1
2 .. _addsyscalls:
3
4 Adding a New System Call
5 ========================
6
7 This document describes what's involved in adding a new system call to the
8 Linux kernel, over and above the normal submission advice in
9 :ref:`Documentation/process/submitting-patches.rst <submittingpatches>`.
10
11
12 System Call Alternatives
13 ------------------------
14
15 The first thing to consider when adding a new system call is whether one of
16 the alternatives might be suitable instead. Although system calls are the
17 most traditional and most obvious interaction points between userspace and the
18 kernel, there are other possibilities -- choose what fits best for your
19 interface.
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.
25
26 - 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
29 ``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
35 - If you're just exposing runtime system information, a new node in sysfs
36 (see ``Documentation/filesystems/sysfs.rst``) or the ``/proc`` filesystem may
37 be more appropriate. However, access to these mechanisms requires that the
38 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
42 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
44 this option is best for when the new function is closely analogous to
45 existing :manpage:`fcntl(2)` functionality, or the new functionality is very simple
46 (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
48 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.
52
53
54 Designing the API: Planning for Extension
55 -----------------------------------------
56
57 A new system call forms part of the API of the kernel, and has to be supported
58 indefinitely. As such, it's a very good idea to explicitly discuss the
59 interface on the kernel mailing list, and it's important to plan for future
60 extensions of the interface.
61
62 (The syscall table is littered with historical examples where this wasn't done,
63 together with the corresponding follow-up system calls --
64 ``eventfd``/``eventfd2``, ``dup2``/``dup3``, ``inotify_init``/``inotify_init1``,
65 ``pipe``/``pipe2``, ``renameat``/``renameat2`` -- so
66 learn from the history of the kernel and plan for extensions from the start.)
67
68 For simpler system calls that only take a couple of arguments, the preferred
69 way to allow for future extensibility is to include a flags argument to the
70 system call. To make sure that userspace programs can safely use flags
71 between kernel versions, check whether the flags value holds any unknown
72 flags, and reject the system call (with ``EINVAL``) if it does::
73
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
79 For more sophisticated system calls that involve a larger number of arguments,
80 it's preferred to encapsulate the majority of the arguments into a structure
81 that is passed in by pointer. Such a structure can cope with future extension
82 by including a size argument in the structure::
83
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
91 As long as any subsequently added field, say ``param_4``, is designed so that a
92 zero value gives the previous behaviour, then this allows both directions of
93 version 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
97 expects is zero (effectively checking that ``param_4 == 0``).
98 - 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
100 setting ``param_4 = 0``).
101
102 See :manpage:`perf_event_open(2)` and the ``perf_copy_attr()`` function (in
103 ``kernel/events/core.c``) for an example of this approach.
104
105
106 Designing the API: Other Considerations
107 ---------------------------------------
108
109 If your new system call allows userspace to refer to a kernel object, it
110 should use a file descriptor as the handle for that object -- don't invent a
111 new type of userspace object handle when the kernel already has mechanisms and
112 well-defined semantics for using file descriptors.
113
114 If your new :manpage:`xyzzy(2)` system call does return a new file descriptor,
115 then 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
117 the 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
120 the exec'ed program. (However, resist the temptation to re-use the actual value
121 of the ``O_CLOEXEC`` constant, as it is architecture-specific and is part of a
122 numbering space of ``O_*`` flags that is fairly full.)
123
124 If your system call returns a new file descriptor, you should also consider
125 what it means to use the :manpage:`poll(2)` family of system calls on that file
126 descriptor. Making a file descriptor ready for reading or writing is the
127 normal way for the kernel to indicate to userspace that an event has
128 occurred on the corresponding kernel object.
129
130 If your new :manpage:`xyzzy(2)` system call involves a filename argument::
131
132 int sys_xyzzy(const char __user *path, ..., unsigned int flags);
133
134 you should also consider whether an :manpage:`xyzzyat(2)` version is more appropriate::
135
136 int sys_xyzzyat(int dfd, const char __user *path, ..., unsigned int flags);
137
138 This allows more flexibility for how userspace specifies the file in question;
139 in particular it allows userspace to request the functionality for an
140 already-opened file descriptor using the ``AT_EMPTY_PATH`` flag, effectively
141 giving an :manpage:`fxyzzy(3)` operation for free::
142
143 - xyzzyat(AT_FDCWD, path, ..., 0) is equivalent to xyzzy(path,...)
