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2.. _addsyscalls:
3
David Drysdale4983953d2015-08-10 09:00:44 +01004Adding a New System Call
5========================
6
7This document describes what's involved in adding a new system call to the
8Linux kernel, over and above the normal submission advice in
Mauro Carvalho Chehab8c27ceff32016-10-18 10:12:27 -02009:ref:`Documentation/process/submitting-patches.rst <submittingpatches>`.
David Drysdale4983953d2015-08-10 09:00:44 +010010
11
12System Call Alternatives
13------------------------
14
15The first thing to consider when adding a new system call is whether one of
16the alternatives might be suitable instead. Although system calls are the
17most traditional and most obvious interaction points between userspace and the
18kernel, there are other possibilities -- choose what fits best for your
19interface.
20
21 - If the operations involved can be made to look like a filesystem-like
22 object, it may make more sense to create a new filesystem or device. This
23 also makes it easier to encapsulate the new functionality in a kernel module
24 rather than requiring it to be built into the main kernel.
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -030025
David Drysdale4983953d2015-08-10 09:00:44 +010026 - If the new functionality involves operations where the kernel notifies
27 userspace that something has happened, then returning a new file
28 descriptor for the relevant object allows userspace to use
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -030029 ``poll``/``select``/``epoll`` to receive that notification.
30 - However, operations that don't map to
31 :manpage:`read(2)`/:manpage:`write(2)`-like operations
32 have to be implemented as :manpage:`ioctl(2)` requests, which can lead
33 to a somewhat opaque API.
34
David Drysdale4983953d2015-08-10 09:00:44 +010035 - If you're just exposing runtime system information, a new node in sysfs
Mauro Carvalho Chehab0c1bc6b2020-04-14 18:48:37 +020036 (see ``Documentation/filesystems/sysfs.rst``) or the ``/proc`` filesystem may
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -030037 be more appropriate. However, access to these mechanisms requires that the
David Drysdale4983953d2015-08-10 09:00:44 +010038 relevant filesystem is mounted, which might not always be the case (e.g.
39 in a namespaced/sandboxed/chrooted environment). Avoid adding any API to
40 debugfs, as this is not considered a 'production' interface to userspace.
41 - If the operation is specific to a particular file or file descriptor, then
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -030042 an additional :manpage:`fcntl(2)` command option may be more appropriate. However,
43 :manpage:`fcntl(2)` is a multiplexing system call that hides a lot of complexity, so
David Drysdale4983953d2015-08-10 09:00:44 +010044 this option is best for when the new function is closely analogous to
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -030045 existing :manpage:`fcntl(2)` functionality, or the new functionality is very simple
David Drysdale4983953d2015-08-10 09:00:44 +010046 (for example, getting/setting a simple flag related to a file descriptor).
47 - If the operation is specific to a particular task or process, then an
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -030048 additional :manpage:`prctl(2)` command option may be more appropriate. As
49 with :manpage:`fcntl(2)`, this system call is a complicated multiplexor so
50 is best reserved for near-analogs of existing ``prctl()`` commands or
51 getting/setting a simple flag related to a process.
David Drysdale4983953d2015-08-10 09:00:44 +010052
53
54Designing the API: Planning for Extension
55-----------------------------------------
56
57A new system call forms part of the API of the kernel, and has to be supported
58indefinitely. As such, it's a very good idea to explicitly discuss the
59interface on the kernel mailing list, and it's important to plan for future
60extensions of the interface.
61
62(The syscall table is littered with historical examples where this wasn't done,
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -030063together with the corresponding follow-up system calls --
64``eventfd``/``eventfd2``, ``dup2``/``dup3``, ``inotify_init``/``inotify_init1``,
65``pipe``/``pipe2``, ``renameat``/``renameat2`` -- so
David Drysdale4983953d2015-08-10 09:00:44 +010066learn from the history of the kernel and plan for extensions from the start.)
67
68For simpler system calls that only take a couple of arguments, the preferred
69way to allow for future extensibility is to include a flags argument to the
70system call. To make sure that userspace programs can safely use flags
71between kernel versions, check whether the flags value holds any unknown
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -030072flags, and reject the system call (with ``EINVAL``) if it does::
David Drysdale4983953d2015-08-10 09:00:44 +010073
74 if (flags & ~(THING_FLAG1 | THING_FLAG2 | THING_FLAG3))
75 return -EINVAL;
76
77(If no flags values are used yet, check that the flags argument is zero.)
