Add implementations for some of the CloudABI file descriptor system calls.

[ed]

Summary

All of the CloudABI system calls that operate on file descriptors of an arbitrary type are prefixed with fd_. This change adds wrappers for these system calls around their FreeBSD equivalents.

The dup2() system call present on CloudABI deviates from POSIX, in the sense that it can only be used to replace existing file descriptor. It cannot be used to create new ones. The reason for this is that this is inherently thread-unsafe. Furthermore, there is no need on CloudABI to use fixed file descriptor numbers. File descriptors 0, 1 and 2 have no special meaning.

This change exposes the kern_dup() through <sys/syscallsubr.h> and puts the FDDUP_* flags in <sys/filedesc.h>. It then adds a new flag, FDDUP_MUSTREPLACE to force that file descriptors are replaced -- not allocated.

Test Plan

This change passes the CloudABI test suite.

[mjg]

I made a commit with part of the work concerning do_dup here: https://svnweb.freebsd.org/base/head/sys/kern/kern_descrip.c?r1=285321&r2=285320&pathrev=285321

To quote the commit message:

• rename it [do_dup] to kern_dup.
• prefix flags with FD
• assert that correct flags were passed

sys/kern/kern_descrip.c

838 ↗

(On Diff #6814)

The only caller passes DUP_FIXED | DUP_REPLACE. I don't object, but in this case the code should assert that DUP_FIXED was also provided.

I would prefer if DUP_REPLACE was made a conflicting flag with _FIXED instead. Maybe renaming it to _MUSTREPLACE (or compatible) would express the intent better. Then assert that at most one is provided (e.g. MPASS((flags & (FDDUP_FIXED | FDDUP_MUSTREPLACE)) != (FDDUP_FIXED | FDDUP_MUSTREPLACE))).

sys/sys/syscallsubr.h

263 ↗

(On Diff #6814)

This file pretty much lacks other macros. I don't mind them landing here, but I'm quite sure someone will object and I won't defend it :)

I suggest moving these macros to sys/filedesc.h instead, at least for the time being.

[ed]

Thanks for the review!

sys/kern/kern_descrip.c

830 ↗

(On Diff #6823)

Sounds perfectly reasonable! I've renamed the flag to FDDUP_MUSTREPLACE and it is now independent of FDDUP_FIXED.

sys/sys/syscallsubr.h

91 ↗

(On Diff #6823)

Done!

[mjg]

ACK in general

You missed the assertion. MPASS((flags & ~(FDDUP_FIXED | FDDUP_FCNTL | FDDUP_CLOEXEC)) == 0); now needs to be extended with FDDUP_MUSTREPLACE, but there is no need to upload an updated patch here.

[ed]

In D3035#59963, @mjg wrote:

ACK in general

You missed the assertion. MPASS((flags & ~(FDDUP_FIXED | FDDUP_FCNTL | FDDUP_CLOEXEC)) == 0); now needs to be extended with FDDUP_MUSTREPLACE, but there is no need to upload an updated patch here.

Whoops. Fixed!