144 - xyzzyat(fd, "", ..., AT_EMPTY_PATH) is equivalent to fxyzzy(fd, ...)
145
146 (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.)
149
150 If your new :manpage:`xyzzy(2)` system call involves a parameter describing an
151 offset within a file, make its type ``loff_t`` so that 64-bit offsets can be
152 supported even on 32-bit architectures.
153
154 If your new :manpage:`xyzzy(2)` system call involves privileged functionality,
155 it needs to be governed by the appropriate Linux capability bit (checked with
156 a call to ``capable()``), as described in the :manpage:`capabilities(7)` man
157 page. Choose an existing capability bit that governs related functionality,
158 but try to avoid combining lots of only vaguely related functions together
159 under the same bit, as this goes against capabilities' purpose of splitting
160 the power of root. In particular, avoid adding new uses of the already
161 overly-general ``CAP_SYS_ADMIN`` capability.
162
163 If your new :manpage:`xyzzy(2)` system call manipulates a process other than
164 the calling process, it should be restricted (using a call to
165 ``ptrace_may_access()``) so that only a calling process with the same
166 permissions as the target process, or with the necessary capabilities, can
167 manipulate the target process.
168
169 Finally, be aware that some non-x86 architectures have an easier time if
170 system call parameters that are explicitly 64-bit fall on odd-numbered
171 arguments (i.e. parameter 1, 3, 5), to allow use of contiguous pairs of 32-bit
172 registers. (This concern does not apply if the arguments are part of a
173 structure that's passed in by pointer.)
174
175
176 Proposing the API
177 -----------------
178
179 To make new system calls easy to review, it's best to divide up the patchset
180 into separate chunks. These should include at least the following items as
181 distinct 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
188 selftest in ``tools/testing/selftests/``.
189 - 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
192 New system call proposals, like any change to the kernel's API, should always
193 be cc'ed to linux-api@vger.kernel.org.
194
195
196 Generic System Call Implementation
197 ----------------------------------
198
199 The 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
202 number of arguments to the system call, and the macro takes the system call name
203 followed by the (type, name) pairs for the parameters as arguments. Using
204 this macro allows metadata about the new system call to be made available for
205 other tools.
206
207 The new entry point also needs a corresponding function prototype, in
208 ``include/linux/syscalls.h``, marked as asmlinkage to match the way that system
209 calls are invoked::
210
211 asmlinkage long sys_xyzzy(...);
212
213 Some architectures (e.g. x86) have their own architecture-specific syscall
214 tables, but several other architectures share a generic syscall table. Add your
215 new system call to the generic list by adding an entry to the list in
216 ``include/uapi/asm-generic/unistd.h``::
217
218 #define __NR_xyzzy 292
219 __SYSCALL(__NR_xyzzy, sys_xyzzy)
220
221 Also update the __NR_syscalls count to reflect the additional system call, and
222 note that if multiple new system calls are added in the same merge window,
223 your new syscall number may get adjusted to resolve conflicts.
224
225 The file ``kernel/sys_ni.c`` provides a fallback stub implementation of each
226 system call, returning ``-ENOSYS``. Add your new system call here too::
227
228 COND_SYSCALL(xyzzy);
229
230 Your new kernel functionality, and the system call that controls it, should
231 normally be optional, so add a ``CONFIG`` option (typically to
232 ``init/Kconfig``) for it. As usual for new ``CONFIG`` options:
233
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
238 option in the Makefile (e.g. ``obj-$(CONFIG_XYZZY_SYSCALL) += xyzzy.o``).
239 - Double check that the kernel still builds with the new CONFIG option turned
240 off.
241
242 To summarize, you need a commit that includes:
243
244 - ``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``
249
250
251 x86 System Call Implementation
252 ------------------------------
253
254 To wire up your new system call for x86 platforms, you need to update the
255 master syscall tables. Assuming your new system call isn't special in some
256 way (see below), this involves a "common" entry (for x86_64 and x32) in
257 arch/x86/entry/syscalls/syscall_64.tbl::
258
259 333 common xyzzy sys_xyzzy
260
261 and an "i386" entry in ``arch/x86/entry/syscalls/syscall_32.tbl``::
262
263 380 i386 xyzzy sys_xyzzy
264
265 Again, these numbers are liable to be changed if there are conflicts in the
266 relevant merge window.
