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

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