode_name(cap_mode_t mode);

       int cap_set_mode(cap_mode_t mode);

       #include <sys/types.h>

       cap_t cap_get_pid(pid_t pid);

       int cap_setuid(uid_t uid);

       int cap_setgroups(gid_t gid, size_t ngroups, const gid_t groups);

       Link with -lcap.


       cap_get_proc() allocates a capability state in working storage, sets its state to that  of
       the  calling  process,  and returns a pointer to this newly created capability state.  The
       caller should free any releasable memory, when the capability state in working storage  is
       no longer required, by calling cap_free() with the cap_t as an argument.

       cap_set_proc()  sets  the  values  for  all  capability  flags for all capabilities to the
       capability state identified by cap_p.  The new capability state of  the  process  will  be
       completely  determined by the contents of cap_p upon successful return from this function.
       If any flag in cap_p is set for any capability not currently  permitted  for  the  calling
       process,  the  function  will  fail,  and  the capability state of the process will remain

       cap_get_pid() returns cap_t, see cap_init(3), with the process capabilities of the process
       indicated  by  pid.   (If pid is 0, then the calling process's capabilities are returned.)
       This information can also be obtained from the /proc/<pid>/status file.

       cap_get_bound() with a cap as an argument returns the current value of this  bounding  set
       capability flag in effect for the calling process. This operation is unprivileged. Note, a
       macro function CAP_IS_SUPPORTED(cap_value_t cap) is provided that evaluates to true (1) if
       the  system  supports  the  specified capability, cap.  If the system does not support the
       capability, this function returns 0. This macro works by testing for  an  error  condition
       with cap_get_bound().

       cap_drop_bound()  can  be  used  to  lower the specified bounding set capability, cap.  To
       complete successfully,  the  prevailing  effective  capability  set  must  have  a  raised

       cap_get_ambient()  returns the prevailing value of the specified ambient capability, or -1
       if the capability is not supported by the running kernel.  A macro CAP_AMBIENT_SUPPORTED()
       uses this function to determine if ambient capabilities are supported by the kernel.

       cap_set_ambient()  sets  the specified ambient capability to a specific value. To complete
       successfully, the prevailing effective capability set  must  have  a  raised  CAP_SETPCAP.
       Further,  to raise a specific ambient capability the inheritable and permitted sets of the
       calling process must contain the specified capability, and raised ambient bits  will  only
       be retained as long as this remains true.

       cap_reset_ambient()  resets  all  of  the  ambient capabilities for the calling process to
       their lowered value. To complete successfully, the  prevailing  effective  capability  set
       must  have a raised CAP_SETPCAP.  Note, the ambient set is intended to operate in a legacy
       environment where the application  has  limited  awareness  of  capabilities  in  general.
       Executing a file with associated filesystem capabilities, the kernel will implicitly reset
       the ambient set of the process. Also, changes to the inheritable set by the  program  code
       without explicitly fixing up the ambient set can also drop ambient bits.

       cap_get_secbits() returns the securebits of the calling process. These bits affect the way
       in which the  calling  process  implements  things  like  setuid-root  fixup  and  ambient

       cap_set_secbits()  attempts  to  modify  the  securebits  of  the  calling  process.  Note
       CAP_SETPCAP must be in the effective  capability  set  for  this  to  be  effective.  Some
       settings lock the sub-states of the securebits, so attempts to set values may be denied by
       the kernel even when the CAP_SETPCAP capability is raised.

       To help manage the complexity of the securebits, libcap provides a combined securebit  and
       capability  set  concept  called  a libcap mode.  cap_get_mode() attempts to summarize the
       prevailing security environment in the form  of  a  numerical  cap_mode_t  value.  A  text
       representation  of  the  mode  can  be obtained via the cap_mode_name() function. The vast
       majority of combinations of these values are not well defined in terms of a  libcap  mode,
       and  for these states cap_get_mode() returns (cap_mode_t)0 which cap_get_name() identifies
       as  ``UNCERTAIN''.   Supported  modes  are:  CAP_MODE_NOPRIV,   CAP_MODE_PURE1E_INIT   and

       cap_set_mode()  can  be used to set the desired mode. The permitted capability CAP_SETPCAP
       is required for this function to succeed.

       cap_setuid() is a convenience function for the setuid(2) system call.  Where  cap_setuid()
       arranges  for  the  right effective capability to be raised in order to perform the system
       call, and also arranges to preserve the availability of permitted capabilities  after  the
       uid has changed. Following this call all effective capabilities are lowered.

       cap_setgroups()  is  a convenience function for performing both setgid(2) and setgroups(2)
       calls in one call. The cap_setgroups() call raises the right effective capability for  the
       duration of the call, and empties the effective capability set before returning.


       The  functions  cap_get_proc()  and  cap_get_pid() return a non-NULL value on success, and
       NULL on failure.

