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capget, capset — set/get capabilities of thread(s)
#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); |
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).
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.
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.
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).
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.
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.
One of the arguments was invalid.
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.
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.)
No such thread.
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
This page is part of release 3.52 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/.
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written by Andrew Morgan <morgankernel.org> %%%LICENSE_START(GPL_NOVERSION_ONELINE) may be distributed as per GPL %%%LICENSE_END Modified by David A. Wheeler <dwheelerida.org> Modified 2004-05-27, mtk Modified 2004-06-21, aeb Modified 2008-04-28, morgan of kernel.org Update in line with addition of file capabilities and 64-bit capability sets in kernel 2.6.2[45]. Modified 2009-01-26, andi kleen |