From owner-freebsd-fs@FreeBSD.ORG Sun Sep 18 22:57:57 2005
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Date: Sun, 18 Sep 2005 18:57:52 -0400
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From: Erez Zadok <ezk@cs.sunysb.edu>
To: freebsd-fs@freebsd.org
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Cc: christos@zoulas.com
Subject: turning off the NFS attribute cache
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Summary:
Freebsd doesn't seem to have a way to turn off the NFS attribute cache,
which breaks the Amd automounter so badly that I do not recommend using Amd
on FreeBSD for heavy use, not until this is fixed.
Details:
I'm the lead maintainer of the am-utils package (www.am-utils.org), the
so-called Amd Berkeley Automounter. Amd is a user-level NFSv2 server that
manages automounts of all other file systems. The kernel contacts Amd via
RPCs, and Amd in turn performs the actual mounts, and then responds back to
the kernel's RPCs. Every kernel caches attributes of files, in a Directory
Name Lookup Cache (DNLC).
Amd manages its namespace in the user level, but the kernel caches names
itself. So the two must coordinate to ensure that both namespaces are in
sync. If the kernel uses a cached entry from the DNLC, without consulting
Amd, users may see corruption of the automounter namespace (symlinks
pointing to the wrong places, ESTALE errors, and more). For example,
suppose Amd timed out an entry and removed the entry from Amd's namespace.
Amd has to tell the kernel to purge its corresponding DNLC entry too. The
way Amd often does that is by incrementing the mtime of the parent
directory. This is the most common method for kernels to check if their
DNLC entries are stale: if the parent dir mtime is newer, the kernel will
discard all cached entries for that directory, and will re-issue lookup
methods. Those lookups will result in NFS_GETATTR/NFS_LOOKUP calls sent to
Amd, and Amd can then properly inform the kernel of the new state of
automounted entries.
In order to ensure that Amd is "in charge" of its namespace w/o interference
from the kernel, Amd will try to turn off the NFS attribute cache. It does
so by using the NFSMNT_NOAC flag, if it exists, or setting various "cache
timeout" fields in struct nfs_args to 0.
We have released a major new version of am-utils, version 6.1, a few months
ago. Since then, a lot of people have experimented with Amd, in
anticipation of migrating from the very old am-utils 6.0 to the new 6.1.
For a couple of months now, we have received reports of problems with Amd,
especially under heavy use. Users reported getting ESTALE errors from time
to time, or seeing automounted entries whose symlinks don't point to where
it should be. After much debugging, we traced it to a few places in Amd
where it wasn't updating the parent dir mtime as it should have. So we
fixed it and verified that it was working (on Solaris and Linux, where the
actual user bug reports came from).
After fixing this in Amd, I went on to verify that things work for other
OSs. When I got to FreeBSD 4.6, I found that it always caches directory
entries, and there is no way to turn it off completely. Specifically, if I
set the ac{reg,dir}{min,max} fields in struct nfs_args all to zero, the
kernel seems to cache the entries for a default number of seconds (I counted
something like 5 seconds). On some OSs, setting these four fields to 0
turns off the attribute cache, but not on FreeBSD 4.6. I was able to verify
this using Amd and a script that exercises the interaction of the kernel's
attrcache and Amd. I didn't look at the kernel sources (yet) but I'm pretty
certain of this behavior.
I then experimented by setting the ac{reg,dir}{min,max} fields in struct
nfs_args all to 1, the smallest non-zero value I could. When I ran my Amd
exercising script, I found that the value of 1 reduced the race between the
DNLC and Amd, and the script took a little longer to run before it detected
an incoherency. That makes sense: the smaller the DNLC cache interval is,
the shorter the window of vulnerability is. (BTW, the mount_nfs man page
says that the ac{reg,dir}{min,max} fields use a 1 second resolution, but my
experimentation indicated it was in 0.1 second units -- is that right?)
Clearly, setting the ac{reg,dir}{min,max} fields to 0 is worse than setting
it to 1 on FreeBSD. So the current (ugly) workaround I've used in am-utils
is to set the global parameter auto_attrcache=1 in the /etc/amd.conf file.
The near term solution is for FreeBSD to support a true 'noac' flag, which
can be added fairly easily. This'd make Amd work reliably.
