linux-ext4 - Re: [PATCH v15 00/22] Richacls (Core and Ext4)

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Date:	Tue, 10 Nov 2015 18:58:19 +0100
From:	Andreas Gruenbacher <agruenba@...hat.com>
To:	"J. Bruce Fields" <bfields@...ldses.org>
Cc:	Steve French <smfrench@...il.com>,
	Christoph Hellwig <hch@...radead.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	"Theodore Ts'o" <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Jeff Layton <jlayton@...chiereds.net>,
	Trond Myklebust <trond.myklebust@...marydata.com>,
	Anna Schumaker <anna.schumaker@...app.com>,
	Dave Chinner <david@...morbit.com>,
	linux-ext4 <linux-ext4@...r.kernel.org>,
	XFS Developers <xfs@....sgi.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux NFS Mailing List <linux-nfs@...r.kernel.org>,
	"linux-cifs@...r.kernel.org" <linux-cifs@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH v15 00/22] Richacls (Core and Ext4)

On Tue, Nov 10, 2015 at 6:07 PM, J. Bruce Fields <bfields@...ldses.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@...hat.com> wrote:
>> > On Tue, Nov 10, 2015 at 12:29 PM, Christoph Hellwig <hch@...radead.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
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