path: root/man2/capget.2

.\" Copyright: written by Andrew Morgan <morgan@kernel.org>
.\" and Copyright 2006, 2008, Michael Kerrisk <tmk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: GPL-1.0-or-later
.\"
.\" Modified by David A. Wheeler <dwheeler@ida.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 Linux 2.6.2[45].
.\" Modified 2009-01-26, andi kleen
.\"
.TH capget 2 2023-05-03 "Linux man-pages 6.05.01"
.SH NAME
capget, capset \- set/get capabilities of thread(s)
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.BR "#include <linux/capability.h>" " /* Definition of " CAP_* " and"
.BR "                                 _LINUX_CAPABILITY_*" " constants */"
.BR "#include <sys/syscall.h>" "      /* Definition of " SYS_* " constants */"
.B #include <unistd.h>
.PP
.BI "int syscall(SYS_capget, cap_user_header_t " hdrp ,
.BI "            cap_user_data_t " datap );
.BI "int syscall(SYS_capset, cap_user_header_t " hdrp ,
.BI "            const cap_user_data_t " datap );
.fi
.PP
.IR Note :
glibc provides no wrappers for these system calls,
necessitating the use of
.BR syscall (2).
.SH DESCRIPTION
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
.I cap_user_*_t
types) is subject to extension with each kernel revision,
but old programs will keep working.
.PP
The portable interfaces are
.BR cap_set_proc (3)
and
.BR cap_get_proc (3);
if possible, you should use those interfaces in applications; see NOTES.
.\"
.SS Current details
Now that you have been warned, some current kernel details.
The structures are defined as follows.
.PP
.in +4n
.EX
#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
.\" commit e338d263a76af78fe8f38a72131188b58fceb591
.\" Added 64 bit capability support
#define _LINUX_CAPABILITY_U32S_2     2
\&
        /* V3 added in Linux 2.6.26 */
#define _LINUX_CAPABILITY_VERSION_3  0x20080522
.\" commit ca05a99a54db1db5bca72eccb5866d2a86f8517f
#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;
.EE
.in
.PP
The
.IR effective ,
.IR permitted ,
and
.I inheritable
fields are bit masks of the capabilities defined in
.BR capabilities (7).
Note that the
.B 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
.I struct __user_cap_header_struct
and
.I struct __user_cap_data_struct
names because the typedefs are only pointers.
.PP
Kernels prior to Linux 2.6.25 prefer
32-bit capabilities with version
.BR _LINUX_CAPABILITY_VERSION_1 .
Linux 2.6.25 added 64-bit capability sets, with version
.BR _LINUX_CAPABILITY_VERSION_2 .
There was, however, an API glitch, and Linux 2.6.26 added
.B _LINUX_CAPABILITY_VERSION_3
to fix the problem.
.PP
Note that 64-bit capabilities use
.I datap[0]
and
.IR datap[1] ,
whereas 32-bit capabilities use only
.IR datap[0] .
.PP
On kernels that support file capabilities (VFS capabilities 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.
.PP
For
.BR capget ()
calls, one can probe the capabilities of any process by specifying its
process ID with the
.I hdrp\->pid
field value.
.PP
For details on the data, see
.BR capabilities (7).
.\"
.SS With VFS capabilities support
VFS capabilities employ a file extended attribute (see
.BR xattr (7))
to allow capabilities to be attached to executables.
This privilege model obsoletes kernel support for one process
asynchronously setting the capabilities of another.
That is, on kernels that have VFS capabilities support, when calling
.BR capset (),
the only permitted values for
.I hdrp\->pid
are 0 or, equivalently, the value returned by
.BR gettid (2).
.\"
.SS Without VFS capabilities support
On older kernels that do not provide VFS capabilities support
.BR capset ()
can, if the caller has the
.B CAP_SETPCAP
capability, be used to change not only the caller's own capabilities,
but also the capabilities of other threads.
The call operates on the capabilities of the thread specified by the
.I pid
field of
.I hdrp
when that is nonzero, or on the capabilities of the calling thread if
.I pid
is 0.
If
.I pid
refers to a single-threaded process, then
.I 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
.BR gettid (2).
For
.BR capset (),
.I pid
can also be: \-1, meaning perform the change on all threads except the
caller and
.BR 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 \-\fIpid\fP.
.SH RETURN VALUE
On success, zero is returned.
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.PP
The calls fail with the error
.BR EINVAL ,
and set the
.I version
field of
.I hdrp
to the kernel preferred value of
.B _LINUX_CAPABILITY_VERSION_?
when an unsupported
.I version
value is specified.
In this way, one can probe what the current
preferred capability revision is.
.SH ERRORS
.TP
.B EFAULT
Bad memory address.
.I hdrp
must not be NULL.
.I datap
may be NULL only when the user is trying to determine the preferred
capability version format supported by the kernel.
.TP
.B EINVAL
One of the arguments was invalid.
.TP
.B EPERM
An attempt was made to add a capability to the permitted set, or to set
a capability in the effective set that is not in the
permitted set.
.TP
.B EPERM
An attempt was made to add a capability to the inheritable set, and either:
.RS
.IP \[bu] 3
that capability was not in the caller's bounding set; or
.IP \[bu]
the capability was not in the caller's permitted set
and the caller lacked the
.B CAP_SETPCAP
capability in its effective set.
.RE
.TP
.B EPERM
The caller attempted to use
.BR 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
.B CAP_SETPCAP
capability is required.
(A bug in kernels before Linux 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
.I pid
field as a nonzero value (i.e., the value returned by
.BR getpid (2))
instead of 0.)
.TP
.B ESRCH
No such thread.
.SH STANDARDS
Linux.
.SH NOTES
The portable interface to the capability querying and setting
functions is provided by the
.I libcap
library and is available here:
.br
.UR http://git.kernel.org/cgit\:/linux\:/kernel\:/git\:/morgan\:\:/libcap.git
.UE
.SH SEE ALSO
.BR clone (2),
.BR gettid (2),
.BR capabilities (7)