*at family of syscalls in FreeBSD

[Roman Divacky]

Hi,

Starting from Linux 2.6.16, there is available so called *at syscalls. For example
openat(), linkat() etc. Those syscalls are used to avoid races in threaded programs
and to implement per-thread CWD. in other words they are usefull. Thats why Linux
implemented them.

I am currently implementing those in our Linuxulator. As suggested by Robert Watson
I implemented general kern_fooat() functions and wrapped them around those kern_fooat()
functions. It works ok and everything. But I want to introduce native *at syscalls
for FreeBSD binaries. Hence I am here to discuss the API.

My suggestion:

use Linux API with some slight naming changes:

syscalls:
	
	openat(), mkdirat(), mknodat(), chownat(), utimesat(), statat(), unlinkat(), renameat(),
	linkat(), symlinkat(), readlinkat(), chmodat(), accessat().

example of a syscall:

	int openat(int dirfd, char *path, int flags, int mode);

ie. exactly the same API as Linux have with the exception of naming them sanely, ie. instead
for example fchownat() we have chownat() because it does not operate on a fd but on a path.
I am not sure about compatibility but we can always introduce a weak reference like fchownat() ->
chownat().
   
I want to have special AT_FDCWD -100 define for "cwd" argument, just like linux.

I am currently finishing my linuxulator-side work and I'd like to see this in 7.0R so please
comment on the API and the idea of introducing those syscalls in FreeBSD.

thank you

Roman Divacky