Manpage of CAPGET

CAPGET

Section: Linux Programmer's Manual (2)
Updated: 2015-07-23
Index
 

NAME

capget, capset - set/get capabilities of thread(s)  

SYNOPSIS

#include <sys/capability.h>

int capget(cap_user_header_t hdrp, cap_user_data_t datap);

int capset(cap_user_header_t hdrp, const cap_user_data_t 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 effective capabilities identifying which capabilities (if any) it may currently exercise. Each thread also has a set of inheritable capabilities that may be passed through an execve(2) call, and a set of permitted 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 system calls (in particular the format of the cap_user_*_ttypes) is subject to extension with each kernel revision, but old programs will keep working.

The portable interfaces are cap_set_proc(3) and cap_get_proc(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 structures are defined as follows.

#define _LINUX_CAPABILITY_VERSION_1  0x19980330
#define _LINUX_CAPABILITY_U32S_1     1

        /* V2 added in Linux 2.6.25; deprecated */
#define _LINUX_CAPABILITY_VERSION_2  0x20071026
#define _LINUX_CAPABILITY_U32S_2     2

        /* V3 added in Linux 2.6.26 */
#define _LINUX_CAPABILITY_VERSION_3  0x20080522
#define _LINUX_CAPABILITY_U32S_3     2

typedef struct __user_cap_header_struct {
   __u32 version;
   int pid;
} *cap_user_header_t;

typedef struct __user_cap_data_struct {
   __u32 effective;
   __u32 permitted;
   __u32 inheritable;
} *cap_user_data_t;

The effective, permitted, and inheritablefields are bit masks of the capabilities defined in capabilities(7). Note that 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 __user_cap_header_structand struct __user_cap_data_structnames because the typedefs are only pointers.

Kernels prior to 2.6.25 prefer 32-bit capabilities with version _LINUX_CAPABILITY_VERSION_1. Linux 2.6.25 added 64-bit capability sets, with version _LINUX_CAPABILITY_VERSION_2. There was, however, an API glitch, and Linux 2.6.26 added _LINUX_CAPABILITY_VERSION_3to fix the problem.

Note that 64-bit capabilities use datap[0] and datap[1], whereas 32-bit capabilities use only datap[0].

On kernels that support file capabilities (VFS capability support), these system calls behave slightly differently. This support was added as an option in Linux 2.6.24, and became fixed (nonoptional) in Linux 2.6.33.

For capget() calls, one can probe the capabilities of any process by specifying its process ID with the hdrp->pidfield value.  

With VFS capability support

VFS Capability support creates a file-attribute method for adding capabilities 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->pidare 0 or gettid(2), which are equivalent.  

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 pidfield of hdrpwhen that is nonzero, or on the capabilities of the calling thread if pidis 0. If pidrefers to a single-threaded process, then pidcan be specified as a traditional process ID; operating on a thread of a multithreaded process requires a thread ID of the type returned by gettid(2). For capset(), pidcan also be: -1, meaning perform the change on all threads except the caller and init(1); 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 errnois set appropriately.

The calls will fail with the error EINVAL, and set the versionfield of hdrpto the kernel preferred value of _LINUX_CAPABILITY_VERSION_?when an unsupported versionvalue is specified. In this way, one can probe what the current preferred capability revision is.  

ERRORS

EFAULT
Bad memory address. hdrpmust not be NULL. datapmay be NULL only when the user is trying to determine the preferred capability 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 permitted. For kernels lacking VFS support, the CAP_SETPCAPcapability 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 pidfield 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 libcaplibrary and is available here:
 

SEE ALSO

clone(2), gettid(2), capabilities(7)


 

Index

NAME
SYNOPSIS
DESCRIPTION
Current details
With VFS capability support
Without VFS capability support
RETURN VALUE
ERRORS
CONFORMING TO
NOTES
SEE ALSO

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Time: 22:27:44 GMT, June 20, 2016