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The previous patch renamed several files that are cross-referenced along the Kernel documentation. Adjust the links to point to the right places. Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
543 lines
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ReStructuredText
543 lines
25 KiB
ReStructuredText
Adding a New System Call
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========================
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This document describes what's involved in adding a new system call to the
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Linux kernel, over and above the normal submission advice in
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:ref:`Documentation/process/submitting-patches.rst <submittingpatches>`.
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System Call Alternatives
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------------------------
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The first thing to consider when adding a new system call is whether one of
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the alternatives might be suitable instead. Although system calls are the
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most traditional and most obvious interaction points between userspace and the
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kernel, there are other possibilities -- choose what fits best for your
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interface.
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- If the operations involved can be made to look like a filesystem-like
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object, it may make more sense to create a new filesystem or device. This
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also makes it easier to encapsulate the new functionality in a kernel module
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rather than requiring it to be built into the main kernel.
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- If the new functionality involves operations where the kernel notifies
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userspace that something has happened, then returning a new file
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descriptor for the relevant object allows userspace to use
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``poll``/``select``/``epoll`` to receive that notification.
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- However, operations that don't map to
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:manpage:`read(2)`/:manpage:`write(2)`-like operations
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have to be implemented as :manpage:`ioctl(2)` requests, which can lead
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to a somewhat opaque API.
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- If you're just exposing runtime system information, a new node in sysfs
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(see ``Documentation/filesystems/sysfs.txt``) or the ``/proc`` filesystem may
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be more appropriate. However, access to these mechanisms requires that the
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relevant filesystem is mounted, which might not always be the case (e.g.
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in a namespaced/sandboxed/chrooted environment). Avoid adding any API to
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debugfs, as this is not considered a 'production' interface to userspace.
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- If the operation is specific to a particular file or file descriptor, then
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an additional :manpage:`fcntl(2)` command option may be more appropriate. However,
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:manpage:`fcntl(2)` is a multiplexing system call that hides a lot of complexity, so
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this option is best for when the new function is closely analogous to
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existing :manpage:`fcntl(2)` functionality, or the new functionality is very simple
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(for example, getting/setting a simple flag related to a file descriptor).
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- If the operation is specific to a particular task or process, then an
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additional :manpage:`prctl(2)` command option may be more appropriate. As
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with :manpage:`fcntl(2)`, this system call is a complicated multiplexor so
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is best reserved for near-analogs of existing ``prctl()`` commands or
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getting/setting a simple flag related to a process.
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Designing the API: Planning for Extension
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-----------------------------------------
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A new system call forms part of the API of the kernel, and has to be supported
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indefinitely. As such, it's a very good idea to explicitly discuss the
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interface on the kernel mailing list, and it's important to plan for future
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extensions of the interface.
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(The syscall table is littered with historical examples where this wasn't done,
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together with the corresponding follow-up system calls --
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``eventfd``/``eventfd2``, ``dup2``/``dup3``, ``inotify_init``/``inotify_init1``,
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``pipe``/``pipe2``, ``renameat``/``renameat2`` -- so
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learn from the history of the kernel and plan for extensions from the start.)
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For simpler system calls that only take a couple of arguments, the preferred
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way to allow for future extensibility is to include a flags argument to the
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system call. To make sure that userspace programs can safely use flags
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between kernel versions, check whether the flags value holds any unknown
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flags, and reject the system call (with ``EINVAL``) if it does::
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if (flags & ~(THING_FLAG1 | THING_FLAG2 | THING_FLAG3))
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return -EINVAL;
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(If no flags values are used yet, check that the flags argument is zero.)
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For more sophisticated system calls that involve a larger number of arguments,
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it's preferred to encapsulate the majority of the arguments into a structure
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that is passed in by pointer. Such a structure can cope with future extension
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by including a size argument in the structure::
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struct xyzzy_params {
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u32 size; /* userspace sets p->size = sizeof(struct xyzzy_params) */
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u32 param_1;
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u64 param_2;
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u64 param_3;
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};
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As long as any subsequently added field, say ``param_4``, is designed so that a
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zero value gives the previous behaviour, then this allows both directions of
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version mismatch:
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- To cope with a later userspace program calling an older kernel, the kernel
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code should check that any memory beyond the size of the structure that it
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expects is zero (effectively checking that ``param_4 == 0``).
