Provided by: libcap-dev_2.66-5ubuntu2.1_amd64 
NAME
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_prctl, cap_prctlw, cap_setgroups - capability manipulation on processes
SYNOPSIS
#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);
CAP_AMBIENT_SUPPORTED();
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_prctl(long int pr_cmd, long int arg1, long int arg2,
long int arg3, long int arg4, long int arg5);
int cap_prctlw(long int pr_cmd, long int arg1, long int arg2,
long int arg3, long int arg4, long int arg5);
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.
DESCRIPTION
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 unchanged.
cap_get_pid() returns a cap_t, see cap_init(3), with the process capabilities of the process known to the
caller as 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. (The entries in that file can be translated with the
capsh --decode=XXX command line.) When the caller is operating within a (CLONE_NEWPID) namespace, the
numerical pid argument is interpreted in the range of that namespace. As such, the caller's idea of the
target pid may differ from that of the target process when they are operating in different pid
namespaces. See pid_namespaces(7) for details. Further, the returned cap_t value holds the capabilities
that the target pid thinks it has. If the target is operating in a (CLONE_NEWUSER) namespace, the system
wide privilege of those user namespace capabilities my be substantially reduced. See user_namespaces(7)
for details.
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_SETPCAP.
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. 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. Further, 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 capabilities.
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_MODE_PURE1E.
cap_prctl() can be used to read state via the prctl() system call.
cap_prctlw() can be used to write state via the prctl() system call.
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.
RETURN VALUE
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.
CONFORMING TO
cap_set_proc() and cap_get_proc() are specified in the withdrawn POSIX.1e draft specification.
cap_get_pid() is a Linux extension.
NOTES
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
or,
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. Notably, this also ensures that cap_prctlw() can be used to ensure process
control bits are shared over all threads of a single process.
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 process 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.
EXAMPLE
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};
if (!CAP_IS_SUPPORTED(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 capsh --user=nobody --mode=NOPRIV --print
where --print displays the resulting privilege state.
SEE ALSO
libcap(3), libpsx(3), capsh(1), cap_clear(3), cap_copy_ext(3), cap_from_text(3), cap_get_file(3),
cap_init(3), namespaces(7), pid_namespaces(7), user_namespaces(7), psx_syscall(3), capabilities(7).
2022-04-28 CAP_GET_PROC(3)