Guess they finally saw sense and realized that the when platforms say the syscall ABI is unstable, it really is unstable…
Usually the only time the SYSCALL ABI breaks, it's because kernel authors intentionally chose to do it for no apparent reason. For example, OpenBSD at some point changed how mmap() was defined so that it takes seven arguments:
void *sys_mmap(void *addr, size_t len, int prot,
int flags, int fd, long pad, off_t pos);
I work on a project called Cosmopolitan Libc which lets you create static binaries that just work on Linux + Mac + Windows + FreeBSD + OpenBSD. It was only possible to do this because Unix systems generally agree on definitions. I worked really hard to support OpenBSD since I believe in the project. I just hope they keep the ABI stable going forward.
For example, I'm really happy that this restriction only applies to dynamic binaries. It seems perfectly reasonable that they'd want to make the assumption that if a program chooses to link OpenBSD's Libc that it intends to use it. That's fine just so long as we continue having the ability to build static binaries with an alternative cross-platform Libc like Cosmopolitan.
Speaking of which, I think I might actually implement some of OpenBSD's ideas in Cosmopolitan. I could probably track down all the functions that need raw SYSCALL and use __section__ so they're all linked to the same part of the binary and then call the msyscall() function to limit it just to that page. That way as a guest libc author I'm upholding the spirit of the intent. When in Rome do as the Romans.
That's a really weird way to phrase it. Apple says that interface is unstable. Go uses it anyway. The unstable interface turns out to be unstable.
It wasn't Apple that broke Go binaries. It was Go.
Windows never had a stable syscall ABI either.
Almost only Linux does it... because the kernel and libc are maintained as separate projects in the Linux world.
That's because Linux is just that: a kernel. And people are free to build their userspace on top of it.
Practically as long as it's trivially callable from C I'm not bothered.
It seems like that was the case from day one, when a copy of the NetBSD code was imported to later become OpenBSD[1]. And if my reasoning is correct, saying that OpenBSD “changed it at some point” is not quite correct.
https://github.com/openbsd/src/blob/df930be708d50e9715f173ca...
return((caddr_t)(long)__syscall((quad_t)SYS_mmap, addr, len, prot,
flags, fd, 0, offset));
The main advantage of not having a stable kernel ABI is that you have the freedom to change it for security/performance reasons. This means the OS can change how they implement things over time without breaking things as easily. MS is notorious for this and for using this fact to allow them to 'emulate' older ways of doing things even when the real implementation has long since moved on.
The main advantage of having a stable kernel ABI is that the userspace can be whatever the developer wants it to be realistically. If they want it to be just a single GO program and literally nothing else they can do that (routers are a good example of devices that do this).
Unix systems have never had that restriction. Because if you use the official libc and pass -static to gcc then that means the "stable api" creates a binary that depends on the kernel abi. If the kernel authors break the abi then it means you need to recompile all your software in order to upgrade.
Yes they do.
> Only Windows forbids developers from linking static binaries. That's because they change the SYSCALL ordinals every few months.
That Windows somewhat actively precludes raw syscalls doesn’t mean they are supported elsewhere. It’s always been bsd (and especially macOS) policy that syscalls are an implementation detail.
> Because if you use the official libc and pass -static to gcc then that means the "stable api" creates a binary that depends on the kernel abi.
Try doing that on macOS, you’ll find out that there is no static version of libSystem, or crt0.
As far as I can tell it might be used to align the memory layout of the following 64 bit arguments to 64 bits. Or at least ensure that there is no auto generated padding that might contain random values.
Edit: The link provided by ainar-g 3 mentions that the way gcc handled padding of the offset field changed between gcc 1 and 2.
There is a way out. I've previously proposed on LKML that the Linux kernel team provide an official userspace system call library that sits below libc and that all libc implementations would share. We'd forbid new system calls being called except through this library. Optionally, we'd enforce this constraint on all system calls.
This is how Fuchsia works, by the way: all Fuchsia system calls must go through one giant vDSO.
