Adding a New System Call======================== This document describes what's involved in adding a new system call to theLinux kernel, over and above the normal submission advice inDocumentation/SubmittingPatches.  System Call Alternatives------------------------ The first thing to consider when adding a new system call is whether one ofthe alternatives might be suitable instead.  Although system calls are themost traditional and most obvious interaction points between userspace and thekernel, there are other possibilities -- choose what fits best for yourinterface.  - 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 supportedindefinitely.  As such, it's a very good idea to explicitly discuss theinterface on the kernel mailing list, and it's important to plan for futureextensions 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 -- solearn 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 preferredway to allow for future extensibility is to include a flags argument to thesystem call.  To make sure that userspace programs can safely use flagsbetween kernel versions, check whether the flags value holds any unknownflags, 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 structurethat is passed in by pointer.  Such a structure can cope with future extensionby 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 azero value gives the previous behaviour, then this allows both directions ofversion 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 (inkernel/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, itshould use a file descriptor as the handle for that object -- don't invent anew type of userspace object handle when the kernel already has mechanisms andwell-defined semantics for using file descriptors. If your new xyzzy(2) system call does return a new file descriptor, then theflags argument should include a value that is equivalent to setting O_CLOEXECon the new FD.  This makes it possible for userspace to close the timingwindow between xyzzy() and calling fcntl(fd, F_SETFD, FD_CLOEXEC), where anunexpected fork() and execve() in another thread could leak a descriptor tothe exec'ed program. (However, resist the temptation to re-use the actual valueof the O_CLOEXEC constant, as it is architecture-specific and is part of anumbering space of O_* flags that is fairly full.) If your system call returns a new file descriptor, you should also considerwhat it means to use the poll(2) family of system calls on that filedescriptor. Making a file descriptor ready for reading or writing is thenormal way for the kernel to indicate to userspace that an event hasoccurred 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 analready-opened file descriptor using the AT_EMPTY_PATH flag, effectively givingan 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) manpage; for an example of AT_EMPTY_PATH, see the fstatat(2) man page.) If your new xyzzy(2) system call involves a parameter describing an offsetwithin a file, make its type loff_t so that 64-bit offsets can be supportedeven on 32-bit architectures. If your new xyzzy(2) system call involves privileged functionality, it needsto be governed by the appropriate Linux capability bit (checked with a call tocapable()), as described in the capabilities(7) man page.  Choose an existingcapability bit that governs related functionality, but try to avoid combininglots of only vaguely related functions together under the same bit, as thisgoes against capabilities' purpose of splitting the power of root.  Inparticular, avoid adding new uses of the already overly-general CAP_SYS_ADMINcapability. If your new xyzzy(2) system call manipulates a process other than the callingprocess, it should be restricted (using a call to ptrace_may_access()) so thatonly a calling process with the same permissions as the target process, orwith the necessary capabilities, can manipulate the target process. Finally, be aware that some non-x86 architectures have an easier time ifsystem call parameters that are explicitly 64-bit fall on odd-numberedarguments (i.e. parameter 1, 3, 5), to allow use of contiguous pairs of 32-bitregisters.  (This concern does not apply if the arguments are part of astructure that's passed in by pointer.)  Proposing the API----------------- To make new system calls easy to review, it's best to divide up the patchsetinto separate chunks.  These should include at least the following items asdistinct 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 alwaysbe 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 calledsys_xyzzy(), but you add this entry point with the appropriateSYSCALL_DEFINEn() macro rather than explicitly.  The 'n' indicates the numberof arguments to the system call, and the macro takes the system call namefollowed by the (type, name) pairs for the parameters as arguments.  Usingthis macro allows metadata about the new system call to be made available forother tools. The new entry point also needs a corresponding function prototype, ininclude/linux/syscalls.h, marked as asmlinkage to match the way that systemcalls are invoked:     asmlinkage long sys_xyzzy(...); Some architectures (e.g. x86) have their own architecture-specific syscalltables, but several other architectures share a generic syscall table. Add yournew system call to the generic list by adding an entry to the list ininclude/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, andnote 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 systemcall, returning -ENOSYS.  Add your new system call here too:     cond_syscall(sys_xyzzy); Your new kernel functionality, and the system call that controls it, shouldnormally be optional, so add a CONFIG option (typically to init/Kconfig) forit. 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 themaster syscall tables.  Assuming your new system call isn't special in someway (see below), this involves a "common" entry (for x86_64 and x32) inarch/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 therelevant merge window.  