78
79For more sophisticated system calls that involve a larger number of arguments,
80it's preferred to encapsulate the majority of the arguments into a structure
81that is passed in by pointer. Such a structure can cope with future extension
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -030082by including a size argument in the structure::
David Drysdale4983953d2015-08-10 09:00:44 +010083
84 struct xyzzy_params {
85 u32 size; /* userspace sets p->size = sizeof(struct xyzzy_params) */
86 u32 param_1;
87 u64 param_2;
88 u64 param_3;
89 };
90
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -030091As long as any subsequently added field, say ``param_4``, is designed so that a
David Drysdale4983953d2015-08-10 09:00:44 +010092zero value gives the previous behaviour, then this allows both directions of
93version mismatch:
94
95 - To cope with a later userspace program calling an older kernel, the kernel
96 code should check that any memory beyond the size of the structure that it
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -030097 expects is zero (effectively checking that ``param_4 == 0``).
David Drysdale4983953d2015-08-10 09:00:44 +010098 - To cope with an older userspace program calling a newer kernel, the kernel
99 code can zero-extend a smaller instance of the structure (effectively
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300100 setting ``param_4 = 0``).
David Drysdale4983953d2015-08-10 09:00:44 +0100101
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300102See :manpage:`perf_event_open(2)` and the ``perf_copy_attr()`` function (in
103``kernel/events/core.c``) for an example of this approach.
David Drysdale4983953d2015-08-10 09:00:44 +0100104
105
106Designing the API: Other Considerations
107---------------------------------------
108
109If your new system call allows userspace to refer to a kernel object, it
110should use a file descriptor as the handle for that object -- don't invent a
111new type of userspace object handle when the kernel already has mechanisms and
112well-defined semantics for using file descriptors.
113
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300114If your new :manpage:`xyzzy(2)` system call does return a new file descriptor,
115then the flags argument should include a value that is equivalent to setting
116``O_CLOEXEC`` on the new FD. This makes it possible for userspace to close
117the timing window between ``xyzzy()`` and calling
118``fcntl(fd, F_SETFD, FD_CLOEXEC)``, where an unexpected ``fork()`` and
119``execve()`` in another thread could leak a descriptor to
David Drysdale4983953d2015-08-10 09:00:44 +0100120the exec'ed program. (However, resist the temptation to re-use the actual value
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300121of the ``O_CLOEXEC`` constant, as it is architecture-specific and is part of a
122numbering space of ``O_*`` flags that is fairly full.)
David Drysdale4983953d2015-08-10 09:00:44 +0100123
124If your system call returns a new file descriptor, you should also consider
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300125what it means to use the :manpage:`poll(2)` family of system calls on that file
David Drysdale4983953d2015-08-10 09:00:44 +0100126descriptor. Making a file descriptor ready for reading or writing is the
127normal way for the kernel to indicate to userspace that an event has
128occurred on the corresponding kernel object.
129
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300130If your new :manpage:`xyzzy(2)` system call involves a filename argument::
David Drysdale4983953d2015-08-10 09:00:44 +0100131
132 int sys_xyzzy(const char __user *path, ..., unsigned int flags);
133
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300134you should also consider whether an :manpage:`xyzzyat(2)` version is more appropriate::
David Drysdale4983953d2015-08-10 09:00:44 +0100135
136 int sys_xyzzyat(int dfd, const char __user *path, ..., unsigned int flags);
137
138This allows more flexibility for how userspace specifies the file in question;
139in particular it allows userspace to request the functionality for an
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300140already-opened file descriptor using the ``AT_EMPTY_PATH`` flag, effectively
141giving an :manpage:`fxyzzy(3)` operation for free::
David Drysdale4983953d2015-08-10 09:00:44 +0100142
143 - xyzzyat(AT_FDCWD, path, ..., 0) is equivalent to xyzzy(path,...)
144 - xyzzyat(fd, "", ..., AT_EMPTY_PATH) is equivalent to fxyzzy(fd, ...)
145
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300146(For more details on the rationale of the \*at() calls, see the
147:manpage:`openat(2)` man page; for an example of AT_EMPTY_PATH, see the
148:manpage:`fstatat(2)` man page.)