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- To cope with an older userspace program calling a newer kernel, the kernel
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code can zero-extend a smaller instance of the structure (effectively
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setting ``param_4 = 0``).
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See :manpage:`perf_event_open(2)` and the ``perf_copy_attr()`` function (in
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``kernel/events/core.c``) for an example of this approach.
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Designing the API: Other Considerations
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---------------------------------------
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If your new system call allows userspace to refer to a kernel object, it
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should use a file descriptor as the handle for that object -- don't invent a
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new type of userspace object handle when the kernel already has mechanisms and
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well-defined semantics for using file descriptors.
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If your new :manpage:`xyzzy(2)` system call does return a new file descriptor,
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then the flags argument should include a value that is equivalent to setting
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``O_CLOEXEC`` on the new FD. This makes it possible for userspace to close
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the timing window between ``xyzzy()`` and calling
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``fcntl(fd, F_SETFD, FD_CLOEXEC)``, where an unexpected ``fork()`` and
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``execve()`` in another thread could leak a descriptor to
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the exec'ed program. (However, resist the temptation to re-use the actual value
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of the ``O_CLOEXEC`` constant, as it is architecture-specific and is part of a
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numbering space of ``O_*`` flags that is fairly full.)
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If your system call returns a new file descriptor, you should also consider
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what it means to use the :manpage:`poll(2)` family of system calls on that file
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descriptor. Making a file descriptor ready for reading or writing is the
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normal way for the kernel to indicate to userspace that an event has
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occurred on the corresponding kernel object.
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If your new :manpage:`xyzzy(2)` system call involves a filename argument::
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int sys_xyzzy(const char __user *path, ..., unsigned int flags);
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you should also consider whether an :manpage:`xyzzyat(2)` version is more appropriate::
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int sys_xyzzyat(int dfd, const char __user *path, ..., unsigned int flags);
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This allows more flexibility for how userspace specifies the file in question;
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in particular it allows userspace to request the functionality for an
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already-opened file descriptor using the ``AT_EMPTY_PATH`` flag, effectively
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giving an :manpage:`fxyzzy(3)` operation for free::
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- xyzzyat(AT_FDCWD, path, ..., 0) is equivalent to xyzzy(path,...)
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- xyzzyat(fd, "", ..., AT_EMPTY_PATH) is equivalent to fxyzzy(fd, ...)
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(For more details on the rationale of the \*at() calls, see the
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:manpage:`openat(2)` man page; for an example of AT_EMPTY_PATH, see the
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:manpage:`fstatat(2)` man page.)
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If your new :manpage:`xyzzy(2)` system call involves a parameter describing an
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offset within a file, make its type ``loff_t`` so that 64-bit offsets can be
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supported even on 32-bit architectures.
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If your new :manpage:`xyzzy(2)` system call involves privileged functionality,
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it needs to be governed by the appropriate Linux capability bit (checked with
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a call to ``capable()``), as described in the :manpage:`capabilities(7)` man
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page. Choose an existing capability bit that governs related functionality,
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but try to avoid combining lots of only vaguely related functions together
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under the same bit, as this goes against capabilities' purpose of splitting
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the power of root. In particular, avoid adding new uses of the already
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overly-general ``CAP_SYS_ADMIN`` capability.
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If your new :manpage:`xyzzy(2)` system call manipulates a process other than
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the calling process, it should be restricted (using a call to
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``ptrace_may_access()``) so that only a calling process with the same
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permissions as the target process, or with the necessary capabilities, can
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manipulate the target process.
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Finally, be aware that some non-x86 architectures have an easier time if
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system call parameters that are explicitly 64-bit fall on odd-numbered
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arguments (i.e. parameter 1, 3, 5), to allow use of contiguous pairs of 32-bit
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registers. (This concern does not apply if the arguments are part of a
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structure that's passed in by pointer.)
