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Date: Tue, 10 Nov 2015 18:58:19 +0100
Message-ID: <CAHc6FU6aWsWO1SOSyNVnbObE0GoE4dr5a3WVTgvX4xn0wum1Fg@mail.gmail.com>
Subject: Re: [PATCH v15 00/22] Richacls (Core and Ext4)
From: Andreas Gruenbacher <agruenba@redhat.com>
To: "J. Bruce Fields" <bfields@fieldses.org>
Cc: Steve French <smfrench@gmail.com>, Christoph Hellwig <hch@infradead.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        "Theodore Ts'o" <tytso@mit.edu>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        Jeff Layton <jlayton@poochiereds.net>,
        Trond Myklebust <trond.myklebust@primarydata.com>,
        Anna Schumaker <anna.schumaker@netapp.com>,
        Dave Chinner <david@fromorbit.com>,
        linux-ext4 <linux-ext4@vger.kernel.org>,
        XFS Developers <xfs@oss.sgi.com>, LKML <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux NFS Mailing List <linux-nfs@vger.kernel.org>,
        "linux-cifs@vger.kernel.org" <linux-cifs@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>
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On Tue, Nov 10, 2015 at 6:07 PM, J. Bruce Fields <bfields@fieldses.org> wrote:
> On Tue, Nov 10, 2015 at 10:43:46AM -0600, Steve French wrote:
>> On Tue, Nov 10, 2015 at 6:39 AM, Andreas Gruenbacher
>> <agruenba@redhat.com> wrote:
>> > On Tue, Nov 10, 2015 at 12:29 PM, Christoph Hellwig <hch@infradead.org> wrote:
>> >> On Mon, Nov 09, 2015 at 12:08:41PM +0100, Andreas Gruenbacher wrote:
>> >>> Here is another update to the richacl patch queue.  This posting contains
>> >>> the patches ready to be merged; the patches later in the queue still need
>> >>> some more review.
>> <snip>
>> >> and still abuses xattrs instead of a proper syscall interface.
>> >> That's far from being ready to merge.
>> >
>> > The xattr syscall interface is what's used for very similar kinds of
>> > things today; using it for richacls as well sure does not count as
>> > abuse. Things could be improved in the xattr interface and in its
>> > implementation, but we need more substantial reasons than that for
>> > reimplementing the wheel once again.
>>
>> I don't have strong disagreement with using pseudo-xattrs to
>> store/retrieve ACLs (we already do this) but retrieving/setting an ACL
>> all at once can be awkward  when ACLs are quite large e.g. when it
>> encodes to over 1MB
>
> At least in the NFS case, that's also a limitation of the protocol.

I couldn't find a limit in the NFSv4 specification, but the client and
server implementations both define arbitrary ACL size limits. In
addition, the xattr syscalls allow attributes to be up to 64k long.

> If
> we really wanted to support massive ACLs then we'd need both syscall and
> NFS interfaces to allow incrementally reading and writing ACLs, and I
> don't even know what those would look like.
>
> So this is a fine limitation as far as I'm concerned.

The bigger problem would be incrementally setting ACLs. To prevent
processes from racing with each other, we would need a locking
mechanism. In addition, the memory overhead would be prohibitive and
access decisions would become extremely slow; we would have to come up
with mechanisms to avoid those problems.

Thanks,
Andreas