[PATCH RFC 1/1] Documentation: describe how to add a system call

[David Drysdale]

Add a document describing the process of adding a new system call,
including the need for a flags argument for future compatibility, and
covering 32-bit/64-bit concerns (albeit in an x86-centric way).

Signed-off-by: David Drysdale <drysdale@xxxxxxxxxx>
Reviewed-by: Michael Kerrisk <mtk.manpages@xxxxxxxxx>

---
 Documentation/adding-syscalls.txt | 454 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 454 insertions(+)
 create mode 100644 Documentation/adding-syscalls.txt

diff --git a/Documentation/adding-syscalls.txt b/Documentation/adding-syscalls.txt
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+Adding a New System Call
+========================
+
+This document describes what's involved in adding a new system call to the
+Linux kernel, over and above the normal submission advice in
+Documentation/SubmittingPatches.
+
+
+System Call Alternatives
+------------------------
+
+The first thing to consider when adding a new system call is whether one of
+the alternatives might be suitable instead.  Although system calls are the
+most traditional and most obvious interaction points between userspace and the
+kernel, there are other possibilities -- choose what fits best for your
+interface.
+
+ - If the operations involved can be made to look like a filesystem-like
+   object, it may make more sense to create a new filesystem or device.  This
+   also makes it easier to encapsulate the new functionality in a kernel module
+   rather than requiring it to be built into the main kernel.
+     - If the new functionality involves operations where the kernel notifies
+       userspace that something has happened, then returning a new file
+       descriptor for the relevant object allows userspace to use
+       poll/select/epoll to receive that notification.
+     - However, operations that don't map to read(2)/write(2)-like operations
+       have to be implemented as ioctl(2) requests, which can lead to a
+       somewhat opaque API.
+ - If you're just exposing runtime system information, a new node in sysfs
+   (see Documentation/filesystems/sysfs.txt) or the /proc filesystem may be
+   more appropriate.  However, access to these mechanisms requires that the
+   relevant filesystem is mounted, which might not always be the case (e.g.
+   in a namespaced/sandboxed/chrooted environment).
+ - If the operation is specific to a particular file or file descriptor, then
+   an additional fcntl(2) command option may be more appropriate.  However,
+   fcntl(2) is a multiplexing system call that hides a lot of complexity, so
+   this option is best for when the new function is closely analogous to
+   existing fcntl(2) functionality, or the new functionality is very simple
+   (for example, getting/setting a simple flag related to a file descriptor).
+ - If the operation is specific to a particular task or process, then an
+   additional prctl(2) command option may be more appropriate.  As with
+   fcntl(2), this system call is a complicated multiplexor so is best reserved
+   for near-analogs of existing prctl() commands or getting/setting a simple
+   flag related to a process.
+
+
+Designing the API
+-----------------
+
+A new system call forms part of the API of the kernel, and has to be supported
+indefinitely.  As such, it's a very good idea to explicitly discuss the
+interface on the kernel mailing list, and to plan for future extensions of the
+interface.  In particular:
+
+  **Include a flags argument for every new system call**
+
+The syscall table is littered with historical examples where this wasn't done,
+together with the corresponding follow-up system calls (eventfd/eventfd2,
+dup2/dup3, inotify_init/inotify_init1,  pipe/pipe2, renameat/renameat2), so
+learn from the history of the kernel and include a flags argument from the
+start.
+
+Also, to make sure that userspace programs can safely use flags between kernel
+versions, check whether the flags value holds any unknown flags, and reject the
+sycall (with EINVAL) if it does:
+
+    if (flags & ~(THING_FLAG1 | THING_FLAG2 | THING_FLAG3))
+        return -EINVAL;
+
+If your new xyzzy(2) system call returns a new file descriptor, then the flags
+argument should include a value that is equivalent to setting O_CLOEXEC on the
+new FD.  This makes it possible for userspace to close the timing window
+between xyzzy() and calling fcntl(fd, F_SETFD, FD_CLOEXEC), where an
+unexpected fork() and execve() in another thread could leak a descriptor to
+the exec'ed program. (However, resist the temptation to re-use the actual value
+of the O_CLOEXEC constant, as it is architecture-specific and is part of a
+numbering space of O_* flags that is fairly full.)