The long term solution is to implement Autofs support for all OSs and to
support it in Amd. Luckily, there is autofs support for freebsd now, but
Amd doesn't support autofs yet on freebsd (we support the solaris and linux
autofs as of now). Still, we found that even with autofs support, many
sysadmins still prefer to use the good 'ol non-autofs mode.
For what it's worth, I've confirmed that OpenBSD 3.7 and NetBSD 2.0.2 also
do not have a way to turn off the attribute cache and both suffer from the
same problem. I've reported it to Christos Zoulas (NetBSD) who said that
they will fix this. It is my hope that FreeBSD will implement a "noac" flag
ASAP; it's crucial if you want people to use FreeBSD+Amd in heavy-use
production-level environments.
I also plan to test this attrcache behavior on all OSs I have access to and
report the same to each.
Finally, I'll be happy to work with anyone to provide more details, scripts
to exercise these bugs, submit official bug reports, and even implement a
"noac" flag for any BSD kernel.
Sincerely,
Erez.
From owner-freebsd-fs@FreeBSD.ORG Sun Sep 18 23:07:17 2005
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Date: Sun, 18 Sep 2005 19:07:05 -0400
From: Kris Kennaway <kris@obsecurity.org>
To: Erez Zadok <ezk@cs.sunysb.edu>
Message-ID: <20050918230705.GA54054@xor.obsecurity.org>
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Cc: freebsd-fs@freebsd.org, christos@zoulas.com
Subject: Re: turning off the NFS attribute cache
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On Sun, Sep 18, 2005 at 06:57:52PM -0400, Erez Zadok wrote:
> After fixing this in Amd, I went on to verify that things work for other
> OSs. When I got to FreeBSD 4.6, I found that it always caches directory
> entries, and there is no way to turn it off completely. Specifically, if I
> set the ac{reg,dir}{min,max} fields in struct nfs_args all to zero, the
> kernel seems to cache the entries for a default number of seconds (I counted
> something like 5 seconds). On some OSs, setting these four fields to 0
> turns off the attribute cache, but not on FreeBSD 4.6. I was able to verify
> this using Amd and a script that exercises the interaction of the kernel's
> attrcache and Amd. I didn't look at the kernel sources (yet) but I'm pretty
> certain of this behavior.
FreeBSD 4.6 is, frankly, ancient..are you certain this still applies
to modern versions?
Kris
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From owner-freebsd-fs@FreeBSD.ORG Sun Sep 18 23:16:39 2005
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Date: Sun, 18 Sep 2005 19:16:30 -0400
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From: Erez Zadok <ezk@cs.sunysb.edu>
To: Kris Kennaway <kris@obsecurity.org>
In-reply-to: Your message of "Sun, 18 Sep 2005 19:07:05 EDT."
<20050918230705.GA54054@xor.obsecurity.org>
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Cc: freebsd-fs@freebsd.org, Erez Zadok <ezk@cs.sunysb.edu>, christos@zoulas.com
Subject: Re: turning off the NFS attribute cache
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In message <20050918230705.GA54054@xor.obsecurity.org>, Kris Kennaway writes:
> FreeBSD 4.6 is, frankly, ancient..are you certain this still applies
> to modern versions?
>
> Kris
Argh. My bad. I tested in on 5.4, but my finger wrote 4.6. Too many OSs;
too many versions. :-)
Yes, the problem is definitely there in 5.4. I didn't test 6.0 yet.
Erez.
From owner-freebsd-fs@FreeBSD.ORG Sun Sep 18 23:57:32 2005
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Date: Sun, 18 Sep 2005 19:57:24 -0400
From: Kris Kennaway <kris@obsecurity.org>
To: Erez Zadok <ezk@cs.sunysb.edu>
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Cc: freebsd-fs@freebsd.org, christos@zoulas.com,
Kris Kennaway <kris@obsecurity.org>
Subject: Re: turning off the NFS attribute cache
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On Sun, Sep 18, 2005 at 07:16:30PM -0400, Erez Zadok wrote:
> In message <20050918230705.GA54054@xor.obsecurity.org>, Kris Kennaway wri=
tes:
>=20
> > FreeBSD 4.6 is, frankly, ancient..are you certain this still applies
> > to modern versions?
> >=20
> > Kris
>=20
> Argh. My bad. I tested in on 5.4, but my finger wrote 4.6. Too many OS=
s;
> too many versions. :-)
>=20
> Yes, the problem is definitely there in 5.4. I didn't test 6.0 yet.