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Proposing the API
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-----------------
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To make new system calls easy to review, it's best to divide up the patchset
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into separate chunks. These should include at least the following items as
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distinct commits (each of which is described further below):
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- The core implementation of the system call, together with prototypes,
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generic numbering, Kconfig changes and fallback stub implementation.
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- Wiring up of the new system call for one particular architecture, usually
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x86 (including all of x86_64, x86_32 and x32).
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- A demonstration of the use of the new system call in userspace via a
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selftest in ``tools/testing/selftests/``.
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- A draft man-page for the new system call, either as plain text in the
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cover letter, or as a patch to the (separate) man-pages repository.
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New system call proposals, like any change to the kernel's API, should always
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be cc'ed to linux-api@vger.kernel.org.
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Generic System Call Implementation
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----------------------------------
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The main entry point for your new :manpage:`xyzzy(2)` system call will be called
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``sys_xyzzy()``, but you add this entry point with the appropriate
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``SYSCALL_DEFINEn()`` macro rather than explicitly. The 'n' indicates the
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number of arguments to the system call, and the macro takes the system call name
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followed by the (type, name) pairs for the parameters as arguments. Using
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this macro allows metadata about the new system call to be made available for
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other tools.
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The new entry point also needs a corresponding function prototype, in
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``include/linux/syscalls.h``, marked as asmlinkage to match the way that system
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calls are invoked::
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asmlinkage long sys_xyzzy(...);
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Some architectures (e.g. x86) have their own architecture-specific syscall
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tables, but several other architectures share a generic syscall table. Add your
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new system call to the generic list by adding an entry to the list in
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``include/uapi/asm-generic/unistd.h``::
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#define __NR_xyzzy 292
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__SYSCALL(__NR_xyzzy, sys_xyzzy)
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Also update the __NR_syscalls count to reflect the additional system call, and
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note that if multiple new system calls are added in the same merge window,
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your new syscall number may get adjusted to resolve conflicts.
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The file ``kernel/sys_ni.c`` provides a fallback stub implementation of each
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system call, returning ``-ENOSYS``. Add your new system call here too::
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cond_syscall(sys_xyzzy);
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Your new kernel functionality, and the system call that controls it, should
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normally be optional, so add a ``CONFIG`` option (typically to
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``init/Kconfig``) for it. As usual for new ``CONFIG`` options:
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- Include a description of the new functionality and system call controlled
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by the option.
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- Make the option depend on EXPERT if it should be hidden from normal users.
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- Make any new source files implementing the function dependent on the CONFIG
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option in the Makefile (e.g. ``obj-$(CONFIG_XYZZY_SYSCALL) += xyzzy.c``).
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- Double check that the kernel still builds with the new CONFIG option turned
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off.
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To summarize, you need a commit that includes:
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- ``CONFIG`` option for the new function, normally in ``init/Kconfig``
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- ``SYSCALL_DEFINEn(xyzzy, ...)`` for the entry point
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- corresponding prototype in ``include/linux/syscalls.h``
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- generic table entry in ``include/uapi/asm-generic/unistd.h``
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- fallback stub in ``kernel/sys_ni.c``
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x86 System Call Implementation
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------------------------------
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To wire up your new system call for x86 platforms, you need to update the
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master syscall tables. Assuming your new system call isn't special in some
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way (see below), this involves a "common" entry (for x86_64 and x32) in
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arch/x86/entry/syscalls/syscall_64.tbl::
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333 common xyzzy sys_xyzzy
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and an "i386" entry in ``arch/x86/entry/syscalls/syscall_32.tbl``::
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380 i386 xyzzy sys_xyzzy
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Again, these numbers are liable to be changed if there are conflicts in the
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relevant merge window.
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Compatibility System Calls (Generic)
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------------------------------------
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For most system calls the same 64-bit implementation can be invoked even when
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the userspace program is itself 32-bit; even if the system call's parameters
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include an explicit pointer, this is handled transparently.