+
+If your new xyzzy(2) system call involves a filename argument:
+
+    int sys_xyzzy(const char __user *path, ..., unsigned int flags);
+
+you should also consider whether an xyzzyat(2) version is more appropriate:
+
+    int sys_xyzzyat(int dfd, const char __user *path, ..., unsigned int flags);
+
+This allows more flexibility for how userspace specifies the file in question;
+in particular it allows userspace to request the functionality for an
+already-opened file descriptor using the AT_EMPTY_PATH flag, effectively giving
+an fxyzzy(3) operation for free:
+
+ - xyzzyat(AT_FDCWD, path, ..., 0) is equivalent to xyzzy(path,...)
+ - xyzzyat(fd, "", ..., AT_EMPTY_PATH) is equivalent to fxyzzy(fd, ...)
+
+(For more details on the rationale of the *at() calls, see the openat(2) man
+page; for an example of AT_EMPTY_PATH, see the statat(2) man page.)
+
+If your new xyzzy(2) system call involves a parameter describing an offset
+within a file, make its type loff_t so that 64-bit offsets can be supported
+even on 32-bit architectures.
+
+If your new xyzzy(2) system call involves administrative functionality, it
+needs to be governed by the appropriate Linux capability bit, as described in
+the capabilities(7) man page.
+
+ - If there is an existing capability that governs related functionality, then
+   use that.  However, avoid combining lots of only vaguely related functions
+   together under the same bit, as this goes against capabilities' purpose of
+   splitting the power of root.  In particular, avoid adding new uses of the
+   already overly-general CAP_SYS_ADMIN capability.
+ - If there is no related capability, then consider adding a new capability
+   bit -- but bear in mind that the numbering space is limited, and each new
+   bit needs to be understood and administered by sysadmins.
+
+Finally, be aware that some non-x86 architectures have an easier time if
+system call parameters that are explicitly 64-bit fall on odd-numbered
+arguments (i.e. parameter 1, 3, 5), to allow use of contiguous pairs of 32-bit
+registers.
+
+
+Proposing the API
+-----------------
+
+To make new system calls easy to review, it's best to divide up the patchset
+into separate chunks.  These should include at least the following items as
+distinct commits (each of which is described further below):
+
+ - The core implementation of the system call together with prototypes, generic
+   numbering and fallback stub implementation.
+ - Wiring up of the new system call for one particular architecture, usually
+   x86 (including all of x86_64, x86_32 and x32).
+ - A demonstration of the use of the new system call in userspace via a
+   selftest.
+ - A draft man-page for the new system call.
+
+Changes to the kernel's API should always be cc'ed to linux-api@xxxxxxxxxxxxxxx
+
+
+Generic System Call Implementation
+----------------------------------
+
+The main entry point for your new xyzzy(2) system call will be called
+sys_xyzzy(), but you add this entry point with the appropriate
+SYSCALL_DEFINEn() macro rather than explicitly.  The 'n' indicates the number
+of arguments to the system call, and the macro takes the system call name
+followed by the (type, name) pairs for the parameters as arguments.  Using
+this macro allows metadata about the new system call to be made available for
+other tools.
+
+The new entry point also needs a corresponding function prototype, in
+include/linux/syscalls.h, marked as asmlinkage to match the way that system
+calls are invoked:
+
+    asmlinkage long sys_xyzzy(...);
+
+Some architectures (e.g. x86) have their own architecture-specific syscall
+tables, but several other architectures share a generic syscall table. Add your
+new system call to the generic list by adding an entry to the list in
+include/uapi/asm-generic/unistd.h:
+
+    #define __NR_xyzzy 292
+    __SYSCALL(__NR_xyzzy, sys_xyzzy)
+
+Also update the __NR_syscalls count to reflect the additional system call, and
+note that if multiple new system calls are added in the same merge window,
+your new syscall number may get adjusted to resolve conflicts.
