1 ======================
2 ioctl based interfaces
3 ======================
4
5 ioctl() is the most common way for applications to interface
6 with device drivers. It is flexible and easily extended by adding new
7 commands and can be passed through character devices, block devices as
8 well as sockets and other special file descriptors.
9
10 However, it is also very easy to get ioctl command definitions wrong,
11 and hard to fix them later without breaking existing applications,
12 so this documentation tries to help developers get it right.
13
14 Command number definitions
15 ==========================
16
17 The command number, or request number, is the second argument passed to
18 the ioctl system call. While this can be any 32-bit number that uniquely
19 identifies an action for a particular driver, there are a number of
20 conventions around defining them.
21
22 ``include/uapi/asm-generic/ioctl.h`` provides four macros for defining
23 ioctl commands that follow modern conventions: ``_IO``, ``_IOR``,
24 ``_IOW``, and ``_IOWR``. These should be used for all new commands,
25 with the correct parameters:
26
27 _IO/_IOR/_IOW/_IOWR
28 The macro name specifies how the argument will be used. It may be a
29 pointer to data to be passed into the kernel (_IOW), out of the kernel
30 (_IOR), or both (_IOWR). _IO can indicate either commands with no
31 argument or those passing an integer value instead of a pointer.
32 It is recommended to only use _IO for commands without arguments,
33 and use pointers for passing data.
34
35 type
36 An 8-bit number, often a character literal, specific to a subsystem
37 or driver, and listed in Documentation/userspace-api/ioctl/ioctl-number.rst
38
39 nr
40 An 8-bit number identifying the specific command, unique for a give
41 value of 'type'
42
43 data_type
44 The name of the data type pointed to by the argument, the command number
45 encodes the ``sizeof(data_type)`` value in a 13-bit or 14-bit integer,
46 leading to a limit of 8191 bytes for the maximum size of the argument.
47 Note: do not pass sizeof(data_type) type into _IOR/_IOW/IOWR, as that
48 will lead to encoding sizeof(sizeof(data_type)), i.e. sizeof(size_t).
49 _IO does not have a data_type parameter.
50
51
52 Interface versions
53 ==================
54
55 Some subsystems use version numbers in data structures to overload
56 commands with different interpretations of the argument.
57
58 This is generally a bad idea, since changes to existing commands tend
59 to break existing applications.
60
61 A better approach is to add a new ioctl command with a new number. The
62 old command still needs to be implemented in the kernel for compatibility,
63 but this can be a wrapper around the new implementation.
64
65 Return code
66 ===========
67
68 ioctl commands can return negative error codes as documented in errno(3);
69 these get turned into errno values in user space. On success, the return
70 code should be zero. It is also possible but not recommended to return
71 a positive 'long' value.
72
73 When the ioctl callback is called with an unknown command number, the
74 handler returns either -ENOTTY or -ENOIOCTLCMD, which also results in
75 -ENOTTY being returned from the system call. Some subsystems return
76 -ENOSYS or -EINVAL here for historic reasons, but this is wrong.
77
78 Prior to Linux 5.5, compat_ioctl handlers were required to return
79 -ENOIOCTLCMD in order to use the fallback conversion into native
80 commands. As all subsystems are now responsible for handling compat
81 mode themselves, this is no longer needed, but it may be important to
82 consider when backporting bug fixes to older kernels.
83
84 Timestamps
85 ==========
86
87 Traditionally, timestamps and timeout values are passed as ``struct
88 timespec`` or ``struct timeval``, but these are problematic because of
89 incompatible definitions of these structures in user space after the
90 move to 64-bit time_t.
91
92 The ``struct __kernel_timespec`` type can be used instead to be embedded
93 in other data structures when separate second/nanosecond values are
94 desired, or passed to user space directly. This is still not ideal though,
95 as the structure matches neither the kernel's timespec64 nor the user
96 space timespec exactly. The get_timespec64() and put_timespec64() helper
97 functions can be used to ensure that the layout remains compatible with
98 user space and the padding is treated correctly.
99
100 As it is cheap to convert seconds to nanoseconds, but the opposite
101 requires an expensive 64-bit division, a simple __u64 nanosecond value
102 can be simpler and more efficient.
103
104 Timeout values and timestamps should ideally use CLOCK_MONOTONIC time,
105 as returned by ktime_get_ns() or ktime_get_ts64(). Unlike
106 CLOCK_REALTIME, this makes the timestamps immune from jumping backwards
107 or forwards due to leap second adjustments and clock_settime() calls.
108
109 ktime_get_real_ns() can be used for CLOCK_REALTIME timestamps that
110 need to be persistent across a reboot or between multiple machines.
111
112 32-bit compat mode
113 ==================
114
115 In order to support 32-bit user space running on a 64-bit machine, each
116 subsystem or driver that implements an ioctl callback handler must also
117 implement the corresponding compat_ioctl handler.
118
119 As long as all the rules for data structures are followed, this is as
120 easy as setting the .compat_ioctl pointer to a helper function such as
121 compat_ptr_ioctl() or blkdev_compat_ptr_ioctl().