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However, there are a couple of situations where a compatibility layer is
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needed to cope with size differences between 32-bit and 64-bit.
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The first is if the 64-bit kernel also supports 32-bit userspace programs, and
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so needs to parse areas of (``__user``) memory that could hold either 32-bit or
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64-bit values. In particular, this is needed whenever a system call argument
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is:
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- a pointer to a pointer
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- a pointer to a struct containing a pointer (e.g. ``struct iovec __user *``)
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- a pointer to a varying sized integral type (``time_t``, ``off_t``,
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``long``, ...)
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- a pointer to a struct containing a varying sized integral type.
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The second situation that requires a compatibility layer is if one of the
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system call's arguments has a type that is explicitly 64-bit even on a 32-bit
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architecture, for example ``loff_t`` or ``__u64``. In this case, a value that
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arrives at a 64-bit kernel from a 32-bit application will be split into two
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32-bit values, which then need to be re-assembled in the compatibility layer.
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(Note that a system call argument that's a pointer to an explicit 64-bit type
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does **not** need a compatibility layer; for example, :manpage:`splice(2)`'s arguments of
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type ``loff_t __user *`` do not trigger the need for a ``compat_`` system call.)
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The compatibility version of the system call is called ``compat_sys_xyzzy()``,
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and is added with the ``COMPAT_SYSCALL_DEFINEn()`` macro, analogously to
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SYSCALL_DEFINEn. This version of the implementation runs as part of a 64-bit
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kernel, but expects to receive 32-bit parameter values and does whatever is
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needed to deal with them. (Typically, the ``compat_sys_`` version converts the
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values to 64-bit versions and either calls on to the ``sys_`` version, or both of
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them call a common inner implementation function.)
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The compat entry point also needs a corresponding function prototype, in
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``include/linux/compat.h``, marked as asmlinkage to match the way that system
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calls are invoked::
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asmlinkage long compat_sys_xyzzy(...);
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If the system call involves a structure that is laid out differently on 32-bit
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and 64-bit systems, say ``struct xyzzy_args``, then the include/linux/compat.h
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header file should also include a compat version of the structure (``struct
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compat_xyzzy_args``) where each variable-size field has the appropriate
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``compat_`` type that corresponds to the type in ``struct xyzzy_args``. The
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``compat_sys_xyzzy()`` routine can then use this ``compat_`` structure to
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parse the arguments from a 32-bit invocation.
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For example, if there are fields::
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struct xyzzy_args {
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const char __user *ptr;
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__kernel_long_t varying_val;
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u64 fixed_val;
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/* ... */
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};
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in struct xyzzy_args, then struct compat_xyzzy_args would have::
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struct compat_xyzzy_args {
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compat_uptr_t ptr;
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compat_long_t varying_val;
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u64 fixed_val;
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/* ... */
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};
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The generic system call list also needs adjusting to allow for the compat
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version; the entry in ``include/uapi/asm-generic/unistd.h`` should use
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``__SC_COMP`` rather than ``__SYSCALL``::
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#define __NR_xyzzy 292
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__SC_COMP(__NR_xyzzy, sys_xyzzy, compat_sys_xyzzy)
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To summarize, you need:
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- a ``COMPAT_SYSCALL_DEFINEn(xyzzy, ...)`` for the compat entry point
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- corresponding prototype in ``include/linux/compat.h``
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- (if needed) 32-bit mapping struct in ``include/linux/compat.h``
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- instance of ``__SC_COMP`` not ``__SYSCALL`` in
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``include/uapi/asm-generic/unistd.h``
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Compatibility System Calls (x86)
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--------------------------------
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To wire up the x86 architecture of a system call with a compatibility version,
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the entries in the syscall tables need to be adjusted.
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First, the entry in ``arch/x86/entry/syscalls/syscall_32.tbl`` gets an extra
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column to indicate that a 32-bit userspace program running on a 64-bit kernel
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should hit the compat entry point::
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380 i386 xyzzy sys_xyzzy compat_sys_xyzzy
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Second, you need to figure out what should happen for the x32 ABI version of
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the new system call. There's a choice here: the layout of the arguments
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should either match the 64-bit version or the 32-bit version.