David Drysdale4983953d2015-08-10 09:00:44 +0100149
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300150If your new :manpage:`xyzzy(2)` system call involves a parameter describing an
151offset within a file, make its type ``loff_t`` so that 64-bit offsets can be
152supported even on 32-bit architectures.
David Drysdale4983953d2015-08-10 09:00:44 +0100153
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300154If your new :manpage:`xyzzy(2)` system call involves privileged functionality,
155it needs to be governed by the appropriate Linux capability bit (checked with
156a call to ``capable()``), as described in the :manpage:`capabilities(7)` man
157page. Choose an existing capability bit that governs related functionality,
158but try to avoid combining lots of only vaguely related functions together
159under the same bit, as this goes against capabilities' purpose of splitting
160the power of root. In particular, avoid adding new uses of the already
161overly-general ``CAP_SYS_ADMIN`` capability.
David Drysdale4983953d2015-08-10 09:00:44 +0100162
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300163If your new :manpage:`xyzzy(2)` system call manipulates a process other than
164the calling process, it should be restricted (using a call to
165``ptrace_may_access()``) so that only a calling process with the same
166permissions as the target process, or with the necessary capabilities, can
167manipulate the target process.
David Drysdale4983953d2015-08-10 09:00:44 +0100168
169Finally, be aware that some non-x86 architectures have an easier time if
170system call parameters that are explicitly 64-bit fall on odd-numbered
171arguments (i.e. parameter 1, 3, 5), to allow use of contiguous pairs of 32-bit
172registers. (This concern does not apply if the arguments are part of a
173structure that's passed in by pointer.)
174
175
176Proposing the API
177-----------------
178
179To make new system calls easy to review, it's best to divide up the patchset
180into separate chunks. These should include at least the following items as
181distinct commits (each of which is described further below):
182
183 - The core implementation of the system call, together with prototypes,
184 generic numbering, Kconfig changes and fallback stub implementation.
185 - Wiring up of the new system call for one particular architecture, usually
186 x86 (including all of x86_64, x86_32 and x32).
187 - A demonstration of the use of the new system call in userspace via a
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300188 selftest in ``tools/testing/selftests/``.
David Drysdale4983953d2015-08-10 09:00:44 +0100189 - A draft man-page for the new system call, either as plain text in the
190 cover letter, or as a patch to the (separate) man-pages repository.
191
192New system call proposals, like any change to the kernel's API, should always
193be cc'ed to linux-api@vger.kernel.org.
194
195
196Generic System Call Implementation
197----------------------------------
198
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300199The main entry point for your new :manpage:`xyzzy(2)` system call will be called
200``sys_xyzzy()``, but you add this entry point with the appropriate
201``SYSCALL_DEFINEn()`` macro rather than explicitly. The 'n' indicates the
202number of arguments to the system call, and the macro takes the system call name
David Drysdale4983953d2015-08-10 09:00:44 +0100203followed by the (type, name) pairs for the parameters as arguments. Using
204this macro allows metadata about the new system call to be made available for
205other tools.
206
207The new entry point also needs a corresponding function prototype, in
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300208``include/linux/syscalls.h``, marked as asmlinkage to match the way that system
209calls are invoked::
David Drysdale4983953d2015-08-10 09:00:44 +0100210
211 asmlinkage long sys_xyzzy(...);
212
213Some architectures (e.g. x86) have their own architecture-specific syscall
214tables, but several other architectures share a generic syscall table. Add your
215new system call to the generic list by adding an entry to the list in
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300216``include/uapi/asm-generic/unistd.h``::
David Drysdale4983953d2015-08-10 09:00:44 +0100217
218 #define __NR_xyzzy 292
219 __SYSCALL(__NR_xyzzy, sys_xyzzy)
220
221Also update the __NR_syscalls count to reflect the additional system call, and
222note that if multiple new system calls are added in the same merge window,
223your new syscall number may get adjusted to resolve conflicts.
