1CAPGET(2) Linux Programmer's Manual CAPGET(2)
2
6 capget, capset - set/get capabilities of thread(s)
7
9 #include <sys/capability.h>
10 int capget(cap_user_header_t hdrp, cap_user_data_t datap);
12 int capset(cap_user_header_t hdrp, const cap_user_data_t datap);
14
16 Since Linux 2.2, the power of the superuser (root) has been partitioned
17 into a set of discrete capabilities. Each thread has a set of effec‐
18 tive capabilities identifying which capabilities (if any) it may cur‐
19 rently exercise. Each thread also has a set of inheritable capabili‐
20 ties that may be passed through an execve(2) call, and a set of permit‐
21 ted capabilities that it can make effective or inheritable.
22 These two system calls are the raw kernel interface for getting and
24 setting thread capabilities. Not only are these system calls specific
25 to Linux, but the kernel API is likely to change and use of these sys‐
26 tem calls (in particular the format of the cap_user_*_t types) is sub‐
27 ject to extension with each kernel revision, but old programs will keep
28 working.
29 The portable interfaces are cap_set_proc(3) and cap_get_proc(3); if
31 possible, you should use those interfaces in applications. If you wish
32 to use the Linux extensions in applications, you should use the easier-
33 to-use interfaces capsetp(3) and capgetp(3).
34 Current details
36 Now that you have been warned, some current kernel details. The struc‐
37 tures are defined as follows.
38 #define _LINUX_CAPABILITY_VERSION_1 0x19980330
40 #define _LINUX_CAPABILITY_U32S_1 1
41 /* V2 added in Linux 2.6.25; deprecated */
43 #define _LINUX_CAPABILITY_VERSION_2 0x20071026
44 #define _LINUX_CAPABILITY_U32S_2 2
45 /* V3 added in Linux 2.6.26 */
47 #define _LINUX_CAPABILITY_VERSION_3 0x20080522
48 #define _LINUX_CAPABILITY_U32S_3 2
49 typedef struct __user_cap_header_struct {
51 __u32 version;
52 int pid;
53 } *cap_user_header_t;
54 typedef struct __user_cap_data_struct {
56 __u32 effective;
57 __u32 permitted;
58 __u32 inheritable;
59 } *cap_user_data_t;
60 The effective, permitted, and inheritable fields are bit masks of the
62 capabilities defined in capabilities(7). Note that the CAP_* values
63 are bit indexes and need to be bit-shifted before ORing into the bit
64 fields. To define the structures for passing to the system call, you
65 have to use the struct __user_cap_header_struct and struct
66 __user_cap_data_struct names because the typedefs are only pointers.
67 Kernels prior to 2.6.25 prefer 32-bit capabilities with version
69 _LINUX_CAPABILITY_VERSION_1. Linux 2.6.25 added 64-bit capability
70 sets, with version _LINUX_CAPABILITY_VERSION_2. There was, however, an
71 API glitch, and Linux 2.6.26 added _LINUX_CAPABILITY_VERSION_3 to fix
72 the problem.
73 Note that 64-bit capabilities use datap[0] and datap[1], whereas 32-bit
75 capabilities use only datap[0].
76 On kernels that support file capabilities (VFS capabilities support),
78 these system calls behave slightly differently. This support was added
79 as an option in Linux 2.6.24, and became fixed (nonoptional) in Linux
80 2.6.33.
81 For capget() calls, one can probe the capabilities of any process by
83 specifying its process ID with the hdrp->pid field value.
84 With VFS capabilities support
86 VFS capabilities employ a file extended attribute (see xattr(7)) to
87 allow capabilities to be attached to executables. This privilege model
88 obsoletes kernel support for one process asynchronously setting the
89 capabilities of another. That is, on kernels that have VFS capabili‐
90 ties support, when calling capset(), the only permitted values for
91 hdrp->pid are 0 or, equivalently, the value returned by gettid(2).
92 Without VFS capabilities support
94 On older kernels that do not provide VFS capabilities support capset()
95 can, if the caller has the CAP_SETPCAP capability, be used to change
96 not only the caller's own capabilities, but also the capabilities of
97 other threads. The call operates on the capabilities of the thread
98 specified by the pid field of hdrp when that is nonzero, or on the
99 capabilities of the calling thread if pid is 0. If pid refers to a
100 single-threaded process, then pid can be specified as a traditional
101 process ID; operating on a thread of a multithreaded process requires a
102 thread ID of the type returned by gettid(2). For capset(), pid can
103 also be: -1, meaning perform the change on all threads except the call‐
104 er and init(1); or a value less than -1, in which case the change is
105 applied to all members of the process group whose ID is -pid.
106 For details on the data, see capabilities(7).
108
110 On success, zero is returned. On error, -1 is returned, and errno is
111 set appropriately.
112 The calls fail with the error EINVAL, and set the version field of hdrp
114 to the kernel preferred value of _LINUX_CAPABILITY_VERSION_? when an
115 unsupported version value is specified. In this way, one can probe
116 what the current preferred capability revision is.
117
119 EFAULT Bad memory address. hdrp must not be NULL. datap may be NULL
120 only when the user is trying to determine the preferred capabil‐
121 ity version format supported by the kernel.
122 EINVAL One of the arguments was invalid.
124 EPERM An attempt was made to add a capability to the Permitted set, or
126 to set a capability in the Effective or Inheritable sets that is
127 not in the Permitted set.
128 EPERM The caller attempted to use capset() to modify the capabilities
130 of a thread other than itself, but lacked sufficient privilege.
131 For kernels supporting VFS capabilities, this is never permit‐
132 ted. For kernels lacking VFS support, the CAP_SETPCAP capabil‐
133 ity is required. (A bug in kernels before 2.6.11 meant that
134 this error could also occur if a thread without this capability
135 tried to change its own capabilities by specifying the pid field
136 as a nonzero value (i.e., the value returned by getpid(2))
137 instead of 0.)
138 ESRCH No such thread.
140
142 These system calls are Linux-specific.
143
145 The portable interface to the capability querying and setting functions
146 is provided by the libcap library and is available here:
147 ⟨http://git.kernel.org/cgit/linux/kernel/git/morgan/libcap.git⟩
148
150 clone(2), gettid(2), capabilities(7)
151
153 This page is part of release 4.16 of the Linux man-pages project. A
154 description of the project, information about reporting bugs, and the
155 latest version of this page, can be found at
156 https://www.kernel.org/doc/man-pages/.
157Linux 2017-09-15 CAPGET(2)