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+Adding a New System Call
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+========================
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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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+Documentation/SubmittingPatches.
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+
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+
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+System Call Alternatives
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+------------------------
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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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+
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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 read(2)/write(2)-like operations
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+ have to be implemented as ioctl(2) requests, which can lead to a
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+ 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 be
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+ 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 fcntl(2) command option may be more appropriate. However,
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+ 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 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 prctl(2) command option may be more appropriate. As with
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+ fcntl(2), this system call is a complicated multiplexor so is best reserved
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+ for near-analogs of existing prctl() commands or getting/setting a simple
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+ flag related to a process.
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+
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+
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+Designing the API: Planning for Extension
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+-----------------------------------------
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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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+
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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 -- eventfd/eventfd2,
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+dup2/dup3, inotify_init/inotify_init1, 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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+
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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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+
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+ if (flags & ~(THING_FLAG1 | THING_FLAG2 | THING_FLAG3))
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+ return -EINVAL;
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+
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+(If no flags values are used yet, check that the flags argument is zero.)
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+
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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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+
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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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+
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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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+
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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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+
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+See 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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+
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+
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+Designing the API: Other Considerations
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+---------------------------------------
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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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+
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+If your new xyzzy(2) system call does return a new file descriptor, then the
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+flags argument should include a value that is equivalent to setting O_CLOEXEC
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+on the new FD. This makes it possible for userspace to close the timing
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+window between xyzzy() and calling fcntl(fd, F_SETFD, FD_CLOEXEC), where an
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+unexpected fork() and 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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+
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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 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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+
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+If your new xyzzy(2) system call involves a filename argument:
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+
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+ int sys_xyzzy(const char __user *path, ..., unsigned int flags);
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+
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+you should also consider whether an xyzzyat(2) version is more appropriate:
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+
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+ int sys_xyzzyat(int dfd, const char __user *path, ..., unsigned int flags);
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+
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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 giving
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+an fxyzzy(3) operation for free:
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+
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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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+
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+(For more details on the rationale of the *at() calls, see the openat(2) man
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+page; for an example of AT_EMPTY_PATH, see the statat(2) man page.)
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+
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+If your new xyzzy(2) system call involves a parameter describing an offset
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+within a file, make its type loff_t so that 64-bit offsets can be supported
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+even on 32-bit architectures.
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+
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+If your new xyzzy(2) system call involves privileged functionality, it needs
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+to be governed by the appropriate Linux capability bit (checked with a call to
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+capable()), as described in the capabilities(7) man page. Choose an existing
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+capability bit that governs related functionality, but try to avoid combining
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+lots of only vaguely related functions together under the same bit, as this
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+goes against capabilities' purpose of splitting the power of root. In
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+particular, avoid adding new uses of the already overly-general CAP_SYS_ADMIN
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+capability.
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+
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+If your new xyzzy(2) system call manipulates a process other than the calling
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+process, it should be restricted (using a call to ptrace_may_access()) so that
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+only a calling process with the same permissions as the target process, or
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+with the necessary capabilities, can manipulate the target process.
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+
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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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+
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+
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+Proposing the API
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+-----------------
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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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+
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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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+
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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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+
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+
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+Generic System Call Implementation
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+----------------------------------
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+
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+The main entry point for your new 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 number
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+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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+
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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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+
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+ asmlinkage long sys_xyzzy(...);
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+
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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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+
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+ #define __NR_xyzzy 292
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+ __SYSCALL(__NR_xyzzy, sys_xyzzy)
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+
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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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+
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+The file kernel/sys_ni.c provides a fallback stub implementation of each system
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+call, returning -ENOSYS. Add your new system call here too:
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+
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+ cond_syscall(sys_xyzzy);
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+
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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 init/Kconfig) for
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+it. As usual for new CONFIG options:
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+
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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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+
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+To summarize, you need a commit that includes:
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+
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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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+
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+
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+x86 System Call Implementation
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+------------------------------
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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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+
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+ 333 common xyzzy sys_xyzzy
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+
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+and an "i386" entry in arch/x86/entry/syscalls/syscall_32.tbl:
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+
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+ 380 i386 xyzzy sys_xyzzy
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+
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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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+
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+
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+Compatibility System Calls (Generic)
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+------------------------------------
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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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+
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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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+
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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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+
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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, long, ...)
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+ - a pointer to a struct containing a varying sized integral type.
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+
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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 arrives
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+at a 64-bit kernel from a 32-bit application will be split into two 32-bit
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+values, which then need to be re-assembled in the compatibility layer.
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+
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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, 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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+
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+The compatibility version of the system call is called compat_sys_xyzzy(), and
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+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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+
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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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+
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+ asmlinkage long compat_sys_xyzzy(...);
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+
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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 compat_
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+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 parse the
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+arguments from a 32-bit invocation.
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+
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+For example, if there are fields:
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+
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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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+
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+in struct xyzzy_args, then struct compat_xyzzy_args would have:
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+
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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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+
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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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+
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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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+
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+To summarize, you need:
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+
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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 include/uapi/asm-generic/unistd.h
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+
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+
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+Compatibility System Calls (x86)
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+--------------------------------
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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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+
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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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+
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+ 380 i386 xyzzy sys_xyzzy compat_sys_xyzzy
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+
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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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+
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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 the
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+compatibility wrapper:
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+
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+ 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
|
Linus Torvalds