224
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300225The file ``kernel/sys_ni.c`` provides a fallback stub implementation of each
226system call, returning ``-ENOSYS``. Add your new system call here too::
David Drysdale4983953d2015-08-10 09:00:44 +0100227
Dominik Brodowski67a7acd2018-03-04 19:06:35 +0100228 COND_SYSCALL(xyzzy);
David Drysdale4983953d2015-08-10 09:00:44 +0100229
230Your new kernel functionality, and the system call that controls it, should
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300231normally be optional, so add a ``CONFIG`` option (typically to
232``init/Kconfig``) for it. As usual for new ``CONFIG`` options:
David Drysdale4983953d2015-08-10 09:00:44 +0100233
234 - Include a description of the new functionality and system call controlled
235 by the option.
236 - Make the option depend on EXPERT if it should be hidden from normal users.
237 - Make any new source files implementing the function dependent on the CONFIG
Guillaume Dore418ca3d2018-10-18 17:47:50 +0200238 option in the Makefile (e.g. ``obj-$(CONFIG_XYZZY_SYSCALL) += xyzzy.o``).
David Drysdale4983953d2015-08-10 09:00:44 +0100239 - Double check that the kernel still builds with the new CONFIG option turned
240 off.
241
242To summarize, you need a commit that includes:
243
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300244 - ``CONFIG`` option for the new function, normally in ``init/Kconfig``
245 - ``SYSCALL_DEFINEn(xyzzy, ...)`` for the entry point
246 - corresponding prototype in ``include/linux/syscalls.h``
247 - generic table entry in ``include/uapi/asm-generic/unistd.h``
248 - fallback stub in ``kernel/sys_ni.c``
David Drysdale4983953d2015-08-10 09:00:44 +0100249
250
251x86 System Call Implementation
252------------------------------
253
254To wire up your new system call for x86 platforms, you need to update the
255master syscall tables. Assuming your new system call isn't special in some
256way (see below), this involves a "common" entry (for x86_64 and x32) in
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300257arch/x86/entry/syscalls/syscall_64.tbl::
David Drysdale4983953d2015-08-10 09:00:44 +0100258
259 333 common xyzzy sys_xyzzy
260
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300261and an "i386" entry in ``arch/x86/entry/syscalls/syscall_32.tbl``::
David Drysdale4983953d2015-08-10 09:00:44 +0100262
263 380 i386 xyzzy sys_xyzzy
264
265Again, these numbers are liable to be changed if there are conflicts in the
266relevant merge window.
267
268
269Compatibility System Calls (Generic)
270------------------------------------
271
272For most system calls the same 64-bit implementation can be invoked even when
273the userspace program is itself 32-bit; even if the system call's parameters
274include an explicit pointer, this is handled transparently.
275
276However, there are a couple of situations where a compatibility layer is
277needed to cope with size differences between 32-bit and 64-bit.
278
279The first is if the 64-bit kernel also supports 32-bit userspace programs, and
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300280so needs to parse areas of (``__user``) memory that could hold either 32-bit or
David Drysdale4983953d2015-08-10 09:00:44 +010028164-bit values. In particular, this is needed whenever a system call argument
282is:
283
284 - a pointer to a pointer
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300285 - a pointer to a struct containing a pointer (e.g. ``struct iovec __user *``)
286 - a pointer to a varying sized integral type (``time_t``, ``off_t``,
287 ``long``, ...)
David Drysdale4983953d2015-08-10 09:00:44 +0100288 - a pointer to a struct containing a varying sized integral type.
289
290The second situation that requires a compatibility layer is if one of the
291system call's arguments has a type that is explicitly 64-bit even on a 32-bit
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300292architecture, for example ``loff_t`` or ``__u64``. In this case, a value that
293arrives at a 64-bit kernel from a 32-bit application will be split into two
29432-bit values, which then need to be re-assembled in the compatibility layer.
David Drysdale4983953d2015-08-10 09:00:44 +0100295
296(Note that a system call argument that's a pointer to an explicit 64-bit type
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300297does **not** need a compatibility layer; for example, :manpage:`splice(2)`'s arguments of
298type ``loff_t __user *`` do not trigger the need for a ``compat_`` system call.)
