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diff --git a/Documentation/adding-syscalls.txt b/Documentation/adding-syscalls.txt new file mode 100644 index 000000000..cc2d4ac4f --- /dev/null +++ b/Documentation/adding-syscalls.txt @@ -0,0 +1,527 @@ +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). Avoid adding any API to + debugfs, as this is not considered a 'production' interface to userspace. + - 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: Planning for Extension +----------------------------------------- + +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 it's important to plan for future +extensions of the interface. + +(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 plan for extensions from the start.) + +For simpler system calls that only take a couple of arguments, the preferred +way to allow for future extensibility is to include a flags argument to the +system call. 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 system call (with EINVAL) if it does: + + if (flags & ~(THING_FLAG1 | THING_FLAG2 | THING_FLAG3)) + return -EINVAL; + +(If no flags values are used yet, check that the flags argument is zero.) + +For more sophisticated system calls that involve a larger number of arguments, +it's preferred to encapsulate the majority of the arguments into a structure +that is passed in by pointer. Such a structure can cope with future extension +by including a size argument in the structure: + + struct xyzzy_params { + u32 size; /* userspace sets p->size = sizeof(struct xyzzy_params) */ + u32 param_1; + u64 param_2; + u64 param_3; + }; + +As long as any subsequently added field, say param_4, is designed so that a +zero value gives the previous behaviour, then this allows both directions of +version mismatch: + + - To cope with a later userspace program calling an older kernel, the kernel + code should check that any memory beyond the size of the structure that it + expects is zero (effectively checking that param_4 == 0). + - To cope with an older userspace program calling a newer kernel, the kernel + code can zero-extend a smaller instance of the structure (effectively + setting param_4 = 0). + +See perf_event_open(2) and the perf_copy_attr() function (in +kernel/events/core.c) for an example of this approach. + + +Designing the API: Other Considerations +--------------------------------------- + +If your new system call allows userspace to refer to a kernel object, it +should use a file descriptor as the handle for that object -- don't invent a +new type of userspace object handle when the kernel already has mechanisms and +well-defined semantics for using file descriptors. + +If your new xyzzy(2) system call does return 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 system call returns a new file descriptor, you should also consider +what it means to use the poll(2) family of system calls on that file +descriptor. Making a file descriptor ready for reading or writing is the +normal way for the kernel to indicate to userspace that an event has +occurred on the corresponding kernel object. + +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 privileged functionality, it needs +to be governed by the appropriate Linux capability bit (checked with a call to +capable()), as described in the capabilities(7) man page. Choose an existing +capability bit that governs related functionality, but try to 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 your new xyzzy(2) system call manipulates a process other than the calling +process, it should be restricted (using a call to ptrace_may_access()) so that +only a calling process with the same permissions as the target process, or +with the necessary capabilities, can manipulate the target process. + +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. (This concern does not apply if the arguments are part of a +structure that's passed in by pointer.) + + +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, Kconfig changes 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 in tools/testing/selftests/. + - A draft man-page for the new system call, either as plain text in the + cover letter, or as a patch to the (separate) man-pages repository. + +New system call proposals, like any change to the kernel's API, should always +be cc'ed to linux-api@vger.kernel.org. + + +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); + +Your new kernel functionality, and the system call that controls it, should +normally be optional, so add a CONFIG option (typically to init/Kconfig) for +it. As usual for new CONFIG options: + + - Include a description of the new functionality and system call controlled + by the option. + - Make the option depend on EXPERT if it should be hidden from normal users. + - Make any new source files implementing the function dependent on the CONFIG + option in the Makefile (e.g. "obj-$(CONFIG_XYZZY_SYSCALL) += xyzzy.c"). + - Double check that the kernel still builds with the new CONFIG option turned + off. + +To summarize, you need a commit that includes: + + - CONFIG option for the new function, normally in init/Kconfig + - 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 +stack the same and most of the registers the same 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/. + +For a new system call, there will obviously be no libc wrapper function and so +the test will need to invoke it using syscall(); also, if the system call +involves a new userspace-visible structure, the corresponding header will need +to be installed to compile the test. + +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. + +For more extensive and thorough testing of new functionality, you should also +consider adding tests to the Linux Test Project, or to the xfstests project +for filesystem-related changes. + - https://linux-test-project.github.io/ + - git://git.kernel.org/pub/scm/fs/xfs/xfstests-dev.git + + +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@vger.kernel.org +For more details, see https://www.kernel.org/doc/man-pages/patches.html + +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 + - Suggestion from Ingo Molnar that system calls that involve multiple + arguments should encapsulate those arguments in a struct, which includes a + size field for future extensibility: https://lkml.org/lkml/2015/7/30/117 + - 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 |