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If there's a pointer-to-a-pointer involved, the decision is easy: x32 is
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ILP32, so the layout should match the 32-bit version, and the entry in
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``arch/x86/entry/syscalls/syscall_64.tbl`` is split so that x32 programs hit
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the compatibility wrapper::
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333 64 xyzzy sys_xyzzy
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...
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555 x32 xyzzy compat_sys_xyzzy
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If no pointers are involved, then it is preferable to re-use the 64-bit system
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call for the x32 ABI (and consequently the entry in
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arch/x86/entry/syscalls/syscall_64.tbl is unchanged).
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In either case, you should check that the types involved in your argument
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layout do indeed map exactly from x32 (-mx32) to either the 32-bit (-m32) or
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64-bit (-m64) equivalents.
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System Calls Returning Elsewhere
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--------------------------------
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For most system calls, once the system call is complete the user program
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continues exactly where it left off -- at the next instruction, with the
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stack the same and most of the registers the same as before the system call,
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and with the same virtual memory space.
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However, a few system calls do things differently. They might return to a
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different location (``rt_sigreturn``) or change the memory space
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(``fork``/``vfork``/``clone``) or even architecture (``execve``/``execveat``)
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of the program.
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To allow for this, the kernel implementation of the system call may need to
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save and restore additional registers to the kernel stack, allowing complete
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control of where and how execution continues after the system call.
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This is arch-specific, but typically involves defining assembly entry points
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that save/restore additional registers and invoke the real system call entry
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point.
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For x86_64, this is implemented as a ``stub_xyzzy`` entry point in
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``arch/x86/entry/entry_64.S``, and the entry in the syscall table
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(``arch/x86/entry/syscalls/syscall_64.tbl``) is adjusted to match::
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333 common xyzzy stub_xyzzy
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The equivalent for 32-bit programs running on a 64-bit kernel is normally
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called ``stub32_xyzzy`` and implemented in ``arch/x86/entry/entry_64_compat.S``,
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with the corresponding syscall table adjustment in
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``arch/x86/entry/syscalls/syscall_32.tbl``::
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380 i386 xyzzy sys_xyzzy stub32_xyzzy
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If the system call needs a compatibility layer (as in the previous section)
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then the ``stub32_`` version needs to call on to the ``compat_sys_`` version
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of the system call rather than the native 64-bit version. Also, if the x32 ABI
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implementation is not common with the x86_64 version, then its syscall
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table will also need to invoke a stub that calls on to the ``compat_sys_``
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version.
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For completeness, it's also nice to set up a mapping so that user-mode Linux
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still works -- its syscall table will reference stub_xyzzy, but the UML build
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doesn't include ``arch/x86/entry/entry_64.S`` implementation (because UML
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simulates registers etc). Fixing this is as simple as adding a #define to
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``arch/x86/um/sys_call_table_64.c``::
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#define stub_xyzzy sys_xyzzy
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Other Details
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-------------
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Most of the kernel treats system calls in a generic way, but there is the
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occasional exception that may need updating for your particular system call.
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The audit subsystem is one such special case; it includes (arch-specific)
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functions that classify some special types of system call -- specifically
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file open (``open``/``openat``), program execution (``execve``/``exeveat``) or
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socket multiplexor (``socketcall``) operations. If your new system call is
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analogous to one of these, then the audit system should be updated.
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More generally, if there is an existing system call that is analogous to your
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new system call, it's worth doing a kernel-wide grep for the existing system
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call to check there are no other special cases.
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Testing
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-------
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A new system call should obviously be tested; it is also useful to provide
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reviewers with a demonstration of how user space programs will use the system
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call. A good way to combine these aims is to include a simple self-test
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program in a new directory under ``tools/testing/selftests/``.