+
+The file kernel/sys_ni.c provides a fallback stub implementation of each system
+call, returning -ENOSYS.  Add your new system call here too:
+
+    cond_syscall(sys_xyzzy);
+
+To summarize, you need a commit that includes:
+
+ - SYSCALL_DEFINEn(xyzzy, ...) for the entry point
+ - corresponding prototype in include/linux/syscalls.h
+ - generic table entry in include/uapi/asm-generic/unistd.h
+ - fallback stub in kernel/sys_ni.c
+
+
+x86 System Call Implementation
+------------------------------
+
+To wire up your new system call for x86 platforms, you need to update the
+master syscall tables.  Assuming your new system call isn't special in some
+way (see below), this involves a "common" entry (for x86_64 and x32) in
+arch/x86/entry/syscalls/syscall_64.tbl:
+
+    333   common   xyzzy     sys_xyzzy
+
+and an "i386" entry in arch/x86/entry/syscalls/syscall_32.tbl:
+
+    380   i386     xyzzy     sys_xyzzy
+
+Again, these numbers are liable to be changed if there are conflicts in the
+relevant merge window.
+
+
+Compatibility System Calls (Generic)
+------------------------------------
+
+For most system calls the same 64-bit implementation can be invoked even when
+the userspace program is itself 32-bit; even if the system call's parameters
+include an explicit pointer, this is handled transparently.
+
+However, there are a couple of situations where a compatibility layer is
+needed to cope with size differences between 32-bit and 64-bit.
+
+The first is if the 64-bit kernel also supports 32-bit userspace programs, and
+so needs to parse areas of (__user) memory that could hold either 32-bit or
+64-bit values.  In particular, this is needed whenever a system call argument
+is:
+
+ - a pointer to a pointer
+ - a pointer to a struct containing a pointer (e.g. struct iovec __user *)
+ - a pointer to a varying sized integral type (time_t, off_t, long, ...)
+ - a pointer to a struct containing a varying sized integral type.
+
+The second situation that requires a compatibility layer is if one of the
+system call's arguments has a type that is explicitly 64-bit even on a 32-bit
+architecture, for example loff_t or __u64.  In this case, a value that arrives
+at a 64-bit kernel from a 32-bit application will be split into two 32-bit
+values, which then need to be re-assembled in the compatibility layer.
+
+(Note that a system call argument that's a pointer to an explicit 64-bit type
+does *not* need a compatibility layer; for example, splice(2)'s arguments of
+type loff_t __user * do not trigger the need for a compat_ system call.)
+
+The compatibility version of the system call is called compat_sys_xyzzy(), and
+is added with the COMPAT_SYSCALL_DEFINEn() macro, analogously to
+SYSCALL_DEFINEn.  This version of the implementation runs as part of a 64-bit
+kernel, but expects to receive 32-bit parameter values and does whatever is
+needed to deal with them.  (Typically, the compat_sys_ version converts the
+values to 64-bit versions and either calls on to the sys_ version, or both of
+them call a common inner implementation function.)
+
+The compat entry point also needs a corresponding function prototype, in
+include/linux/compat.h, marked as asmlinkage to match the way that system
+calls are invoked:
+
+    asmlinkage long compat_sys_xyzzy(...);
+
+If the system call involves a structure that is laid out differently on 32-bit
+and 64-bit systems, say struct xyzzy_args, then the include/linux/compat.h
+header file should also include a compat version of the structure (struct
+compat_xyzzy_args) where each variable-size field has the appropriate compat_
+type that corresponds to the type in struct xyzzy_args.  The
+compat_sys_xyzzy() routine can then use this compat_ structure to parse the
+arguments from a 32-bit invocation.
+
+For example, if there are fields:
+
+    struct xyzzy_args {
+        const char __user *ptr;
+        __kernel_long_t varying_val;
+        u64 fixed_val;
+        /* ... */
+    };
+
+in struct xyzzy_args, then struct compat_xyzzy_args would have:
+
+    struct compat_xyzzy_args {
+        compat_uptr_t ptr;
+        compat_long_t varying_val;
+        u64 fixed_val;
+        /* ... */
+    };
+
+The generic system call list also needs adjusting to allow for the compat
+version; the entry in include/uapi/asm-generic/unistd.h should use
+__SC_COMP rather than __SYSCALL:
+
+    #define __NR_xyzzy 292
+    __SC_COMP(__NR_xyzzy, sys_xyzzy, compat_sys_xyzzy)
+
+To summarize, you need:
+
+ - a COMPAT_SYSCALL_DEFINEn(xyzzy, ...) for the compat entry point
+ - corresponding prototype in include/linux/compat.h
+ - (if needed) 32-bit mapping struct in include/linux/compat.h
+ - instance of __SC_COMP not __SYSCALL in include/uapi/asm-generic/unistd.h
+
+
+Compatibility System Calls (x86)
+--------------------------------
+
+To wire up the x86 architecture of a system call with a compatibility version,
+the entries in the syscall tables need to be adjusted.