       The function cap_get_bound() returns -1 if the requested capability is unknown,  otherwise
       the  return value reflects the current state of that capability in the prevailing bounding
       set. Note, a macro function,

       The all of the setting functions such as cap_set_proc() and cap_drop_bound()  return  zero
       for success, and -1 on failure.

       On failure, errno is set to EINVAL, EPERM, or ENOMEM.


       cap_set_proc()   and   cap_get_proc()  are  specified  in  the  withdrawn  POSIX.1e  draft
       specification.  cap_get_pid() is a Linux extension.


       Neither glibc, nor the Linux kernel honors POSIX semantics for  setting  capabilities  and
       securebits  in  the  presence  of pthreads. That is, changing capability sets, by default,
       only affect the running thread. To be meaningfully secure, however,  the  capability  sets
       should  be  mirrored by all threads within a common program because threads are not memory
       isolated. As a workaround for this, libcap is packaged with  a  separate  POSIX  semantics
       system  call  library:  libpsx.  If your program uses POSIX threads, to achieve meaningful
       POSIX semantics capability manipulation, you should link your program with:

       ld ... -lcap -lpsx -lpthread --wrap=pthread_create


       gcc ... -lcap -lpsx -lpthread -Wl,-wrap,pthread_create

       When linked this way, due to linker magic, libcap uses psx_syscall(3) and  psx_syscall6(3)
       to perform state setting system calls.

   capgetp() and capsetp()
       The library also supports the deprecated functions:

       int capgetp(pid_t pid, cap_t cap_d);

       int capsetp(pid_t pid, cap_t cap_d);

       capgetp()  attempts  to  obtain  the  capabilities  of  some  other  process;  storing the
       capabilities in a pre-allocated cap_d.  See cap_init() for information  on  allocating  an
       empty capability set. This function is deprecated; you should use cap_get_pid().

       capsetp()  attempts  to  set  the  capabilities  of  the  calling porcess or of some other
       process(es), pid.  Note that setting capabilities of another process is only  possible  on
       older  kernels  that  do  not  provide  VFS  support  for  setting file capabilities.  See
       capset(2) for information on which kernels provide such support.

       If pid is positive it refers to a specific process;  if it  is  zero,  it  refers  to  the
       calling process; -1 refers to all processes other than the calling process and process '1'
       (typically init(8)); other negative values refer to the -pid process group.

       In order to use this function, the kernel must support it and  the  calling  process  must
       have  CAP_SETPCAP  raised  in  its  Effective  capability set. The capabilities set in the
       target process(es) are those contained in cap_d.

       Kernels that support filesystem capabilities redefine the semantics of CAP_SETPCAP and  on
       such  systems, capsetp() will always fail for any target not equal to the calling process.
       capsetp() returns zero for success, and -1 on failure.

       On kernels where it is (was) supported, capsetp() should be used with care.   It  existed,
       primarily,  to  overcome  an  early  lack  of  support for capabilities in the filesystems
       supported by Linux.  Note that on older kernels where capsetp() could be used to  set  the
       capabilities of another process, the only processes that had CAP_SETPCAP available to them
       by default were processes started as kernel threads.  (Typically  this  includes  init(8),
       kflushd and kswapd.) A kernel recompilation was needed to modify this default.


       The  code  segment  below raises the CAP_FOWNER and CAP_SETFCAP effective capabilities for
       the caller:

           cap_t caps;
           const cap_value_t cap_list[2] = {CAP_FOWNER, CAP_SETFCAP};

               /* handle error */

           caps = cap_get_proc();
           if (caps == NULL)
               /* handle error */;

           if (cap_set_flag(caps, CAP_EFFECTIVE, 2, cap_list, CAP_SET) == -1)
               /* handle error */;

           if (cap_set_proc(caps) == -1)
               /* handle error */;

           if (cap_free(caps) == -1)
               /* handle error */;

       Alternatively, to completely drop privilege in a program launched setuid-root but  wanting
       to  run  as a specific user ID etc. in such a way that neither it, nor any of its children
       can acquire privilege again:

           uid_t nobody = 65534;
           const gid_t groups[] = {65534};

           if (cap_setgroups(groups[0], 1, groups) != 0)
               /* handle error */;
           if (cap_setuid(nobody) != 0)
               /* handle error */;

            * privilege is still available here

           if (cap_set_mode(CAP_MODE_NOPRIV) != 0)
               /* handle error */

       Note, the above sequence can be performed by the capsh tool as follows:

       sudo /sbin/capsh --user=nobody --mode=NOPRIV --print

       where --print displays the resulting privilege state.


       libcap(3),   libpsx(3),   capsh(1),   cap_clear(3),   cap_copy_ext(3),   cap_from_text(3),
       cap_get_file(3), cap_init(3), psx_syscall(3), capabilities(7).

                                            2019-12-21                            CAP_GET_PROC(3)