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

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Date:	Tue, 10 Nov 2015 14:17:45 -0500
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Andreas Gruenbacher <agruenba@...hat.com>
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 06:58:19PM +0100, Andreas Gruenbacher wrote:
> 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:
> >> 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.

I don't recall 4.0 specifying any limit, 4.1 does include negotiation of
maximum rpc calls and replies, and that effectively limits ACL sizes
since they have to fit in a single rpc.

> 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.

Right.  Anyway, not worth the trouble, I think.

(Though what might be worth thinking about at some point is just making
sure we fail in helpful ways.)

--b.
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