+
+First, the entry in arch/x86/entry/syscalls/syscall_32.tbl gets an extra
+column to indicate that a 32-bit userspace program running on a 64-bit kernel
+should hit the compat entry point:
+
+    380   i386     xyzzy     sys_xyzzy    compat_sys_xyzzy
+
+Second, you need to figure out what should happen for the x32 ABI version of
+the new system call.  There's a choice here: the layout of the arguments
+should either match the 64-bit version or the 32-bit version.
+
+If there's a pointer-to-a-pointer involved, the decision is easy: x32 is
+ILP32, so the layout should match the 32-bit version, and the entry in
+arch/x86/entry/syscalls/syscall_64.tbl is split so that x32 programs hit the
+compatibility wrapper:
+
+    333   64       xyzzy     sys_xyzzy
+    ...
+    555   x32      xyzzy     compat_sys_xyzzy
+
+If no pointers are involved, then it is preferable to re-use the 64-bit system
+call for the x32 ABI (and consequently the entry in
+arch/x86/entry/syscalls/syscall_64.tbl is unchanged).
+
+In either case, you should check that the types involved in your argument
+layout do indeed map exactly from x32 (-mx32) to either the 32-bit (-m32) or
+64-bit (-m64) equivalents.
+
+
+System Calls Returning Elsewhere
+--------------------------------
+
+For most system calls, once the system call is complete the user program
+continues exactly where it left off -- at the next instruction, with the same
+stack and registers as before the system call, and with the same virtual
+memory space.
+
+However, a few system calls do things differently.  They might return to a
+different location (rt_sigreturn) or change the memory space (fork/vfork/clone)
+or even architecture (execve/execveat) of the program.
+
+To allow for this, the kernel implementation of the system call may need to
+save and restore additional registers to the kernel stack, allowing complete
+control of where and how execution continues after the system call.
+
+This is arch-specific, but typically involves defining assembly entry points
+that save/restore additional registers and invoke the real system call entry
+point.
+
+For x86_64, this is implemented as a stub_xyzzy entry point in
+arch/x86/entry/entry_64.S, and the entry in the syscall table
+(arch/x86/entry/syscalls/syscall_64.tbl) is adjusted to match:
+
+    333   common   xyzzy     stub_xyzzy
+
+The equivalent for 32-bit programs running on a 64-bit kernel is normally
+called stub32_xyzzy and implemented in arch/x86/entry/entry_64_compat.S,
+with the corresponding syscall table adjustment in
+arch/x86/entry/syscalls/syscall_32.tbl:
+
+    380   i386     xyzzy     sys_xyzzy    stub32_xyzzy
+
+If the system call needs a compatibility layer (as in the previous section)
+then the stub32_ version needs to call on to the compat_sys_ version of the
+system call rather than the native 64-bit version.  Also, if the x32 ABI
+implementation is not common with the x86_64 version, then its syscall
+table will also need to invoke a stub that calls on to the compat_sys_
+version.
+
+For completeness, it's also nice to set up a mapping so that user-mode Linux
+still works -- its syscall table will reference stub_xyzzy, but the UML build
+doesn't include arch/x86/entry/entry_64.S implementation (because UML
+simulates registers etc).  Fixing this is as simple as adding a #define to
+arch/x86/um/sys_call_table_64.c:
+
+    #define stub_xyzzy sys_xyzzy
+
+
+Other Details
+-------------
+
+Most of the kernel treats system calls in a generic way, but there is the
+occasional exception that may need updating for your particular system call.