David Drysdale4983953d2015-08-10 09:00:44 +0100299
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300300The compatibility version of the system call is called ``compat_sys_xyzzy()``,
301and is added with the ``COMPAT_SYSCALL_DEFINEn()`` macro, analogously to
David Drysdale4983953d2015-08-10 09:00:44 +0100302SYSCALL_DEFINEn. This version of the implementation runs as part of a 64-bit
303kernel, but expects to receive 32-bit parameter values and does whatever is
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300304needed to deal with them. (Typically, the ``compat_sys_`` version converts the
305values to 64-bit versions and either calls on to the ``sys_`` version, or both of
David Drysdale4983953d2015-08-10 09:00:44 +0100306them call a common inner implementation function.)
307
308The compat entry point also needs a corresponding function prototype, in
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300309``include/linux/compat.h``, marked as asmlinkage to match the way that system
310calls are invoked::
David Drysdale4983953d2015-08-10 09:00:44 +0100311
312 asmlinkage long compat_sys_xyzzy(...);
313
314If the system call involves a structure that is laid out differently on 32-bit
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300315and 64-bit systems, say ``struct xyzzy_args``, then the include/linux/compat.h
316header file should also include a compat version of the structure (``struct
317compat_xyzzy_args``) where each variable-size field has the appropriate
318``compat_`` type that corresponds to the type in ``struct xyzzy_args``. The
319``compat_sys_xyzzy()`` routine can then use this ``compat_`` structure to
320parse the arguments from a 32-bit invocation.
David Drysdale4983953d2015-08-10 09:00:44 +0100321
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300322For example, if there are fields::
David Drysdale4983953d2015-08-10 09:00:44 +0100323
324 struct xyzzy_args {
325 const char __user *ptr;
326 __kernel_long_t varying_val;
327 u64 fixed_val;
328 /* ... */
329 };
330
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300331in struct xyzzy_args, then struct compat_xyzzy_args would have::
David Drysdale4983953d2015-08-10 09:00:44 +0100332
333 struct compat_xyzzy_args {
334 compat_uptr_t ptr;
335 compat_long_t varying_val;
336 u64 fixed_val;
337 /* ... */
338 };
339
340The generic system call list also needs adjusting to allow for the compat
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300341version; the entry in ``include/uapi/asm-generic/unistd.h`` should use
342``__SC_COMP`` rather than ``__SYSCALL``::
David Drysdale4983953d2015-08-10 09:00:44 +0100343
344 #define __NR_xyzzy 292
345 __SC_COMP(__NR_xyzzy, sys_xyzzy, compat_sys_xyzzy)
346
347To summarize, you need:
348
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300349 - a ``COMPAT_SYSCALL_DEFINEn(xyzzy, ...)`` for the compat entry point
350 - corresponding prototype in ``include/linux/compat.h``
351 - (if needed) 32-bit mapping struct in ``include/linux/compat.h``
352 - instance of ``__SC_COMP`` not ``__SYSCALL`` in
353 ``include/uapi/asm-generic/unistd.h``
David Drysdale4983953d2015-08-10 09:00:44 +0100354
355
356Compatibility System Calls (x86)
357--------------------------------
358
359To wire up the x86 architecture of a system call with a compatibility version,
360the entries in the syscall tables need to be adjusted.
361
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300362First, the entry in ``arch/x86/entry/syscalls/syscall_32.tbl`` gets an extra
David Drysdale4983953d2015-08-10 09:00:44 +0100363column to indicate that a 32-bit userspace program running on a 64-bit kernel
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300364should hit the compat entry point::
David Drysdale4983953d2015-08-10 09:00:44 +0100365
Dominik Brodowski5ac9efa2018-04-09 12:51:43 +0200366 380 i386 xyzzy sys_xyzzy __ia32_compat_sys_xyzzy
David Drysdale4983953d2015-08-10 09:00:44 +0100367
368Second, you need to figure out what should happen for the x32 ABI version of
369the new system call. There's a choice here: the layout of the arguments
370should either match the 64-bit version or the 32-bit version.
371
372If there's a pointer-to-a-pointer involved, the decision is easy: x32 is
373ILP32, so the layout should match the 32-bit version, and the entry in
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300374``arch/x86/entry/syscalls/syscall_64.tbl`` is split so that x32 programs hit
375the compatibility wrapper::
David Drysdale4983953d2015-08-10 09:00:44 +0100376
377 333 64 xyzzy sys_xyzzy
378 ...