Thanks for clarifying...6.0 is unlikely to have fixed this.
Kris
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From owner-freebsd-fs@FreeBSD.ORG Mon Sep 19 00:00:17 2005
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From: Erez Zadok <ezk@cs.sunysb.edu>
To: Kris Kennaway <kris@obsecurity.org>
In-reply-to: Your message of "Sun, 18 Sep 2005 19:57:24 EDT."
<20050918235724.GA67621@xor.obsecurity.org>
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Subject: Re: turning off the NFS attribute cache
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In message <20050918235724.GA67621@xor.obsecurity.org>, Kris Kennaway writes:
>
> Thanks for clarifying...6.0 is unlikely to have fixed this.
>
> Kris
Is 6.0 frozen enough that it's unlikely to be fixed in 6.0?
Erez.
From owner-freebsd-fs@FreeBSD.ORG Mon Sep 19 00:01:42 2005
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From: Kris Kennaway <kris@obsecurity.org>
To: Erez Zadok <ezk@cs.sunysb.edu>
Message-ID: <20050919000134.GA67976@xor.obsecurity.org>
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Cc: freebsd-fs@freebsd.org, christos@zoulas.com,
Kris Kennaway <kris@obsecurity.org>
Subject: Re: turning off the NFS attribute cache
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On Sun, Sep 18, 2005 at 08:00:04PM -0400, Erez Zadok wrote:
> In message <20050918235724.GA67621@xor.obsecurity.org>, Kris Kennaway wri=
tes:
> >=20
> > Thanks for clarifying...6.0 is unlikely to have fixed this.
> >=20
> > Kris
>=20
> Is 6.0 frozen enough that it's unlikely to be fixed in 6.0?
At this point, maybe. If someone can come up with a simple fix quick
enough there might be some chance though.
Kris
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From owner-freebsd-fs@FreeBSD.ORG Mon Sep 19 00:12:42 2005
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Date: Sun, 18 Sep 2005 18:12:36 -0600
From: Scott Long <scottl@samsco.org>
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Cc: freebsd-fs@freebsd.org, Erez Zadok <ezk@cs.sunysb.edu>, christos@zoulas.com
Subject: Re: turning off the NFS attribute cache
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Kris Kennaway wrote:
> On Sun, Sep 18, 2005 at 08:00:04PM -0400, Erez Zadok wrote:
>
>>In message <20050918235724.GA67621@xor.obsecurity.org>, Kris Kennaway writes:
>>
>>>Thanks for clarifying...6.0 is unlikely to have fixed this.
>>>
>>>Kris
>>
>>Is 6.0 frozen enough that it's unlikely to be fixed in 6.0?
>
>
> At this point, maybe. If someone can come up with a simple fix quick
> enough there might be some chance though.
>
> Kris
There are about 2 more weeks before the window closes on fixing this,
assuming that whatever fix is to be had is fairly simple and non-disruptive.
Scott
From owner-freebsd-fs@FreeBSD.ORG Mon Sep 19 00:14:39 2005
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Date: Sun, 18 Sep 2005 20:14:29 -0400
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From: Erez Zadok <ezk@cs.sunysb.edu>
To: Scott Long <scottl@samsco.org>
In-reply-to: Your message of "Sun, 18 Sep 2005 18:12:36 MDT."
<432E0274.4010704@samsco.org>
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Cc: freebsd-fs@freebsd.org, Erez Zadok <ezk@cs.sunysb.edu>, christos@zoulas.com,
Kris Kennaway <kris@obsecurity.org>
Subject: Re: turning off the NFS attribute cache
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In message <432E0274.4010704@samsco.org>, Scott Long writes:
> There are about 2 more weeks before the window closes on fixing this,
> assuming that whatever fix is to be had is fairly simple and non-disruptive.
>
> Scott
I'll give it a shot in beta4 and see if I can get a simple fix made. I'll
post what I have here on -fs for comments.
Erez.
From owner-freebsd-fs@FreeBSD.ORG Mon Sep 19 02:25:53 2005
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From: Erez Zadok <ezk@cs.sunysb.edu>
To: Scott Long <scottl@samsco.org>, Kris Kennaway <kris@obsecurity.org>,
Erez Zadok <ezk@cs.sunysb.edu>, freebsd-fs@freebsd.org, christos@zoulas.com
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Subject: Re: turning off the NFS attribute cache
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Good news!
