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

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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_*_t types) 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

#define _LINUX_CAPABILITY_VERSION_2 0x20071026
#define _LINUX_CAPABILITY_U32S_2 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 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
__user_cap_header_struct and struct __user_cap_data_struct names because the typedefs are only pointers.

Kernels prior to 2.6.25 prefer 32-bit capabilities with version _LINUX_CAPABILITY_VERSION_1, and kernels
2.6.25+ prefer 64-bit capabilities with version _LINUX_CAPABILITY_VERSION_2. Note, 64-bit capabilities
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 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->pid are 0 or
getpid(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 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 gettid(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  _LINUX_CAPABILITY_VERSION_?  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 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_SETPCAP capability 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

COLOPHON

       This  page  is  part  of  release 3.54 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)