267
268
269 Compatibility System Calls (Generic)
270 ------------------------------------
271
272 For most system calls the same 64-bit implementation can be invoked even when
273 the userspace program is itself 32-bit; even if the system call's parameters
274 include an explicit pointer, this is handled transparently.
275
276 However, there are a couple of situations where a compatibility layer is
277 needed to cope with size differences between 32-bit and 64-bit.
278
279 The first is if the 64-bit kernel also supports 32-bit userspace programs, and
280 so needs to parse areas of (``__user``) memory that could hold either 32-bit or
281 64-bit values. In particular, this is needed whenever a system call argument
282 is:
283
284 - a pointer to a pointer
285 - 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``, ...)
288 - a pointer to a struct containing a varying sized integral type.
289
290 The second situation that requires a compatibility layer is if one of the
291 system call's arguments has a type that is explicitly 64-bit even on a 32-bit
292 architecture, for example ``loff_t`` or ``__u64``. In this case, a value that
293 arrives at a 64-bit kernel from a 32-bit application will be split into two
294 32-bit values, which then need to be re-assembled in the compatibility layer.
295
296 (Note that a system call argument that's a pointer to an explicit 64-bit type
297 does **not** need a compatibility layer; for example, :manpage:`splice(2)`'s arguments of
298 type ``loff_t __user *`` do not trigger the need for a ``compat_`` system call.)
299
300 The compatibility version of the system call is called ``compat_sys_xyzzy()``,
301 and is added with the ``COMPAT_SYSCALL_DEFINEn()`` macro, analogously to
302 SYSCALL_DEFINEn. This version of the implementation runs as part of a 64-bit
303 kernel, but expects to receive 32-bit parameter values and does whatever is
304 needed to deal with them. (Typically, the ``compat_sys_`` version converts the
305 values to 64-bit versions and either calls on to the ``sys_`` version, or both of
306 them call a common inner implementation function.)
307
308 The compat entry point also needs a corresponding function prototype, in
309 ``include/linux/compat.h``, marked as asmlinkage to match the way that system
310 calls are invoked::
311
312 asmlinkage long compat_sys_xyzzy(...);
313
314 If the system call involves a structure that is laid out differently on 32-bit
315 and 64-bit systems, say ``struct xyzzy_args``, then the include/linux/compat.h
316 header file should also include a compat version of the structure (``struct
317 compat_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
320 parse the arguments from a 32-bit invocation.
321
322 For example, if there are fields::
323
324 struct xyzzy_args {
325 const char __user *ptr;
326 __kernel_long_t varying_val;
327 u64 fixed_val;
328 /* ... */
329 };
330
331 in struct xyzzy_args, then struct compat_xyzzy_args would have::
332
333 struct compat_xyzzy_args {
334 compat_uptr_t ptr;
335 compat_long_t varying_val;
336 u64 fixed_val;
337 /* ... */
338 };
339
340 The generic system call list also needs adjusting to allow for the compat
341 version; the entry in ``include/uapi/asm-generic/unistd.h`` should use
342 ``__SC_COMP`` rather than ``__SYSCALL``::
343
344 #define __NR_xyzzy 292
345 __SC_COMP(__NR_xyzzy, sys_xyzzy, compat_sys_xyzzy)
346
347 To summarize, you need:
348
349 - 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``
354
355
356 Compatibility System Calls (x86)
357 --------------------------------
358
359 To wire up the x86 architecture of a system call with a compatibility version,
360 the entries in the syscall tables need to be adjusted.
361
362 First, the entry in ``arch/x86/entry/syscalls/syscall_32.tbl`` gets an extra
363 column to indicate that a 32-bit userspace program running on a 64-bit kernel
364 should hit the compat entry point::
365
366 380 i386 xyzzy sys_xyzzy __ia32_compat_sys_xyzzy
367
368 Second, you need to figure out what should happen for the x32 ABI version of
369 the new system call. There's a choice here: the layout of the arguments
370 should either match the 64-bit version or the 32-bit version.
371
372 If there's a pointer-to-a-pointer involved, the decision is easy: x32 is
373 ILP32, so the layout should match the 32-bit version, and the entry in
374 ``arch/x86/entry/syscalls/syscall_64.tbl`` is split so that x32 programs hit
375 the compatibility wrapper::
376
377 333 64 xyzzy sys_xyzzy
378 ...
