File System ACLs: Where to go from here in FreeBSD?
From: Robert Watson (rwatson_at_FreeBSD.org)
Date: Sat, 17 Sep 2005 13:19:31 +0100 (BST) To: trustedbsd-discuss@TrustedBSD.org
The FreeBSD ACL implementation is currently based on a late POSIX.1e
draft, and is similar in functionality to the ACL support in Solaris,
IRIX, and Linux. It was developed along a similar timeline to the Linux
ACL support, and Andreas and I chatted a fair amount along the way so the
parallels are strong -- in fact, the Samba ACL support is almost
identical, and the ACL API man pages on Linux are directly derived from
our ACL man pages (and maybe some of the code?). Differences lie
primarily in three areas:
(1) We follow the POSIX.1e specification for file creation modes, in which
the ACL and umask are combined with the cmode to generate a
conservative ACL. This was the same as IRIX, but different from
Solaris; Linux adopted the Solaris model. Since then, IRIX has (I
believe) also switched to the Solaris model. Our model is
"conservative" in that it will never offer broader rights on a file
than the umask permits, but it turns out to be quite useful in some
environments to allow the ACL on a directory to overide the umask of a
program writing files there.
(2) We offer a few fewer support routines in user space, such as routines
to copy an ACL from one file to another. This has been getting
gradually fleshed out over time.
(3) We don't offer Solaris-compatible NFSv3 extensions to allow remote
management of ACLs via NFS, although the ACLs are enforced on the
server so they are "implemented". I'm not sure if these patches were
merged to Linux or not, but they were floating around for quite a
As I see it, there are two directions we can take file system ACL support,
and here-in lies the Big Question:
(a) We can continue down the POSIX.1e branch of the ACL world, continuing
to enhance and refine our support. For example, continuing to flesh
out a few missing spots in user space, move over to the now
predominent model for generating new file permissions
(non-conservative ACL override), implement NFSv3 RPCs. This is some,
work, but not a huge amount of work.
Or, the a new option that has basically become feasible over the last six
years since the POSIX.1e direction was the one we selected:
(b) We can consider a migration to NT/NFSv4-style ACLs, which is the route
that Darwin has taken. They use the FreeBSD user space ACL library
and POSIX.1e interfaces, but use ACLs with more NT-like semantics.
In particular, they have notions of taking ownership, slightly finer
grained directory controls, etc. This is a lot of work.
Option (b) is an interesting new choice as compared to 1999, when NTFS
ACLs were in the distinct minority in terms of the syntax and semantics
they offered. However, they become much more appealing if we consider
that there appears to be a much clearer mapping from NTFS ACLs to NFSv4
ACLs than there is from POSIX.1e ACLs to NFSv4 ACLs. And the fact that
Mike Smith at Apple has taken the time to make it sit behind our library
for the Darwin implementation on HFS+, etc, is also quite interesting.
When I implemented the library, it was my hope that it would support that
sort of thing, but we never actually tried :-).
If we don't start considering a move to Darwin/NTFS ACLs, then we run into
a problem when it comes to implementing NFSv4 ACLs: the mapping and
behavior is rather poor and unclear. There's an ID on the topic, which I
basically read as saying "This is all rather hard and rather non-ideal".
Apple has identified that, for them, compatibility with NT (and possibly
NFSv4?) is the way to go, and they may be right. On the other hand, the
result is much reduced possibility of clean interoperability with Linux,
Solaris, IRIX, and so on. So there's a definite trade-off.
If we do make this change, I'd like to also simultaneously consider a
change to add an array size field in the ACL structure -- right now, we
have a fixed maximum size, and there's a field that says how much of that
space is used, but not how much space is available. If we want to support
longer ACLs in the future, having a variable array size will improve
efficiency and add flexibility.
If we want to consider switching to the Darwin ACL model, it sounds like
the strategy would be something like the following:
(1) Investigate the model closely, and compare it to NTFS. Identify
whether any of the significant semantic differences is a problem.
(2) Investigate the NFSv4 model closely, and decide if there is a clean
and useful mapping or not. If there are nits, approach Apple and
decide whether the nits are necessary or not.
(3) Produce an implementation on top of UFS2 to experiment with, and see
what happens. Specifically, how our current in-kernel APIs and data
structures work with it.
(4) See whether there is a sensible mapping from existing POSIX.1e ACLs to
the newer ACL style, which could be performed at run-time when reading
an existing ACL-enabled partition. Specifically, in the long term
will we need to support two ACL modes -- a legacy POSIX.1e mode and a
new Darwin/NTFS/NFSv4 mode, or can we run entirely in the new mode and
run-time translate old ACLs to support a migration path?
(6) Investigate what the implications are for applications, especially
relating to supporting two ACL models. Will applications get stuck
figuring out how they co-exist, or can the kernel help to hide it?
(7) Investigate what the implications are for users, who may find that the
semantic changes are significant -- and disruptive, potentially.
Apple has chosen to provide separate tools for managing ACLs, rather
than the POSIX.2e ones, and we might find the same is necessary.
It would be interesting to know if systems other than Darwin have started
exploring this route. For example, Sun has always been quite interested
in NFSv4 -- are they considering or have they made an ACL change that
corresponds with the integration of NFSv4 support?
My feeling is that NFSv4 might be the compelling argument to consider a
migration, and that if we are going to migrate, the sooner we get started
with the implementation work, the better. Any thoughts here are welcome.
Robert N M Watson
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