+
+The audit subsystem is one such special case; it includes (arch-specific)
+functions that classify some special types of system call -- specifically
+file open (open/openat), program execution (execve/exeveat) or socket
+multiplexor (socketcall) operations. If your new system call is analogous to
+one of these, then the audit system should be updated.
+
+More generally, if there is an existing system call that is analogous to your
+new system call, it's worth doing a kernel-wide grep for the existing system
+call to check there are no other special cases.
+
+
+Testing
+-------
+
+A new system call should obviously be tested; it is also useful to provide
+reviewers with a demonstration of how user space programs will use the system
+call.  A good way to combine these aims is to include a simple self-test
+program in a new directory under tools/testing/selftests/.
+
+Make sure the selftest runs successfully on all supported architectures.  For
+example, check that it works when compiled as an x86_64 (-m64), x86_32 (-m32)
+and x32 (-mx32) ABI program.
+
+
+Man Page
+--------
+
+All new system calls should come with a complete man page, ideally using groff
+markup, but plain text will do.  If groff is used, it's helpful to include a
+pre-rendered ASCII version of the man page in the cover email for the
+patchset, for the convenience of reviewers.
+
+The man page should be cc'ed to linux-man@xxxxxxxxxxxxxxx
+
+
+References and Sources
+----------------------
+
+ - LWN article from Michael Kerrisk on use of flags argument in system calls:
+   https://lwn.net/Articles/585415/
+ - LWN article from Michael Kerrisk on how to handle unknown flags in a system
+   call: https://lwn.net/Articles/588444/
+ - LWN article from Jake Edge describing constraints on 64-bit system call
+   arguments: https://lwn.net/Articles/311630/
+ - Pair of LWN articles from David Drysdale that describe the system call
+   implementation paths in detail for v3.14:
+    - https://lwn.net/Articles/604287/
+    - https://lwn.net/Articles/604515/
+ - Architecture-specific requirements for system calls are discussed in the
+   syscall(2) man-page:
+   http://man7.org/linux/man-pages/man2/syscall.2.html#NOTES
+ - Collated emails from Linus Torvalds discussing the problems with ioctl():
+   http://yarchive.net/comp/linux/ioctl.html
+ - "How to not invent kernel interfaces", Arnd Bergmann,
+   http://www.ukuug.org/events/linux2007/2007/papers/Bergmann.pdf
+ - LWN article from Michael Kerrisk on avoiding new uses of CAP_SYS_ADMIN:
+   https://lwn.net/Articles/486306/
+
+ - Recommendation from Andrew Morton that all related information for a new
+   system call should come in the same email thread:
+   https://lkml.org/lkml/2014/7/24/641
+ - Recommendation from Michael Kerrisk that a new system call should come with
+   a man page: https://lkml.org/lkml/2014/6/13/309
+ - Suggestion from Thomas Gleixner that x86 wire-up should be in a separate
+   commit: https://lkml.org/lkml/2014/11/19/254
+ - Suggestion from Greg Kroah-Hartman that it's good for new system calls to
+   come with a man-page & selftest: https://lkml.org/lkml/2014/3/19/710
+ - Discussion from Michael Kerrisk of new system call vs. prctl(2) extension:
+   https://lkml.org/lkml/2014/6/3/411
+ - Numbering oddities arising from (re-)use of O_* numbering space flags:
+    - commit 75069f2b5bfb ("vfs: renumber FMODE_NONOTIFY and add to uniqueness
+      check")
+    - commit 12ed2e36c98a ("fanotify: FMODE_NONOTIFY and __O_SYNC in sparc
+      conflict")
+    - commit bb458c644a59 ("Safer ABI for O_TMPFILE")
+ - Discussion from Matthew Wilcox about restrictions on 64-bit arguments:
+   https://lkml.org/lkml/2008/12/12/187
+ - Recommendation from Greg Kroah-Hartman that unknown flags should be
+   policed: https://lkml.org/lkml/2014/7/17/577
+ - Recommendation from Linus Torvalds that x32 system calls should prefer
+   compatibility with 64-bit versions rather than 32-bit versions:
+   https://lkml.org/lkml/2011/8/31/244
--
2.4.3.573.g4eafbef
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