Dominik Brodowski5ac9efa2018-04-09 12:51:43 +0200379 555 x32 xyzzy __x32_compat_sys_xyzzy
David Drysdale4983953d2015-08-10 09:00:44 +0100380
381If no pointers are involved, then it is preferable to re-use the 64-bit system
382call for the x32 ABI (and consequently the entry in
383arch/x86/entry/syscalls/syscall_64.tbl is unchanged).
384
385In either case, you should check that the types involved in your argument
386layout do indeed map exactly from x32 (-mx32) to either the 32-bit (-m32) or
38764-bit (-m64) equivalents.
388
389
390System Calls Returning Elsewhere
391--------------------------------
392
393For most system calls, once the system call is complete the user program
394continues exactly where it left off -- at the next instruction, with the
395stack the same and most of the registers the same as before the system call,
396and with the same virtual memory space.
397
398However, a few system calls do things differently. They might return to a
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300399different location (``rt_sigreturn``) or change the memory space
400(``fork``/``vfork``/``clone``) or even architecture (``execve``/``execveat``)
401of the program.
David Drysdale4983953d2015-08-10 09:00:44 +0100402
403To allow for this, the kernel implementation of the system call may need to
404save and restore additional registers to the kernel stack, allowing complete
405control of where and how execution continues after the system call.
406
407This is arch-specific, but typically involves defining assembly entry points
408that save/restore additional registers and invoke the real system call entry
409point.
410
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300411For x86_64, this is implemented as a ``stub_xyzzy`` entry point in
412``arch/x86/entry/entry_64.S``, and the entry in the syscall table
413(``arch/x86/entry/syscalls/syscall_64.tbl``) is adjusted to match::
David Drysdale4983953d2015-08-10 09:00:44 +0100414
415 333 common xyzzy stub_xyzzy
416
417The equivalent for 32-bit programs running on a 64-bit kernel is normally
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300418called ``stub32_xyzzy`` and implemented in ``arch/x86/entry/entry_64_compat.S``,
David Drysdale4983953d2015-08-10 09:00:44 +0100419with the corresponding syscall table adjustment in
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300420``arch/x86/entry/syscalls/syscall_32.tbl``::
David Drysdale4983953d2015-08-10 09:00:44 +0100421
422 380 i386 xyzzy sys_xyzzy stub32_xyzzy
423
424If the system call needs a compatibility layer (as in the previous section)
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300425then the ``stub32_`` version needs to call on to the ``compat_sys_`` version
426of the system call rather than the native 64-bit version. Also, if the x32 ABI
David Drysdale4983953d2015-08-10 09:00:44 +0100427implementation is not common with the x86_64 version, then its syscall
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300428table will also need to invoke a stub that calls on to the ``compat_sys_``
David Drysdale4983953d2015-08-10 09:00:44 +0100429version.
430
431For completeness, it's also nice to set up a mapping so that user-mode Linux
432still works -- its syscall table will reference stub_xyzzy, but the UML build
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300433doesn't include ``arch/x86/entry/entry_64.S`` implementation (because UML
David Drysdale4983953d2015-08-10 09:00:44 +0100434simulates registers etc). Fixing this is as simple as adding a #define to
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300435``arch/x86/um/sys_call_table_64.c``::
David Drysdale4983953d2015-08-10 09:00:44 +0100436
437 #define stub_xyzzy sys_xyzzy
438
439
440Other Details
441-------------
442
443Most of the kernel treats system calls in a generic way, but there is the
444occasional exception that may need updating for your particular system call.
445
446The audit subsystem is one such special case; it includes (arch-specific)
447functions that classify some special types of system call -- specifically
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300448file open (``open``/``openat``), program execution (``execve``/``exeveat``) or
449socket multiplexor (``socketcall``) operations. If your new system call is
450analogous to one of these, then the audit system should be updated.
David Drysdale4983953d2015-08-10 09:00:44 +0100451
452More generally, if there is an existing system call that is analogous to your
453new system call, it's worth doing a kernel-wide grep for the existing system
454call to check there are no other special cases.
455
456
457Testing
458-------
459
460A new system call should obviously be tested; it is also useful to provide
461reviewers with a demonstration of how user space programs will use the system
462call. A good way to combine these aims is to include a simple self-test
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300463program in a new directory under ``tools/testing/selftests/``.
