capset(2)
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
/* 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 inheritable fields 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_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. 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_3 to 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->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 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 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(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 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⟩
SEE ALSO
clone(2), gettid(2), capabilities(7)
COLOPHON
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