379 555 x32 xyzzy __x32_compat_sys_xyzzy
380
381 If no pointers are involved, then it is preferable to re-use the 64-bit system
382 call for the x32 ABI (and consequently the entry in
383 arch/x86/entry/syscalls/syscall_64.tbl is unchanged).
384
385 In either case, you should check that the types involved in your argument
386 layout do indeed map exactly from x32 (-mx32) to either the 32-bit (-m32) or
387 64-bit (-m64) equivalents.
388
389
390 System Calls Returning Elsewhere
391 --------------------------------
392
393 For most system calls, once the system call is complete the user program
394 continues exactly where it left off -- at the next instruction, with the
395 stack the same and most of the registers the same as before the system call,
396 and with the same virtual memory space.
397
398 However, a few system calls do things differently. They might return to a
399 different location (``rt_sigreturn``) or change the memory space
400 (``fork``/``vfork``/``clone``) or even architecture (``execve``/``execveat``)
401 of the program.
402
403 To allow for this, the kernel implementation of the system call may need to
404 save and restore additional registers to the kernel stack, allowing complete
405 control of where and how execution continues after the system call.
406
407 This is arch-specific, but typically involves defining assembly entry points
408 that save/restore additional registers and invoke the real system call entry
409 point.
410
411 For 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::
414
415 333 common xyzzy stub_xyzzy
416
417 The equivalent for 32-bit programs running on a 64-bit kernel is normally
418 called ``stub32_xyzzy`` and implemented in ``arch/x86/entry/entry_64_compat.S``,
419 with the corresponding syscall table adjustment in
420 ``arch/x86/entry/syscalls/syscall_32.tbl``::
421
422 380 i386 xyzzy sys_xyzzy stub32_xyzzy
423
424 If the system call needs a compatibility layer (as in the previous section)
425 then the ``stub32_`` version needs to call on to the ``compat_sys_`` version
426 of the system call rather than the native 64-bit version. Also, if the x32 ABI
427 implementation is not common with the x86_64 version, then its syscall
428 table will also need to invoke a stub that calls on to the ``compat_sys_``
429 version.
430
431 For completeness, it's also nice to set up a mapping so that user-mode Linux
432 still works -- its syscall table will reference stub_xyzzy, but the UML build
433 doesn't include ``arch/x86/entry/entry_64.S`` implementation (because UML
434 simulates registers etc). Fixing this is as simple as adding a #define to
435 ``arch/x86/um/sys_call_table_64.c``::
436
437 #define stub_xyzzy sys_xyzzy
438
439
440 Other Details
441 -------------
442
443 Most of the kernel treats system calls in a generic way, but there is the
444 occasional exception that may need updating for your particular system call.
445
446 The audit subsystem is one such special case; it includes (arch-specific)
447 functions that classify some special types of system call -- specifically
448 file open (``open``/``openat``), program execution (``execve``/``exeveat``) or
449 socket multiplexor (``socketcall``) operations. If your new system call is
450 analogous to one of these, then the audit system should be updated.
451
452 More generally, if there is an existing system call that is analogous to your
453 new system call, it's worth doing a kernel-wide grep for the existing system
454 call to check there are no other special cases.
455
456
457 Testing
458 -------
459
460 A new system call should obviously be tested; it is also useful to provide
461 reviewers with a demonstration of how user space programs will use the system
462 call. A good way to combine these aims is to include a simple self-test
463 program in a new directory under ``tools/testing/selftests/``.
464
465 For a new system call, there will obviously be no libc wrapper function and so
466 the test will need to invoke it using ``syscall()``; also, if the system call
467 involves a new userspace-visible structure, the corresponding header will need
468 to be installed to compile the test.
469
470 Make sure the selftest runs successfully on all supported architectures. For
471 example, check that it works when compiled as an x86_64 (-m64), x86_32 (-m32)
472 and x32 (-mx32) ABI program.
473
474 For more extensive and thorough testing of new functionality, you should also
475 consider adding tests to the Linux Test Project, or to the xfstests project
476 for filesystem-related changes.
477
478 - https://linux-test-project.github.io/
479 - git://git.kernel.org/pub/scm/fs/xfs/xfstests-dev.git
480
481
482 Man Page
483 --------
484
485 All new system calls should come with a complete man page, ideally using groff
486 markup, but plain text will do. If groff is used, it's helpful to include a
487 pre-rendered ASCII version of the man page in the cover email for the
488 patchset, for the convenience of reviewers.