Compatibility System Calls (Generic)------------------------------------ For most system calls the same 64-bit implementation can be invoked even whenthe userspace program is itself 32-bit; even if the system call's parametersinclude an explicit pointer, this is handled transparently. However, there are a couple of situations where a compatibility layer isneeded 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, andso needs to parse areas of (__user) memory that could hold either 32-bit or64-bit values.  In particular, this is needed whenever a system call argumentis:  - 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 thesystem call's arguments has a type that is explicitly 64-bit even on a 32-bitarchitecture, for example loff_t or __u64.  In this case, a value that arrivesat a 64-bit kernel from a 32-bit application will be split into two 32-bitvalues, 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 typedoes *not* need a compatibility layer; for example, splice(2)'s arguments oftype 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(), andis added with the COMPAT_SYSCALL_DEFINEn() macro, analogously toSYSCALL_DEFINEn.  This version of the implementation runs as part of a 64-bitkernel, but expects to receive 32-bit parameter values and does whatever isneeded to deal with them.  (Typically, the compat_sys_ version converts thevalues to 64-bit versions and either calls on to the sys_ version, or both ofthem call a common inner implementation function.) The compat entry point also needs a corresponding function prototype, ininclude/linux/compat.h, marked as asmlinkage to match the way that systemcalls are invoked:     asmlinkage long compat_sys_xyzzy(...); If the system call involves a structure that is laid out differently on 32-bitand 64-bit systems, say struct xyzzy_args, then the include/linux/compat.hheader file should also include a compat version of the structure (structcompat_xyzzy_args) where each variable-size field has the appropriate compat_type that corresponds to the type in struct xyzzy_args.  Thecompat_sys_xyzzy() routine can then use this compat_ structure to parse thearguments 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 compatversion; 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 extracolumn to indicate that a 32-bit userspace program running on a 64-bit kernelshould 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 ofthe new system call.  There's a choice here: the layout of the argumentsshould 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 isILP32, so the layout should match the 32-bit version, and the entry inarch/x86/entry/syscalls/syscall_64.tbl is split so that x32 programs hit thecompatibility 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 systemcall for the x32 ABI (and consequently the entry inarch/x86/entry/syscalls/syscall_64.tbl is unchanged). In either case, you should check that the types involved in your argumentlayout do indeed map exactly from x32 (-mx32) to either the 32-bit (-m32) or64-bit (-m64) equivalents.  System Calls Returning Elsewhere-------------------------------- For most system calls, once the system call is complete the user programcontinues exactly where it left off -- at the next instruction, with thestack 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 adifferent 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 tosave and restore additional registers to the kernel stack, allowing completecontrol of where and how execution continues after the system call. This is arch-specific, but typically involves defining assembly entry pointsthat save/restore additional registers and invoke the real system call entrypoint. For x86_64, this is implemented as a stub_xyzzy entry point inarch/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 normallycalled stub32_xyzzy and implemented in arch/x86/entry/entry_64_compat.S,with the corresponding syscall table adjustment inarch/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 thesystem call rather than the native 64-bit version.  Also, if the x32 ABIimplementation is not common with the x86_64 version, then its syscalltable 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 Linuxstill works -- its syscall table will reference stub_xyzzy, but the UML builddoesn't include arch/x86/entry/entry_64.S implementation (because UMLsimulates registers etc).  Fixing this is as simple as adding a #define toarch/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 theoccasional 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 -- specificallyfile open (open/openat), program execution (execve/exeveat) or socketmultiplexor (socketcall) operations. If your new system call is analogous toone of these, then the audit system should be updated. More generally, if there is an existing system call that is analogous to yournew system call, it's worth doing a kernel-wide grep for the existing systemcall to check there are no other special cases.  Testing------- A new system call should obviously be tested; it is also useful to providereviewers with a demonstration of how user space programs will use the systemcall.  A good way to combine these aims is to include a simple self-testprogram in a new directory under tools/testing/selftests/. For a new system call, there will obviously be no libc wrapper function and sothe test will need to invoke it using syscall(); also, if the system callinvolves a new userspace-visible structure, the corresponding header will needto be installed to compile the test. Make sure the selftest runs successfully on all supported architectures.  Forexample, 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 alsoconsider adding tests to the Linux Test Project, or to the xfstests projectfor 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 groffmarkup, but plain text will do.  If groff is used, it's helpful to include apre-rendered ASCII version of the man page in the cover email for thepatchset, for the convenience of reviewers. The man page should be cc'ed to linux-man@vger.kernel.orgFor 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/

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