Adding Syscalls To FreeBSD

The process of adding syscalls to FreeBSD has slowly grown more complex over the years. New features such as audit have added extra fields to the syscalls.master files, symbol versioning added another file to edit in src/lib/libc, and 32-bit compatibility for 64-bit versions requires two declarations of each syscall and translation of some arguments.

NOTE WELL: Adding new syscalls is not to be done lightly. Once we have shipped a syscall in a release, we are generally stuck with it or at least with something that implements the same interface forever. When in doubt, request and obtain review from senior developers!

WARNING: Incorrectly implemented system calls may present serious risks to system stability and security. This document does not address many issues that must be considered when adding any new kernel interface and should not be considered complete.

BSDCan 2022 Guide

Consider watching So you want to add a system call by Brooks Davis for an extensive guide on adding FreeBSD syscalls in 2022. Much of this material is also presented as a paper at AsiaBSDCon 2023 so-you-want-to-add-a-system-call.pdf

Differences from the Guide

In FreeBSD 15 or later symbols are now defined in sys/lib/libsys/ The _ and __sys_ prefixed symbols are now exported automatically.

Registering a syscall

Syscalls are declared in sys/kern/syscalls.master with entries like:

485     AUE_NULL        STD {
                int cpuset_setid(
                    cpuwhich_t which,
                    id_t id,
                    cpusetid_t setid

The format of this line is documented at the top of the file. In general, all new syscalls should be of type STD or NOSTD. The function declaration part contains the prototype of the syscall as seen from userspace. This prototype must also be declared somewhere so userland code can call it, typically in a header under sys/sys/. In this case, it is declared in sys/sys/cpuset.h.

After adding an entry to sys/kern/syscalls.master, you must regenerate the generated files in sys/kern and sys/sys:

$ make sysent
make -C /usr/src/sys/kern sysent
/usr/libexec/flua /usr/src/sys/tools/makesyscalls.lua syscalls.master
make -C /usr/src/sys/compat/freebsd32 sysent
/usr/libexec/flua /usr/src/sys/tools/makesyscalls.lua syscalls.master syscalls.conf

Implementing a syscall

The actual system calls of type STD are implemented by a function with a prototype of the form:

/* XXX: Padding members removed for clarity.  See sysproto.h for details. */
struct cpuset_setid_args {
        cpuwhich_t which;
        id_t id;
        cpusetid_t setid;
/* XXX: variable names below are typical, but are not defined in sysproto.h. */
int     cpuset_setid(struct thread *td, struct cpuset_setid_args *uap);

The return value of this function becomes the value of errno in userspace and if non-zero the userspace function returns -1. Explicit return value can be set in td->td_retval[0]. In the cpuset_setid case, this function resides in sys/kern/kern_cpuset.c.


Security event auditing allows fine-grained logging of security-relevant events in the operating system, and is described by the CAPP common criteria protection profile. Detailed information on configuring audit can be found in the FreeBSD Handbook, and an implementation paper can be found on the TrustedBSD website

Most system calls are considered security-relevant, as they frequently involve access control checks, communication between processes, or system configuration, and therefore almost all system calls will be audited. You can find more detailed information on the AddingAuditEvents page. Please do not forget to add appropriate auditing to your new system call, be it a native call or a compat one!

Please e-mail the trustedbsd-audit mailing list, or RobertWatson, if you require help. The audit event identifier space is managed by the OpenBSM project, and you will need to request assignment of a new identifier if an existing one isn't a good match.


If the new syscall is safe for use in Capsicum capability mode, add it to sys/kern/capabilities.conf. Safe syscalls must not provide access to global namespaces such as the process ID namespace or the filesystem namespace (*at calls relative to a directory are OK), and must check capability rights when using file descriptors.

Userspace considerations

In addition to adding the syscall to the kernel's table the symbol must be added to the symbol map in libc. This is done by adding entries to sys/lib/libsys/ (in FreeBSD 14 or less src/lib/libc/sys/ Each syscall results in three symbols. In the example of cpuset_setid, they are cpuset_setid, _cpuset_setid, and __sys_cpuset_setid. The plain cpuset_setid symbol must be added to the most recent namespace map (which is currently FBSD_1.8 for new syscalls introduced in 15.0-CURRENT but can change later) as per The _cpuset_setid and __sys_cpuset_setid symbols other two are only for internal use and should not be in unless libthr (or some other core library) needs them. If they are added, place them in the FBSDprivate_1.0 map.