489
490 The man page should be cc'ed to linux-man@vger.kernel.org
491 For more details, see https://www.kernel.org/doc/man-pages/patches.html
492
493
494 Do not call System Calls in the Kernel
495 --------------------------------------
496
497 System calls are, as stated above, interaction points between userspace and
498 the kernel. Therefore, system call functions such as ``sys_xyzzy()`` or
499 ``compat_sys_xyzzy()`` should only be called from userspace via the syscall
500 table, but not from elsewhere in the kernel. If the syscall functionality is
501 useful to be used within the kernel, needs to be shared between an old and a
502 new syscall, or needs to be shared between a syscall and its compatibility
503 variant, it should be implemented by means of a "helper" function (such as
504 ``ksys_xyzzy()``). This kernel function may then be called within the
505 syscall stub (``sys_xyzzy()``), the compatibility syscall stub
506 (``compat_sys_xyzzy()``), and/or other kernel code.
507
508 At least on 64-bit x86, it will be a hard requirement from v4.17 onwards to not
509 call system call functions in the kernel. It uses a different calling
510 convention for system calls where ``struct pt_regs`` is decoded on-the-fly in a
511 syscall wrapper which then hands processing over to the actual syscall function.
512 This means that only those parameters which are actually needed for a specific
513 syscall are passed on during syscall entry, instead of filling in six CPU
514 registers with random user space content all the time (which may cause serious
515 trouble down the call chain).
516
517 Moreover, rules on how data may be accessed may differ between kernel data and
518 user data. This is another reason why calling ``sys_xyzzy()`` is generally a
519 bad idea.
520
521 Exceptions to this rule are only allowed in architecture-specific overrides,
522 architecture-specific compatibility wrappers, or other code in arch/.
523
524
525 References 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:
536
537 - https://lwn.net/Articles/604287/
538 - https://lwn.net/Articles/604515/
539
540 - Architecture-specific requirements for system calls are discussed in the
541 :manpage:`syscall(2)` man-page:
542 http://man7.org/linux/man-pages/man2/syscall.2.html#NOTES
543 - Collated emails from Linus Torvalds discussing the problems with ``ioctl()``:
544 https://yarchive.net/comp/linux/ioctl.html
545 - "How to not invent kernel interfaces", Arnd Bergmann,
546 https://www.ukuug.org/events/linux2007/2007/papers/Bergmann.pdf
547 - 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:
551 https://lore.kernel.org/r/20140724144747.3041b208832bbdf9fbce5d96@linux-foundation.org
552 - Recommendation from Michael Kerrisk that a new system call should come with
553 a man page: https://lore.kernel.org/r/CAKgNAkgMA39AfoSoA5Pe1r9N+ZzfYQNvNPvcRN7tOvRb8+v06Q@mail.gmail.com
554 - Suggestion from Thomas Gleixner that x86 wire-up should be in a separate
555 commit: https://lore.kernel.org/r/alpine.DEB.2.11.1411191249560.3909@nanos
556 - Suggestion from Greg Kroah-Hartman that it's good for new system calls to
557 come with a man-page & selftest: https://lore.kernel.org/r/20140320025530.GA25469@kroah.com
558 - Discussion from Michael Kerrisk of new system call vs. :manpage:`prctl(2)` extension:
559 https://lore.kernel.org/r/CAHO5Pa3F2MjfTtfNxa8LbnkeeU8=YJ+9tDqxZpw7Gz59E-4AUg@mail.gmail.com
560 - Suggestion from Ingo Molnar that system calls that involve multiple
561 arguments should encapsulate those arguments in a struct, which includes a
562 size field for future extensibility: https://lore.kernel.org/r/20150730083831.GA22182@gmail.com
563 - Numbering oddities arising from (re-)use of O_* numbering space flags:
564
565 - 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")
570
571 - Discussion from Matthew Wilcox about restrictions on 64-bit arguments:
572 https://lore.kernel.org/r/20081212152929.GM26095@parisc-linux.org
573 - Recommendation from Greg Kroah-Hartman that unknown flags should be
574 policed: https://lore.kernel.org/r/20140717193330.GB4703@kroah.com
575 - Recommendation from Linus Torvalds that x32 system calls should prefer
576 compatibility with 64-bit versions rather than 32-bit versions:
577 https://lore.kernel.org/r/CA+55aFxfmwfB7jbbrXxa=K7VBYPfAvmu3XOkGrLbB1UFjX1+Ew@mail.gmail.com
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.