Implement the basic system calls that operate on pathnames.

[ed]

Summary

Unlike FreeBSD, CloudABI does not use null terminated strings for its
pathnames. Introduce a function called copyin_path() that can be used by
all of the filesystem system calls that use pathnames. This change
already implements the system calls that don't depend on any additional
functionality (e.g., conversion of struct stat).

Also implement the socket system calls that operate on pathnames, namely
the ones used by the C library functions bindat() and connectat(). These
don't receive a 'struct sockaddr_un', but just the pathname, meaning
they could be implemented in such a way that they don't depend on the
size of sun_path. For now, just use the existing interfaces.

Add a missing #include to cloudabi_syscalldefs.h to get this code to
build, as one of its macros depends on UINT64_C().

Test Plan

These implementations have already been tested in the CloudABI branch on
GitHub. They pass all of the tests.

[kib]

sys/compat/cloudabi/cloudabi_file.c

66 ↗

(On Diff #6967)

This results in double-copying of the input string, first to the malloced area, then, from the area to the namei structure pbuf.

I am not sure how invasive would be a change to the vfs_lookup.c, but note that both cn and ni already carry the length of the strings.

[ed]

sys/compat/cloudabi/cloudabi_file.c

66 ↗

(On Diff #6967)

Exactly. I'm not happy with this approach. For what it's worth, on NetBSD I was able to do this slightly easier, because they have a structure (pathbuf, IIRC?) that you can pass in, instead of the {stringptr,uioseg} pair that we use right now.

Shall I leave it the way it is for now, but add a TODO to the comment above?

I was just brainstorming a bit: instead of going into NetBSD's direction of adding a struct pathbuf, maybe we could reuse struct uio for this purpose? As in, generalize struct uio to allow both sized and null-terminated buffers and let kern_...() use those instead. Or maybe I'm overthinking things.

[ed]

Add a comment describing that there's room for improvement.

[ed]

sys/compat/cloudabi/cloudabi_file.c

66 ↗

(On Diff #6967)

I've added a TODO() to the comment.