Add syscall prototype to the user-visible portion of appropriate header, hiding it under the proper visibility check. For instance, a non-standard syscall prototype typically needs #if __BSD_VISIBLE/#endif visibility control.

Add man page for the syscall, at lib/libc/sys/YOURSYSCALL.2, and connect it to the build in lib/libc/sys/

Sometimes raw kernel syscalls interfaces cannot be used directly and require 'tiny .c wrappers' in userspace. The wrappers are typically used for one of two purposes:

  1. Convert raw kernel interface into something expected by userspace, often this coversion uses more generic and non-standard interface to implement more usual function. Examples are open(2) or waitid(2) which are really tiny wrappers around openat(2) and wait6(2) in today libc.

  2. Allow libthr to hook into libc to provide additional services. Libthr often has to modify semantic of raw syscall, for instance to implement cancellation, and libc contains the tables redirecting call to the runtime-selected implementation. The tables are patched on libthr load. Since tables must fill entries with some address in case libthr is not loaded, tiny functions which wrap syscalls are created for use in that tables.

Adding 32-bit compatibility

In addition to the main implementation, all new syscalls must include a 32-bit compatibility definition. For syscalls where all arguments are of identical size and layout or where arguments are 32-bit on 32-bit systems and 64-bit on 64-bit systems such as pointers, long ints, or size_t variables, it is sufficient to add a definition of type NOPROTO to sys/compat/freebsd32/syscalls.master:

484     AUE_NULL        NOPROTO { int cpuset(cpusetid_t *setid); }

As with sys/kern/syscalls.master, files in sys/compat/freebsd32 need to be regenerated after a change to sys/compat/freebsd32/syscalls.master:

$ make sysent
make -C /home/bed22/git/freebsd/sys/kern sysent
make -C /home/bed22/git/freebsd/sys/compat/freebsd32 sysent
/usr/libexec/flua /home/bed22/git/freebsd/sys/tools/makesyscalls.lua syscalls.master syscalls.conf

In other cases, things are more complex. Two key cases are 64-bit arguments such as off_t and id_t and pointers to entries of different sizes and/or layouts such as size_t or struct statfs. In the former case, 64-bit arguments are split into two arguments when actually passed and on non-i386 architectures they must be evenly aligned. As a result, the function needs to be defined with appropriate arguments in syscalls.master and use an implementation that stitches the pieces back together. For example, this is the sys/compat/freebsd32/syscalls.master definition for cpuset_setid:

485     AUE_NULL        STD     { int freebsd32_cpuset_setid(cpuwhich_t which, \
                                    int pad, \
                                    uint32_t id1, uint32_t id2, \
                                    cpusetid_t setid); }
485     AUE_NULL        STD     { int freebsd32_cpuset_setid(cpuwhich_t which, \
                                    uint32_t id1, uint32_t id2, \
                                    cpusetid_t setid); }


and this is the implementation in sys/compat/freebsd32/freebsd32_misc.c:

freebsd32_cpuset_setid(struct thread *td,
    struct freebsd32_cpuset_setid_args *uap)
        return (kern_cpuset_setid(td, uap->which,
            PAIR32TO64(id_t, uap->id), uap->setid));

In the case of pointers to differently sized or aligned arguments, things get more complex. Calls to copyin and copyout must be altered so that correctly formatted data is passed to the primary implementation function and returned to userspace. In this case, a kern_syscall function is usually defined which is called to do the actual work by both the normal and 32-bit compatibility interface functions which simply handle marshaling of data. For example, let us consider a hypothetical syscall foo which takes a pointer to a buffer and a pointer to the size of the buffer and copies data to the buffer and the amount of data copied to the size pointer. The declaration of foo in sys/kern/syscalls.master would look like:

666     AUE_NULL        STD     { int foo(char *buf, size_t *bufsize); }

The primary implementation would look like (ignoring kern_foo() since we don't care what it does:

sys_foo(struct thread *td, struct foo_args *uap)
        int error;
        size_t bufsize;

        if ((error = copyin(uap->bufsize, &bufsize, sizeof(bufsize))) != 0)

        if ((error = kern_foo(td, uap->buf, &bufsize)) != 0)

        return (copyout(&bufsize, uap->bufsize, sizeof(bufsize)));

In sys/compat/freebsd32/syscalls.master we would need a somewhat different definition:

666     AUE_NULL        STD     { int freebsd32_foo(char *buf, \
                                    uint32_t *bufsize); }