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For a new system call, there will obviously be no libc wrapper function and so
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the test will need to invoke it using ``syscall()``; also, if the system call
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involves a new userspace-visible structure, the corresponding header will need
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to be installed to compile the test.
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Make sure the selftest runs successfully on all supported architectures. For
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example, check that it works when compiled as an x86_64 (-m64), x86_32 (-m32)
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and x32 (-mx32) ABI program.
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For more extensive and thorough testing of new functionality, you should also
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consider adding tests to the Linux Test Project, or to the xfstests project
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for filesystem-related changes.
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- https://linux-test-project.github.io/
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- git://git.kernel.org/pub/scm/fs/xfs/xfstests-dev.git
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Man Page
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--------
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All new system calls should come with a complete man page, ideally using groff
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markup, but plain text will do. If groff is used, it's helpful to include a
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pre-rendered ASCII version of the man page in the cover email for the
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patchset, for the convenience of reviewers.
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The man page should be cc'ed to linux-man@vger.kernel.org
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For more details, see https://www.kernel.org/doc/man-pages/patches.html
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References and Sources
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----------------------
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- LWN article from Michael Kerrisk on use of flags argument in system calls:
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https://lwn.net/Articles/585415/
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- LWN article from Michael Kerrisk on how to handle unknown flags in a system
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call: https://lwn.net/Articles/588444/
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- LWN article from Jake Edge describing constraints on 64-bit system call
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arguments: https://lwn.net/Articles/311630/
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- Pair of LWN articles from David Drysdale that describe the system call
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implementation paths in detail for v3.14:
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- https://lwn.net/Articles/604287/
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- https://lwn.net/Articles/604515/
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- Architecture-specific requirements for system calls are discussed in the
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:manpage:`syscall(2)` man-page:
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http://man7.org/linux/man-pages/man2/syscall.2.html#NOTES
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- Collated emails from Linus Torvalds discussing the problems with ``ioctl()``:
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http://yarchive.net/comp/linux/ioctl.html
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- "How to not invent kernel interfaces", Arnd Bergmann,
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http://www.ukuug.org/events/linux2007/2007/papers/Bergmann.pdf
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- LWN article from Michael Kerrisk on avoiding new uses of CAP_SYS_ADMIN:
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https://lwn.net/Articles/486306/
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- Recommendation from Andrew Morton that all related information for a new
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system call should come in the same email thread:
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https://lkml.org/lkml/2014/7/24/641
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- Recommendation from Michael Kerrisk that a new system call should come with
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a man page: https://lkml.org/lkml/2014/6/13/309
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- Suggestion from Thomas Gleixner that x86 wire-up should be in a separate
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commit: https://lkml.org/lkml/2014/11/19/254
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- Suggestion from Greg Kroah-Hartman that it's good for new system calls to
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come with a man-page & selftest: https://lkml.org/lkml/2014/3/19/710
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- Discussion from Michael Kerrisk of new system call vs. :manpage:`prctl(2)` extension:
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https://lkml.org/lkml/2014/6/3/411
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- Suggestion from Ingo Molnar that system calls that involve multiple
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arguments should encapsulate those arguments in a struct, which includes a
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size field for future extensibility: https://lkml.org/lkml/2015/7/30/117
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- Numbering oddities arising from (re-)use of O_* numbering space flags:
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- commit 75069f2b5bfb ("vfs: renumber FMODE_NONOTIFY and add to uniqueness
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check")
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- commit 12ed2e36c98a ("fanotify: FMODE_NONOTIFY and __O_SYNC in sparc
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conflict")
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- commit bb458c644a59 ("Safer ABI for O_TMPFILE")
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- Discussion from Matthew Wilcox about restrictions on 64-bit arguments:
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https://lkml.org/lkml/2008/12/12/187
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- Recommendation from Greg Kroah-Hartman that unknown flags should be
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policed: https://lkml.org/lkml/2014/7/17/577
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- Recommendation from Linus Torvalds that x32 system calls should prefer
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compatibility with 64-bit versions rather than 32-bit versions:
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https://lkml.org/lkml/2011/8/31/244
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