David Drysdale4983953d2015-08-10 09:00:44 +0100464
465For a new system call, there will obviously be no libc wrapper function and so
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300466the test will need to invoke it using ``syscall()``; also, if the system call
David Drysdale4983953d2015-08-10 09:00:44 +0100467involves a new userspace-visible structure, the corresponding header will need
468to be installed to compile the test.
469
470Make sure the selftest runs successfully on all supported architectures. For
471example, check that it works when compiled as an x86_64 (-m64), x86_32 (-m32)
472and x32 (-mx32) ABI program.
473
474For more extensive and thorough testing of new functionality, you should also
475consider adding tests to the Linux Test Project, or to the xfstests project
476for filesystem-related changes.
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300477
David Drysdale4983953d2015-08-10 09:00:44 +0100478 - https://linux-test-project.github.io/
479 - git://git.kernel.org/pub/scm/fs/xfs/xfstests-dev.git
480
481
482Man Page
483--------
484
485All new system calls should come with a complete man page, ideally using groff
486markup, but plain text will do. If groff is used, it's helpful to include a
487pre-rendered ASCII version of the man page in the cover email for the
488patchset, for the convenience of reviewers.
489
490The man page should be cc'ed to linux-man@vger.kernel.org
491For more details, see https://www.kernel.org/doc/man-pages/patches.html
492
Dominik Brodowski819671ff2018-03-11 11:34:25 +0100493
494Do not call System Calls in the Kernel
495--------------------------------------
496
497System calls are, as stated above, interaction points between userspace and
498the kernel. Therefore, system call functions such as ``sys_xyzzy()`` or
499``compat_sys_xyzzy()`` should only be called from userspace via the syscall
500table, but not from elsewhere in the kernel. If the syscall functionality is
501useful to be used within the kernel, needs to be shared between an old and a
502new syscall, or needs to be shared between a syscall and its compatibility
503variant, it should be implemented by means of a "helper" function (such as
André Almeidadd58e642021-01-29 22:45:46 -0300504``ksys_xyzzy()``). This kernel function may then be called within the
Dominik Brodowski819671ff2018-03-11 11:34:25 +0100505syscall stub (``sys_xyzzy()``), the compatibility syscall stub
506(``compat_sys_xyzzy()``), and/or other kernel code.
507
508At least on 64-bit x86, it will be a hard requirement from v4.17 onwards to not
509call system call functions in the kernel. It uses a different calling
510convention for system calls where ``struct pt_regs`` is decoded on-the-fly in a
511syscall wrapper which then hands processing over to the actual syscall function.
512This means that only those parameters which are actually needed for a specific
513syscall are passed on during syscall entry, instead of filling in six CPU
514registers with random user space content all the time (which may cause serious
515trouble down the call chain).
516
517Moreover, rules on how data may be accessed may differ between kernel data and
518user data. This is another reason why calling ``sys_xyzzy()`` is generally a
519bad idea.
520
521Exceptions to this rule are only allowed in architecture-specific overrides,
522architecture-specific compatibility wrappers, or other code in arch/.
523
524
David Drysdale4983953d2015-08-10 09:00:44 +0100525References and Sources
526----------------------
527
528 - LWN article from Michael Kerrisk on use of flags argument in system calls:
529 https://lwn.net/Articles/585415/
530 - LWN article from Michael Kerrisk on how to handle unknown flags in a system
531 call: https://lwn.net/Articles/588444/
532 - LWN article from Jake Edge describing constraints on 64-bit system call
533 arguments: https://lwn.net/Articles/311630/
534 - Pair of LWN articles from David Drysdale that describe the system call
535 implementation paths in detail for v3.14:
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300536