122
123 compat_ptr()
124 ------------
125
126 On the s390 architecture, 31-bit user space has ambiguous representations
127 for data pointers, with the upper bit being ignored. When running such
128 a process in compat mode, the compat_ptr() helper must be used to
129 clear the upper bit of a compat_uptr_t and turn it into a valid 64-bit
130 pointer. On other architectures, this macro only performs a cast to a
131 ``void __user *`` pointer.
132
133 In an compat_ioctl() callback, the last argument is an unsigned long,
134 which can be interpreted as either a pointer or a scalar depending on
135 the command. If it is a scalar, then compat_ptr() must not be used, to
136 ensure that the 64-bit kernel behaves the same way as a 32-bit kernel
137 for arguments with the upper bit set.
138
139 The compat_ptr_ioctl() helper can be used in place of a custom
140 compat_ioctl file operation for drivers that only take arguments that
141 are pointers to compatible data structures.
142
143 Structure layout
144 ----------------
145
146 Compatible data structures have the same layout on all architectures,
147 avoiding all problematic members:
148
149 * ``long`` and ``unsigned long`` are the size of a register, so
150 they can be either 32-bit or 64-bit wide and cannot be used in portable
151 data structures. Fixed-length replacements are ``__s32``, ``__u32``,
152 ``__s64`` and ``__u64``.
153
154 * Pointers have the same problem, in addition to requiring the
155 use of compat_ptr(). The best workaround is to use ``__u64``
156 in place of pointers, which requires a cast to ``uintptr_t`` in user
157 space, and the use of u64_to_user_ptr() in the kernel to convert
158 it back into a user pointer.
159
160 * On the x86-32 (i386) architecture, the alignment of 64-bit variables
161 is only 32-bit, but they are naturally aligned on most other
162 architectures including x86-64. This means a structure like::
163
164 struct foo {
165 __u32 a;
166 __u64 b;
167 __u32 c;
168 };
169
170 has four bytes of padding between a and b on x86-64, plus another four
171 bytes of padding at the end, but no padding on i386, and it needs a
172 compat_ioctl conversion handler to translate between the two formats.
173
174 To avoid this problem, all structures should have their members
175 naturally aligned, or explicit reserved fields added in place of the
176 implicit padding. The ``pahole`` tool can be used for checking the
177 alignment.
178
179 * On ARM OABI user space, structures are padded to multiples of 32-bit,
180 making some structs incompatible with modern EABI kernels if they
181 do not end on a 32-bit boundary.
182
183 * On the m68k architecture, struct members are not guaranteed to have an
184 alignment greater than 16-bit, which is a problem when relying on
185 implicit padding.
186
187 * Bitfields and enums generally work as one would expect them to,
188 but some properties of them are implementation-defined, so it is better
189 to avoid them completely in ioctl interfaces.
190
191 * ``char`` members can be either signed or unsigned, depending on
192 the architecture, so the __u8 and __s8 types should be used for 8-bit
193 integer values, though char arrays are clearer for fixed-length strings.
194
195 Information leaks
196 =================
197
198 Uninitialized data must not be copied back to user space, as this can
199 cause an information leak, which can be used to defeat kernel address
200 space layout randomization (KASLR), helping in an attack.
201
202 For this reason (and for compat support) it is best to avoid any
203 implicit padding in data structures. Where there is implicit padding
204 in an existing structure, kernel drivers must be careful to fully
205 initialize an instance of the structure before copying it to user
206 space. This is usually done by calling memset() before assigning to
207 individual members.
208
209 Subsystem abstractions
210 ======================
211
212 While some device drivers implement their own ioctl function, most
213 subsystems implement the same command for multiple drivers. Ideally the
214 subsystem has an .ioctl() handler that copies the arguments from and
215 to user space, passing them into subsystem specific callback functions
216 through normal kernel pointers.
217
218 This helps in various ways:
219
220 * Applications written for one driver are more likely to work for
221 another one in the same subsystem if there are no subtle differences
222 in the user space ABI.
223
224 * The complexity of user space access and data structure layout is done
225 in one place, reducing the potential for implementation bugs.
226
227 * It is more likely to be reviewed by experienced developers
228 that can spot problems in the interface when the ioctl is shared
229 between multiple drivers than when it is only used in a single driver.
230
231 Alternatives to ioctl
232 =====================
233
234 There are many cases in which ioctl is not the best solution for a
235 problem. Alternatives include:
236
237 * System calls are a better choice for a system-wide feature that
238 is not tied to a physical device or constrained by the file system
239 permissions of a character device node
240
241 * netlink is the preferred way of configuring any network related
242 objects through sockets.
243
244 * debugfs is used for ad-hoc interfaces for debugging functionality
245 that does not need to be exposed as a stable interface to applications.
246
247 * sysfs is a good way to expose the state of an in-kernel object
248 that is not tied to a file descriptor.
249
250 * configfs can be used for more complex configuration than sysfs
251
252 * A custom file system can provide extra flexibility with a simple
253 user interface but adds a lot of complexity to the implementation.
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