Lines Matching full:system

4 Adding a New System Call
7 This document describes what's involved in adding a new system call to the
12 System Call Alternatives
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
35  - If you're just exposing runtime system information, a new node in sysfs
43    :manpage:`fcntl(2)` is a multiplexing system call that hides a lot of complexity, so
49    with :manpage:`fcntl(2)`, this system call is a complicated multiplexor so
57 A new system call forms part of the API of the kernel, and has to be supported
63 together with the corresponding follow-up system calls --
68 For simpler system calls that only take a couple of arguments, the preferred
70 system call.  To make sure that userspace programs can safely use flags
72 flags, and reject the system call (with ``EINVAL``) if it does::
79 For more sophisticated system calls that involve a larger number of arguments,
109 If your new system call allows userspace to refer to a kernel object, it
114 If your new :manpage:`xyzzy(2)` system call does return a new file descriptor,
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
130 If your new :manpage:`xyzzy(2)` system call involves a filename argument::
150 If your new :manpage:`xyzzy(2)` system call involves a parameter describing an
154 If your new :manpage:`xyzzy(2)` system call involves privileged functionality,
163 If your new :manpage:`xyzzy(2)` system call manipulates a process other than
170 system call parameters that are explicitly 64-bit fall on odd-numbered
179 To make new system calls easy to review, it's best to divide up the patchset
183  - The core implementation of the system call, together with prototypes,
185  - Wiring up of the new system call for one particular architecture, usually
187  - A demonstration of the use of the new system call in userspace via a
189  - A draft man-page for the new system call, either as plain text in the
192 New system call proposals, like any change to the kernel's API, should always
196 Generic System Call Implementation
199 The main entry point for your new :manpage:`xyzzy(2)` system call will be called
202 number of arguments to the system call, and the macro takes the system call name
204 this macro allows metadata about the new system call to be made available for
208 ``include/linux/syscalls.h``, marked as asmlinkage to match the way that system
215 new system call to the generic list by adding an entry to the list in
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,
226 system call, returning ``-ENOSYS``.  Add your new system call here too::
230 Your new kernel functionality, and the system call that controls it, should
234  - Include a description of the new functionality and system call controlled
251 x86 System Call Implementation
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
269 Compatibility System Calls (Generic)
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
281 64-bit values.  In particular, this is needed whenever a system call argument
291 system call's arguments has a type that is explicitly 64-bit even on a 32-bit
296 (Note that a system call argument that's a pointer to an explicit 64-bit type
298 type ``loff_t __user *`` do not trigger the need for a ``compat_`` system call.)
300 The compatibility version of the system call is called ``compat_sys_xyzzy()``,
309 ``include/linux/compat.h``, marked as asmlinkage to match the way that system
314 If the system call involves a structure that is laid out differently on 32-bit
340 The generic system call list also needs adjusting to allow for the compat
356 Compatibility System Calls (x86)
359 To wire up the x86 architecture of a system call with a compatibility version,
369 the new system call.  There's a choice here: the layout of the arguments
381 If no pointers are involved, then it is preferable to re-use the 64-bit system
390 System Calls Returning Elsewhere
393 For most system calls, once the system call is complete the user program
395 stack the same and most of the registers the same as before the system call,
398 However, a few system calls do things differently.  They might return to a
403 To allow for this, the kernel implementation of the system call may need to
405 control of where and how execution continues after the system call.
408 that save/restore additional registers and invoke the real system call entry
424 If the system call needs a compatibility layer (as in the previous section)
426 of the system call rather than the native 64-bit version.  Also, if the x32 ABI
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.
447 functions that classify some special types of system call -- specifically
449 socket multiplexor (``socketcall``) operations. If your new system call is
450 analogous to one of these, then the audit system should be updated.
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
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
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
485 All new system calls should come with a complete man page, ideally using groff
494 Do not call System Calls in the Kernel
497 System calls are, as stated above, interaction points between userspace and
498 the kernel.  Therefore, system call functions such as ``sys_xyzzy()`` or
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
528  - LWN article from Michael Kerrisk on use of flags argument in system calls:
530  - LWN article from Michael Kerrisk on how to handle unknown flags in a system
532  - LWN article from Jake Edge describing constraints on 64-bit system call
534  - Pair of LWN articles from David Drysdale that describe the system call
540  - Architecture-specific requirements for system calls are discussed in the
550    system call should come in the same email thread:
552  - Recommendation from Michael Kerrisk that a new system call should come with
556  - Suggestion from Greg Kroah-Hartman that it's good for new system calls to
558  - Discussion from Michael Kerrisk of new system call vs. :manpage:`prctl(2)` extension:
560  - Suggestion from Ingo Molnar that system calls that involve multiple
575  - Recommendation from Linus Torvalds that x32 system calls should prefer