linux-kernel - Re: [PATCH v23 08/22] richacl: Compute maximum file masks from an acl

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Date:	Wed, 13 Jul 2016 14:34:31 +0200
From:	Andreas Gruenbacher <agruenba@...hat.com>
To:	Frank Filz <ffilzlnx@...dspring.com>
Cc:	Jeff Layton <jlayton@...hat.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christoph Hellwig <hch@...radead.org>,
	"Theodore Ts'o" <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	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 API <linux-api@...r.kernel.org>
Subject: Re: [PATCH v23 08/22] richacl: Compute maximum file masks from an acl

Frank,

On Tue, Jul 5, 2016 at 7:08 PM, Frank Filz <ffilzlnx@...dspring.com> wrote:
>> > + * Note: functions like richacl_allowed_to_who(),
>> > +richacl_group_class_allowed(),
>> > + * and richacl_compute_max_masks() iterate through the entire acl in
>> > +reverse
>> > + * order as an optimization.
>> > + *
>> > + * In the standard algorithm, aces are considered in forward order.
>> > +When a
>> > + * process matches an ace, the permissions in the ace are either
>> > +allowed or
>> > + * denied depending on the ace type.  Once a permission has been
>> > +allowed or
>> > + * denied, it is no longer considered in further aces.
>> > + *
>> > + * By iterating through the acl in reverse order, we can compute the
>> > +same
>> > + * result without having to keep track of which permissions have been
>> > +allowed
>> > + * and denied already.
>> > + */
>> >
>>
>> Clever!
>
> Hmm, but does that result in examining the whole ACL for most access checks, at least for files where most of the accesses are by the owner, or a member of a specific group (with perhaps a ton of special case users added on the end)?

I don't understand -- what does this algorithm have to do with access checks?

Thanks,
Andreas