As of Linux 2.2, the power of the superuser (root) has been partitioned into
a set of discrete capabilities.
Every thread has a set of effective capabilities identifying
which capabilities (if any) it may currently exercise.
Every 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 functions are the raw kernel interface for getting and
setting capabilities. Not only are these system calls specific to Linux,
but the kernel API is likely to change and use of
these functions (in particular the format of the
cap_user_*_t types) is subject to change with each kernel revision.
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 structs are defined as follows.
#define _LINUX_CAPABILITY_VERSION 0x19980330
typedef struct __user_cap_header_struct {
int version;
int pid;
} *cap_user_header_t;
typedef struct __user_cap_data_struct {
int effective;
int permitted;
int inheritable;
} *cap_user_data_t;
The calls will return EINVAL, and set the
version field of
hdr to _LINUX_CAPABILITY_VERSION when another version was specified.
The calls operate on the capabilities of the thread specified by the
pid field of
hdr when that is non-zero, 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).
Bad memory address. Neither of
hdrp and
datap may be NULL.
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; 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 non-zero value (i.e., the value returned by
getpid(2))
instead of 0.)