David Drysdale4983953d2015-08-10 09:00:44 +0100537 - https://lwn.net/Articles/604287/
538 - https://lwn.net/Articles/604515/
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300539
David Drysdale4983953d2015-08-10 09:00:44 +0100540 - Architecture-specific requirements for system calls are discussed in the
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300541 :manpage:`syscall(2)` man-page:
David Drysdale4983953d2015-08-10 09:00:44 +0100542 http://man7.org/linux/man-pages/man2/syscall.2.html#NOTES
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300543 - Collated emails from Linus Torvalds discussing the problems with ``ioctl()``:
Alexander A. Klimov93431e02020-05-26 08:05:44 +0200544 https://yarchive.net/comp/linux/ioctl.html
David Drysdale4983953d2015-08-10 09:00:44 +0100545 - "How to not invent kernel interfaces", Arnd Bergmann,
Alexander A. Klimov93431e02020-05-26 08:05:44 +0200546 https://www.ukuug.org/events/linux2007/2007/papers/Bergmann.pdf
David Drysdale4983953d2015-08-10 09:00:44 +0100547 - LWN article from Michael Kerrisk on avoiding new uses of CAP_SYS_ADMIN:
548 https://lwn.net/Articles/486306/
549 - Recommendation from Andrew Morton that all related information for a new
550 system call should come in the same email thread:
Joe Perches05a5f512021-01-10 12:41:44 -0800551 https://lore.kernel.org/r/20140724144747.3041b208832bbdf9fbce5d96@linux-foundation.org
David Drysdale4983953d2015-08-10 09:00:44 +0100552 - Recommendation from Michael Kerrisk that a new system call should come with
Joe Perches05a5f512021-01-10 12:41:44 -0800553 a man page: https://lore.kernel.org/r/CAKgNAkgMA39AfoSoA5Pe1r9N+ZzfYQNvNPvcRN7tOvRb8+v06Q@mail.gmail.com
David Drysdale4983953d2015-08-10 09:00:44 +0100554 - Suggestion from Thomas Gleixner that x86 wire-up should be in a separate
Joe Perches05a5f512021-01-10 12:41:44 -0800555 commit: https://lore.kernel.org/r/alpine.DEB.2.11.1411191249560.3909@nanos
David Drysdale4983953d2015-08-10 09:00:44 +0100556 - Suggestion from Greg Kroah-Hartman that it's good for new system calls to
Joe Perches05a5f512021-01-10 12:41:44 -0800557 come with a man-page & selftest: https://lore.kernel.org/r/20140320025530.GA25469@kroah.com
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300558 - Discussion from Michael Kerrisk of new system call vs. :manpage:`prctl(2)` extension:
Joe Perches05a5f512021-01-10 12:41:44 -0800559 https://lore.kernel.org/r/CAHO5Pa3F2MjfTtfNxa8LbnkeeU8=YJ+9tDqxZpw7Gz59E-4AUg@mail.gmail.com
David Drysdale4983953d2015-08-10 09:00:44 +0100560 - Suggestion from Ingo Molnar that system calls that involve multiple
561 arguments should encapsulate those arguments in a struct, which includes a
Joe Perches05a5f512021-01-10 12:41:44 -0800562 size field for future extensibility: https://lore.kernel.org/r/20150730083831.GA22182@gmail.com
David Drysdale4983953d2015-08-10 09:00:44 +0100563 - Numbering oddities arising from (re-)use of O_* numbering space flags:
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300564
David Drysdale4983953d2015-08-10 09:00:44 +0100565 - commit 75069f2b5bfb ("vfs: renumber FMODE_NONOTIFY and add to uniqueness
566 check")
567 - commit 12ed2e36c98a ("fanotify: FMODE_NONOTIFY and __O_SYNC in sparc
568 conflict")
569 - commit bb458c644a59 ("Safer ABI for O_TMPFILE")
Mauro Carvalho Chehab12983bc2016-09-21 13:14:35 -0300570
David Drysdale4983953d2015-08-10 09:00:44 +0100571 - Discussion from Matthew Wilcox about restrictions on 64-bit arguments:
Joe Perches05a5f512021-01-10 12:41:44 -0800572 https://lore.kernel.org/r/20081212152929.GM26095@parisc-linux.org
David Drysdale4983953d2015-08-10 09:00:44 +0100573 - Recommendation from Greg Kroah-Hartman that unknown flags should be
Joe Perches05a5f512021-01-10 12:41:44 -0800574 policed: https://lore.kernel.org/r/20140717193330.GB4703@kroah.com
David Drysdale4983953d2015-08-10 09:00:44 +0100575 - Recommendation from Linus Torvalds that x32 system calls should prefer
576 compatibility with 64-bit versions rather than 32-bit versions:
Joe Perches05a5f512021-01-10 12:41:44 -0800577 https://lore.kernel.org/r/CA+55aFxfmwfB7jbbrXxa=K7VBYPfAvmu3XOkGrLbB1UFjX1+Ew@mail.gmail.com