*at family of syscalls in FreeBSD

[Eric Lemar]

I'm definitely a fan of this API.  Aside from the general thread-related
utility of this API, it provides a reasonable API for accessing
windows-style ADS streams (subfiles) on a filesystem that supports them
and is becoming reasonably cross-platform.  This lets you handle things
like ADS hanging off directories in a comparatively sane manner.

We've actually implemented a subset of these syscalls in-house (Isilon)
for use with our filesystem, largely for the ADS-related functionality.
Generally speaking, in our tree most of the traditional non-'at' syscalls
are just small kernel wrappers around the 'at' interfaces.  Overall ends
up looking fairly clean and we've ended up using them even in places
where we don't need the ADS functionality just because they are so
convenient.

If you're interested in implementing this API I'd be happy to talk about
our implementation and see whether the relevant parts of our implementation
would be useful for the general community.

thanks,
Eric Lemar

--------------------------
Eric Lemar
Software Development Engineer
Isilon Systems
elemar at isilon.com


________________________________

From: owner-freebsd-arch at freebsd.org on behalf of Roman Divacky
Sent: Mon 6/4/2007 9:24 AM
To: arch at freebsd.org
Subject: *at family of syscalls in FreeBSD



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
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