Likewise, the implementation is a little more complex:

freebsd32_foo(struct thread *td, struct foo_args *uap)
        int error;
        uint32_t bufsize32
        size_t bufsize;

        if ((error = copyin(uap->bufsize, &bufsize32, sizeof(bufsize32))) != 0)
        bufsize = bufsize32;

        if ((error = kern_foo(td, uap->buf, &bufsize)) != 0)

        bufsize32 = bufsize;
        return (copyout(&bufsize32, uap->bufsize, sizeof(bufsize32)));

Testing 32-bit compatibility

32-bit compatibility is somewhat tricky so it's important to test it. If you have access to an amd64 machine, this is easy if you have a utility in the base use can use for testing. Just get your new code up and running normally then buildworld with TARGET=i386. You can then grab the executables you need from /usr/obj and run your tests with them.

Backward compatibily

The backward compatibility, i.e. ability to run older binaries on newer kernels, is required. Each case where a decision is made to abandon backward compatibility, requires coordination with re and perhaps buy-in from the commmunity.

The question of backward compatibility typically arises when either semantic or call syntax of existing syscall are changed. Such changes require allocation of new syscall number, while leaving old syscall as is. Old syscall can be marked with compatX, and its implementation wrapped into #ifdef COMPAT_FREEBSDX/#endif braces to save some space for people who know for sure to not need backward compatibility. Old syscall symbol must have same symbol version in libc as it has before the new syscall addition. Handle this in lib/libc/include/compat.h, where the __sym_compat() macro provides a wrapper around the versioning mechanism and is included into all generated syscall stubs.

Forward compatibility

Since it is fairly common to run a new userland with a somewhat older kernel (for example, the ports building cluster often does this), it is recommended to make this work. Running new binaries on old kernels is termed forward compatibility. Increment __FreeBSD_version in sys/sys/param.h and only call the new syscall if __FreeBSD_version is high enough. This check should be in a single place in libc or libthr (for example, override the syscall's symbol with a C function that does the check and calls the __sys_ version if the syscall is available), so it does not spread to everywhere that uses the new call. If your new syscall is used in bootstrap tools, additional work is required since these tools must build on older versions.

If forward compatibility is not possible or incomplete, coordination with portmgr@ and an UPDATING entry are likely required.

Keep in mind that ports may automatically detect and start using new syscalls at build time.

An example of forward compatibility can be found in revision 277610.

Generally speaking, if the new system call is used in the install world or pkg(8) path, that's a good candidate to consider doing forward compat shims for. If it is an obscure area that might be rarely used by most simple uses of the system, compat shims aren't necessary and the ENOSYS / SIGSYS behavior is enough to force the upgrade.

This project requires backward compatibility (running old binaries on new kernels) when adding a new system call. The old interface should be placed under COMPAT_FREEBSDxx (xx is the major version one before where the new API is introduced) so they can be compiled out of the kernel. The project doesn't require forward compatibility (the UPDATING procedures are written so that it isn't required). However, it is allowed at the discretion of the new system call author and is a good idea if a install world w/o a new kernel will result in an unusable system.


Syscalls may be committed in one or two steps. Historically we have committed them separately as the generated files contained $FreeBSD strings from committed files. This is no longer the case and committing in one or two steps is a matter of personal preference.


syzkaller is a coverage-guided kernel fuzzer which works primarily by fuzzing the system call interface. It has found many bugs over the years and is excellent at catching non-trivial bugs, especially in conjunction with kernel sanitizers. Google maintains a public syzkaller instance which regularly pulls the latest FreeBSD sources, builds them, and tests the result. Bugs causing a panic are detected and reported to a mailing list.

syzkaller's effectiveness is improved when it has semantic knowledge of system call argument types. It thus implements a DSL, syzlang, for describing system call interfaces (including multiplexed system calls such as ioctl(2) and setsockopt(2)). When committing new system calls, it is highly recommended to commit syzlang definitions for the new interfaces to the syzkaller repository. The existing definitions provide lots of examples to draw from; feel free to ask for help from markj@ or tuexen@.


Some time prior to an MFC, you should add stub entries for your syscalls to ease rollback. This can help prevent problems for users who upgrade, install applications that use the new system call, and then downgrade their kernels which results in those applications being killed with SIGSYS. If the new feature is used by login or sh this can be very bad. An example of adding such stubs can be found in revision 156791.

AddingSyscalls (last edited 2024-02-09T00:49:41+0000 by BrooksDavis)