Maybe my impression was wrong, but it was my understanding that Go originally preferred direct syscalls because of stack management headaches. You can't know how much stack space a libc syscall wrapper requires, which even for seemingly simple syscalls can be quite complex--e.g. glibc has to emulate POSIX thread semantics. OTOH, treating such libc wrappers like regular C FFI functions would obliterate the design and implementation assumptions around goroutine stack management. Considering that Linux was originally the first (and, let's be honest, only real target), it made perfect sense to rely on Linux syscall ABI promises.
Fast-forward a few years: 1) Go has a more mature binary format and dynamic linking capabilities, shrinking the gap between Go's internal ABI and the native libc ABI. 2) Goroutine stacks switched from split-stacks to movable stacks, and the minimum stack size became larger. 3) Demand and motivation for supporting libc wrappers (i.e. for Windows) grows. Result: Go surmounts one of its original simplifying design compromises. Though, I would assume that libc wrapper support still incurs ongoing maintenance costs on each platform; namely, managing the minimum stack requirement for each particular call, which could change overtime, while it's important not to be too pessimistic so that a syscall doesn't force an unnecessary stack resize.
It sits so close to the kernel that the concerns of changing semantics apply as easily to the kernel as they do to the system call wrappers.
Dynamically link against libc and statically link the rest for all I care, but there's no reason not to talk to libc.
Also: the vDSO is also a form of dynamic linking. Are you opposed to the vDSO?
I'm not opposed to vDSO but I disagree with how Linux maps it into memory by default. Linux should not be putting anything into the address space that the executable does not specify. MMUs are designed to give each process its own address space. Operating systems that violate that assumption are leaky abstractions imposing requirements where they shouldn't.
The main thing dynamic shared objects accomplish is giving upstream dependencies leverage over your software. They have a long history of being mandated by legal requirements such as LGPL and Microsoft EULAs. It's nice to have the freedom to not link the things.
Other people have made software for decades without writing program-specific libc instances. Tell me you at least started with something decent like musl instead of literally writing your own libc from printf on up.
> Linux should not be putting anything into the address space that the executable does not specify
Execution has to start somewhere, and kernels have often reserved parts of the system address space for themselves.
> The main thing dynamic shared objects accomplish is giving upstream dependencies leverage over your software.
Loose binding in interfaces allows systems on both sides of the interface to evolve. If you want 100% complete control over your system for some reason instead of writing programs that play well with others, just ship your thing as a VM image and be done with it.
Trapping (SYSCALL/INT) is a loose binding. The kernel can evolve all it wants internally. It can introduce new ABIs. Processes are also a loose binding. I think communicating with other tools via pipes and sockets is a fantastic model of cooperation. Same goes for vendoring with static linking. Does that mean I'm going to voluntarily load Linux distro DSOs into my address space? Never again. Programs that do that, won't have a future outside Docker containers.
Also, my executables are VM images. They can boot on metal too. Read https://justine.lol/ape.html and https://github.com/jart/cosmopolitan/issues/20#issuecomment-... Except unlike a Docker distro container, my exes are more on the order of 16kb in size. That's how fat an executable needs to be, in order to run on six different operating systems and boot from bios too.
Strong claim. Wrong, but strong claim.
The completely-statically-linked model you're proposing might be acceptable on servers, but on mobile and embedded devices like Android, it's a showstopper: without zygote pre-initialization and DSO page-sharing, Android apps would each be at least 3MB heavier than they are today and take about 1000ms longer to start --- and a typical Android device has a lot of these processes running.
More broadly, yes, in most contexts, I see a general trend away from elaborate code-sharing schemes and towards "island universe" programs that vendor everything. But these universes need to interact with their host system using a stable ABI somehow, I believe that SYSCALL is fundamentally the wrong layer for this interaction, as it's not flexible enough. For example, the Linux gettimeofday() optimization couldn't have been done without the ability to give Linux programs userspace code to run pre-kernel via the vDSO. How do you propose the kernel do things like vDSO gettimeofday optimizations?
Doesn't everything on Android start off with the JVM as a dependency? In that case the freedom to not use DSOs is something that Google has already taken away from you. That's not a platform I'd choose to develop for unless I was being paid to do it.
On x86 RDTSC returns invariant timestamps so you technically don't need shared memory to get nanosecond precision timestamps. XNU does the same thing and they don't call it a DSO. Because that's just shared memory. I have nothing against shared memory.