NAME
capget, capset - set/get capabilities of thread(s) SYNOPSIS
#include
int capget(capuserheadert hdrp, capuserdatat datap); int capset(capuserheadert hdrp, const capuserdatat datap); DESCRIPTION As of Linux 2.2, the power of the superuser (root) has been partitioned into a set of discrete capabilities. Each thread has a set of effec‐ tive capabilities identifying which capabilities (if any) it may cur‐ rently exercise. Each thread also has a set of inheritable capabili‐ ties that may be passed through an execve(2) call, and a set of permit‐ ted capabilities that it can make effective or inheritable. These two system calls are the raw kernel interface for getting and setting thread capabilities. Not only are these system calls specific to Linux, but the kernel API is likely to change and use of these sys‐ tem calls (in particular the format of the capuser*t types) is sub‐ ject to extension with each kernel revision, but old programs will keep working. The portable interfaces are capsetproc(3) and capgetproc(3); if possible you should use those interfaces in applications. If you wish to use the Linux extensions in applications, you should use the easier-
to-use interfaces capsetp(3) and capgetp(3). Current details Now that you have been warned, some current kernel details. The struc‐ tures are defined as follows.
#define LINUXCAPABILITYVERSION1 0x19980330
#define LINUXCAPABILITYU32S1 1
#define LINUXCAPABILITYVERSION2 0x20071026
#define LINUXCAPABILITYU32S2 2 typedef struct usercapheaderstruct { u32 version; int pid; } *capuserheadert; typedef struct usercapdatastruct { u32 effective; u32 permitted; u32 inheritable; } *capuserdatat; The effective, permitted, and inheritable fields are bit masks of the capabilities defined in capability(7). Note the CAP* values are bit
indexes and need to be bit-shifted before ORing into the bit fields. To define the structures for passing to the system call you have to use the struct usercapheaderstruct and struct usercapdatastruct names because the typedefs are only pointers.
Kernels prior to 2.6.25 prefer 32-bit capabilities with version
LINUXCAPABILITYVERSION1, and kernels 2.6.25+ prefer 64-bit capabil‐
ities with version LINUXCAPABILITYVERSION2. Note, 64-bit capabili‐
ties use datap[0] and datap[1], whereas 32-bit capabilities use only datap[0]. Another change affecting the behavior of these system calls is kernel support for file capabilities (VFS capability support). This support is currently a compile time option (added in kernel 2.6.24). For capget() calls, one can probe the capabilities of any process by
specifying its process ID with the hdrp->pid field value. With VFS capability support
VFS Capability support creates a file-attribute method for adding capa‐ bilities to privileged executables. This privilege model obsoletes kernel support for one process asynchronously setting the capabilities of another. That is, with VFS support, for capset() calls the only
permitted values for hdrp->pid are 0 or getpid(2), which are equiva‐ lent. Without VFS capability support When the kernel does not support VFS capabilities, capset() calls can operate on the capabilities of the thread specified by the pid field of hdrp when that is nonzero, or on the capabilities of the calling thread
if pid is 0. If pid refers to a single-threaded process, then pid can be specified as a traditional process ID; operating on a thread of a multithreaded process requires a thread ID of the type returned by get‐
tid(2). For capset(), pid can also be: -1, meaning perform the change
on all threads except the caller and init(8); or a value less than -1, in which case the change is applied to all members of the process group
whose ID is -pid. For details on the data, see capabilities(7). RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is set appropriately. The calls will fail with the error EINVAL, and set the version field of hdrp to the kernel preferred value of LINUXCAPABILITYVERSION? when an unsupported version value is specified. In this way, one can probe what the current preferred capability revision is. ERRORS EFAULT Bad memory address. hdrp must not be NULL. datap may be NULL only when the user is trying to determine the preferred capabil‐ ity version format supported by the kernel. EINVAL One of the arguments was invalid. EPERM An attempt was made to add a capability to the Permitted set, or to set a capability in the Effective or Inheritable sets that is not in the Permitted set. EPERM The caller attempted to use capset() to modify the capabilities of a thread other than itself, but lacked sufficient privilege. For kernels supporting VFS capabilities, this is never permit‐ ted. For kernels lacking VFS support, the CAPSETPCAP capabil‐ ity is required. (A bug in kernels before 2.6.11 meant that this error could also occur if a thread without this capability tried to change its own capabilities by specifying the pid field as a nonzero value (i.e., the value returned by getpid(2)) instead of 0.) ESRCH No such thread. CONFORMING TO
These system calls are Linux-specific. NOTES The portable interface to the capability querying and setting functions is provided by the libcap library and is available here: ⟨http://git.kernel.org/cgit/linux/kernel/git/morgan/libcap.git⟩ SEE ALSO clone(2), gettid(2), capabilities(7) COLOPHON
This page is part of release 3.53 of the Linux man-pages project. A description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2013-03-11 CAPGET(2)