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NAME |SYNOPSIS |DESCRIPTION |RETURN VALUE |CONFORMING TO |NOTES |EXAMPLE |SEE ALSO |COLOPHON | |
CAP_GET_PROC(3) Linux Programmer's ManualCAP_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 callingcap_free() with thecap_t as an argument.cap_set_proc() sets the values for all capability flags for all capabilities to the capability state identified bycap_p. The new capability state of the process will be completely determined by the contents ofcap_p upon successful return from this function. If any flag incap_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 acap_t, seecap_init(3), with the process capabilities of the process known to the caller aspid. Ifpid 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 thecapsh--decode=XXX command line.) When the caller is operating within a (CLONE_NEWPID) namespace, the numericalpid argument is interpreted in the range of that namespace. As such, the caller's idea of the targetpid may differ from that of the target process when they are operating in different pid namespaces. Seepid_namespaces(7) for details. Further, the returnedcap_t value holds the capabilities that the targetpid thinks it has. If the target is operating in a (CLONE_NEWUSER) namespace, the system wide privilege of those user namespace capabilities may be substantially reduced. Seeuser_namespaces(7) for details.cap_get_bound() with acap 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 functionCAP_IS_SUPPORTED(cap_value_tcap)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 withcap_get_bound().cap_drop_bound() can be used to lower the specified bounding set capability,cap. To complete successfully, the prevailingeffective capability set must have a raisedCAP_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 macroCAP_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 raise a specific ambient capability theinheritable andpermitted flags of the calling process must contain the specified capability value. Raised ambient bits will only be retained as long as this remains true of the inheritable and permitted flags.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. NoteCAP_SETPCAPmust 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 theCAP_SETPCAPcapability 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 numericalcap_mode_tvalue. A text representation of the mode can be obtained via thecap_mode_name() function. The vast majority of combinations of these values are not well defined in terms of a libcap mode, and for these statescap_get_mode() returns (cap_mode_t)0 whichcap_get_name() identifies as ``UNCERTAIN''. Supported modes are:CAP_MODE_NOPRIV,CAP_MODE_HYBRID,CAP_MODE_PURE1EandCAP_MODE_PURE1E_INIT.cap_prctl() can be used to read state via theprctl()system call.cap_prctlw() can be used to write state via theprctl()systemcall.cap_set_mode() can be used to set the desired mode. The permitted capabilityCAP_SETPCAPis required for this function to succeed.cap_setuid() is a convenience function for thesetuid(2) system call. Wherecap_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 bothsetgid(2) andsetgroups(2) calls in one call. Thecap_setgroups() call raises the right effective capability for the duration of the call, and empties the effective capability set before returning.
The functionscap_get_proc() andcap_get_pid() return a non-NULL value on success, and NULL on failure. The functioncap_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 ascap_set_proc() andcap_drop_bound() return zero for success, and -1 on failure. On failure,errno is set toEINVAL,EPERM, orENOMEM.
cap_set_proc() andcap_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,libcapis 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 $(pkg-config --libs --cflags libpsx) or,gcc ... -lcap $(pkg-config --libs --cflags libpsx) When linked this way, due to linker magic, libcap usespsx_syscall(3) andpsx_syscall6(3) to perform state setting system calls. Notably, this also ensures thatcap_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_tpid, cap_tcap_d);int capsetp(pid_tpid, cap_tcap_d);capgetp() attempts to obtain the capabilities of some other process; storing the capabilities in a pre-allocatedcap_d. Seecap_init() for information on allocating an empty capability set. This function is deprecated; you should usecap_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. Seecapset(2) for information on which kernels provide such support. Ifpid 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' (typicallyinit(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 haveCAP_SETPCAPraised in its Effective capability set. The capabilities set in the target process(es) are those contained incap_d. Kernels that support filesystem capabilities redefine the semantics ofCAP_SETPCAPand 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 wherecapsetp() could be used to set the capabilities of another process, the only processes that hadCAP_SETPCAPavailable to them by default were processes started as kernel threads. (Typically this includesinit(8), kflushd and kswapd.) A kernel recompilation was needed to modify this default.
The code segment below raises theCAP_FOWNERandCAP_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 thecapshtool as follows:sudo capsh --user=nobody --mode=NOPRIV --print where--printdisplays 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 thelibcap (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 2025-08-11. (At that time, the date of the most recent commit that was found in the repository was 2025-08-10.) 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 isnot part of the original manual page), send a mail to man-pages@man7.org 2024-11-09CAP_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), libpsx(3), capabilities(7)
HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |