Add intrinsics.syscall_bsd

[Feoramund]

I'm working on porting core:net to FreeBSD, and I ran into this predicament where syscall was returning only positive numbers, which isn't too helpful for detecting errors. I know very little about how LLVM works; feedback's appreciated. I used the rcx register to do the negation since it's already clobbered.

[Yawning]

You don't need to save/restore RFLAGS, because the assembly snippet already specifies that it's clobbered, and "Odin's syscall intrinsic will always return -errno on failure" makes preserving CF non-important. Otherwise this looks fine, apologies for being lazy when I dealt with this in the past.

[andreas-jonsson]

Hi @Feoramund, I have been working on a NetBSD port of Odin. I'm currently missing core:net just like the other BSD's.
Let me know if you would like to get some help testing or collaborating on that. I imagine the code would look and work very similar on NetBSD.

[Feoramund]

You don't need to save/restore RFLAGS, because the assembly snippet already specifies that it's clobbered, and "Odin's syscall intrinsic will always return -errno on failure" makes preserving CF non-important. Otherwise this looks fine, apologies for being lazy when I dealt with this in the past.

I found a way to negate the return value without needing to save/reload CF. The not and inc instructions don't touch CF, so I used those to construct the negative value.

Hi @Feoramund, I have been working on a NetBSD port of Odin. I'm currently missing core:net just like the other BSD's. Let me know if you would like to get some help testing or collaborating on that. I imagine the code would look and work very similar on NetBSD.

I have an echo server working off of syscalls right now, so hopefully won't be much longer before I can plug it all into an OS-specific file for core:net and put up a PR for testing. Any extra help poking around would be greatly appreciated.

[Feoramund]

I've hit a snag. While implementing fcntl, I found this:

     F_GETOWN          Get the process ID or process group currently receiving
                       SIGIO and SIGURG signals; process groups are returned
                       as negative values (arg is ignored).

[...] FCNTL(2)

     In addition, if fd refers to a descriptor open on a terminal device (as
     opposed to a descriptor open on a socket), a cmd of F_SETOWN can fail for
     the same reasons as in tcsetpgrp(3), and a cmd of F_GETOWN for the
     reasons as stated in tcgetpgrp(3).

[...] TCGETPGRP(3)

ERRORS
     If an error occurs, tcgetpgrp() returns -1 and the global variable errno
     is set to indicate the error, as follows:

     [EBADF]            The fd argument is not a valid file descriptor.

     [ENOTTY]           The calling process does not have a controlling
                        terminal or the underlying terminal device represented
                        by fd is not the controlling terminal.

Based on this, it's possible for fcntl to return a negative non-error value. I had concerns about this being possible, but I wasn't sure if or where it would show up.

In this one instance, if that value never overlaps with any of the possible error values, then it's not a big concern, however I think it'd be better to solve this more cleanly. I may need to re-think this. I'm wondering if a separate FreeBSD-specific implementation of syscall with a different signature would be for the best. I.e. syscall :: proc(id: uintptr, args: ..uintptr) -> (uintptr, bool) ---

I'll have to figure out how to do that.

EDIT: While reading through the source code of the compiler, I had an idea. Perhaps it might be more useful to implement an intrinsic that returns the state of the flags. I was able to write one pretty quickly.

[Feoramund]

I figured out how to modify syscall to have an alternative signature and return a boolean for the error status on FreeBSD, both for amd64 and arm64 which are considered the FreeBSD tier 1 platforms.

[andreas-jonsson]

I figured out how to modify syscall to have an alternative signature and return a boolean for the error status on FreeBSD

Does this make sense to adopt on other platforms as well? I always assumed it never overlapped with error values.
Should this not be an issue on all Unix systems?

[Yawning]

I figured out how to modify syscall to have an alternative signature and return a boolean for the error status on FreeBSD

Does this make sense to adopt on other platforms as well? I always assumed it never overlapped with error values. Should this not be an issue on all Unix systems?

There is no way to distinguish certain cases of overlap on Linux (depending on architecture), so the workarounds are "Linux-isms at the syscall interface layer" (F_GETOWN_EX). Some architectures do also pass a boolean back (sparc, ppc64), but nothing that Odin currently supports has this extension.

Per the documentation:

A limitation of the Linux system call conventions on some architectures
(notably i386) means that if a (negative) process group ID to be re‐
turned by F_GETOWN falls in the range -1 to -4095, then the return value
is wrongly interpreted by glibc as an error in the system call; that is,
the return value of fcntl() will be -1, and errno will contain the (pos‐
itive) process group ID. The Linux-specific F_GETOWN_EX operation
avoids this problem. Since glibc 2.11, glibc makes the kernel F_GETOWN
problem invisible by implementing F_GETOWN using F_GETOWN_EX.