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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON |
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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_prctl, cap_prctlw, 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_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.
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.
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. 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.
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.
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).
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
[email protected] (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
2024-06-14. (At that time, the date of the most recent commit
that was found in the repository was 2024-05-18.) 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 [email protected]
2022-04-28 CAP_GET_PROC(3)
Pages that refer to this page: capsh(1), capget(2), PR_CAP_AMBIENT(2const), PR_CAPBSET_DROP(2const), PR_CAPBSET_READ(2const), cap_clear(3), cap_copy_ext(3), cap_from_text(3), cap_get_file(3), cap_iab(3), cap_init(3), cap_launch(3), libcap(3), capabilities(7)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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