After browsing the 6.0 kernel sources, I found out that freebsd DOES indeed
have a way of turning off the attribute cache. It's not enough to just set
nfs_args->acregmin and friends, but you also need to
nfs_args->flags |= NFSMNT_ACREGMIN
Otherwise, the kernel ignores what you set in the ->acregmin field. So, to
completely turn off the attrcache on freebsd, you have to set four fields in
struct nfs_args to 0, and turn four flags on in ->flags.
It's cumbersome, but it works. You may still want to consider offering a
"noac" flag for convenience, which'd set all eight values as needed (as some
OSs do). At the very least, please document specifically in the mount_nfs
man page how one turns off the attribute cache completely.
Anyway, I was able to verify this in 5.4 and 6.0 (SNAP001). And it looks
like the NFSMNT_ACREGMIN type of flags have existed as far back as 4.x, so I
suspect that as long as the implementation was correct, even old versions of
BSD should work well now with Amd.
I'll release a quick new version of am-utils (6.1.2.1) with this fix,
specifically because I have a few freebsd users who complained about this
bug.
I would like to ask whomever is responsible for maintaining am-utils for
FreeBSD to consider importing 6.1.2.1 into your ports tree. Currently
you're stuck at an ancient 6.0.10. 6.1.x fixes numerous bugs other than
this latest one, plus it offers a host of new performance and security
features.
Thanks,
Erez.
From owner-freebsd-fs@FreeBSD.ORG Mon Sep 19 03:06:18 2005
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Cc: freebsd-fs@freebsd.org, christos@zoulas.com,
Kris Kennaway <kris@obsecurity.org>
Subject: Re: turning off the NFS attribute cache
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Erez Zadok wrote:
> Good news!
>
> After browsing the 6.0 kernel sources, I found out that freebsd DOES indeed
> have a way of turning off the attribute cache. It's not enough to just set
> nfs_args->acregmin and friends, but you also need to
>
> nfs_args->flags |= NFSMNT_ACREGMIN
>
> Otherwise, the kernel ignores what you set in the ->acregmin field. So, to
> completely turn off the attrcache on freebsd, you have to set four fields in
> struct nfs_args to 0, and turn four flags on in ->flags.
>
> It's cumbersome, but it works. You may still want to consider offering a
> "noac" flag for convenience, which'd set all eight values as needed (as some
> OSs do). At the very least, please document specifically in the mount_nfs
> man page how one turns off the attribute cache completely.
>
> Anyway, I was able to verify this in 5.4 and 6.0 (SNAP001). And it looks
> like the NFSMNT_ACREGMIN type of flags have existed as far back as 4.x, so I
> suspect that as long as the implementation was correct, even old versions of
> BSD should work well now with Amd.
>
> I'll release a quick new version of am-utils (6.1.2.1) with this fix,
> specifically because I have a few freebsd users who complained about this
> bug.
>
> I would like to ask whomever is responsible for maintaining am-utils for
> FreeBSD to consider importing 6.1.2.1 into your ports tree. Currently
> you're stuck at an ancient 6.0.10. 6.1.x fixes numerous bugs other than
> this latest one, plus it offers a host of new performance and security
> features.
>
> Thanks,
> Erez.
Thanks for looking into this and adapting am-utils for it. Would you
mind submitting a report to the bug database so that we can track this?
http://www.freebsd.org/send-pr.html. As for importing a newer am-utils,
it's been talked about, but I don't expect it to happen for 6.0. Thanks!
Scott
From owner-freebsd-fs@FreeBSD.ORG Tue Sep 20 14:59:32 2005
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Cc:
Subject: File System ACLs: Where to go from here in FreeBSD? (fwd)
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FYI, a conversation relating to the following post is on-going on
trustedbsd-discuss. If this is a topic that interests you, please join in
over there :-).
Thanks,
Robert N M Watson
---------- Forwarded message ----------
Date: Sat, 17 Sep 2005 13:19:31 +0100 (BST)
From: Robert Watson <rwatson@FreeBSD.org>
To: trustedbsd-discuss@TrustedBSD.org
Cc: astrodog@gmail.com
Subject: File System ACLs: Where to go from here in FreeBSD?
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
while.
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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