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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON |
CAP_GET_PROC(3) Linux Programmer's Manual CAP_GET_PROC(3)
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);
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_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 unchanged.
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_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. 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 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_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
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.
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};
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 /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).
This page is part of the libcap (capabilities commands and library)
project. Information about the project can be found at
⟨https://git.kernel.org/pub/scm/libs/libcap/libcap.git/⟩. If you have
a bug report for this manual page, send it to morgan@kernel.org
(please put "libcap" in the Subject line). This page was obtained
from the project's upstream Git repository
⟨https://git.kernel.org/pub/scm/libs/libcap/libcap.git/⟩ on
2020-08-13. (At that time, the date of the most recent commit that
was found in the repository was 2020-08-06.) If you discover any
rendering problems in this HTML version of the page, or you believe
there is a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
2019-12-21 CAP_GET_PROC(3)
Pages that refer to this page: capget(2) , capset(2) , cap_clear(3) , cap_clear_flag(3) , cap_compare(3) , cap_copy_ext(3) , cap_copy_int(3) , cap_dup(3) , cap_free(3) , cap_from_name(3) , cap_from_text(3) , cap_get_fd(3) , cap_get_file(3) , cap_get_flag(3) , cap_init(3) , cap_set_fd(3) , cap_set_file(3) , cap_set_flag(3) , cap_size(3) , cap_to_name(3) , cap_to_text(3) , libcap(3) , capabilities(7)