linux-kernel - Re: [PATCH -V7 21/26] richacl: xattr mapping functions

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Date:	Thu, 20 Oct 2011 13:49:15 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:	Christoph Hellwig <hch@...radead.org>, agruen@...nel.org,
	akpm@...ux-foundation.org, viro@...iv.linux.org.uk,
	dhowells@...hat.com, linux-fsdevel@...r.kernel.org,
	linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org,
	ebiederm@...ssion.com
Subject: Re: [PATCH -V7 21/26] richacl: xattr mapping functions

On Thu, Oct 20, 2011 at 04:32:04PM +0530, Aneesh Kumar K.V wrote:
> On Thu, 20 Oct 2011 05:19:46 -0400, Christoph Hellwig <hch@...radead.org> wrote:
> > On Thu, Oct 20, 2011 at 05:14:34AM -0400, J. Bruce Fields wrote:
> > > > > Does it really make sense to use a string here just to pick between the
> > > > > three choices OWNER@, GROUP@, and EVERYONE@?  Why not just another small
> > > > > integer?  Is the goal to expand this somehow eventually?
> > > > 
> > 
> > > > I guess Andreas wanted the disk layout to be able to store user@...ain
> > > > format if needed.
> > > 
> > > Is that likely?  For that to be useful, tasks would need to be able to
> > > run as user@...ain strings.  And we'd probably want owners and groups to
> > > also be user@...ain strings.
> > > 
> > > The container people seem to eventually want to add some kind of
> > > namespace identifier everywhere:
> > > 
> > > 	http://marc.info/?l=linux-kernel&m=131836778427871&w=2
> > > 
> > > in which case I guess we'd likely end up with (uid, user namespace id)
> > > instead of user@...ain?
> > 
> > 
> > Storing strings is an extremly stupid idea.  The only thing that would
> > make sense would be storing a windows-style 128-bit GUID.
> > 
> 
> How about updating the richacl_xattr as below 
> 
> struct richace_xattr {
> 	__le16		e_type;
> 	__le16		e_flags;
> 	__le32		e_mask;
> 	__le32		e_size;
> 	u8		e_id[0];
> };
> 
> now e_flags can contain ACE4_SPECIAL_WHO to indicate value in e_id
> indicate special who values (which could be 1 byte value indicating
> OWNER@, GROUP@ or EVERYONE@), ACE4_UNIXID_WHO, to indicate value
> in e_id is the little endian value of unix id. ACE_WINSID_WHO to
> indicate e_id is the 128 bit array containing SID value. ?

That's effectively still a string.

Would it be so bad to have to introduce another xattr type if we needed
a new id type?  You'll have to modify the filesystem and the userspace
tools and everything anyway, won't you?

But if we decide we don't need strings, then at a minimum let's make
these some fixed small size.

You could do something like:

	struct richace_xattr {
		__le16		e_type;
		__le16		e_flags;
		__le32		e_mask;
		__le32		e_id[4];
	}

and just use e_id[0] for now.  That would still leave room for a 128-bit
id, or for a 32-bit uid + some-size namespace-id.

Cc'ing Eric Biederman in hopes of finding out whether that would satifsy
whatever wacky future ideas might be expected for user namespaces.

--b.
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