linux-ext4 - Re: e2fsprogs: Richacl support

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Message-ID: <CAHc6FU7J9OESpAJzOKOmu-UP-bC_kzKT-oVg-MVKAaVzswOacA@mail.gmail.com>
Date:	Mon, 19 Oct 2015 00:46:49 +0200
From:	Andreas Gruenbacher <agruenba@...hat.com>
To:	"Theodore Ts'o" <tytso@....edu>
Cc:	Dave Chinner <david@...morbit.com>,
	linux-ext4 <linux-ext4@...r.kernel.org>, xfs@....sgi.com
Subject: Re: e2fsprogs: Richacl support

On Sun, Oct 18, 2015 at 11:44 PM, Theodore Ts'o <tytso@....edu> wrote:
> On Sun, Oct 18, 2015 at 10:46:23PM +0200, Andreas Gruenbacher wrote:
>> > The only question is whether we pay attention to the richacl acl's at
>> > all.  One thing that's not clear to me is what VFS is supposed to do
>> > if an inode has both an Posix ACL xattr and a Richacl xattr at the
>> > same time.
>>
>> The VFS will either look for POSIX ACLs or for a richacl; it won't
>> even notice if both are present.
>
> How does this work in practice?  Does it look for richacl's first, and
> if it doesn't find it, it will then look for a Posix ACL, or vice
> versa?

The filesystem sets the MS_POSIXACL super-block flag for POSIX ACLs or
the MS_RICHACL super-block flag for richacls. These flags are checked
with the IS_POSIXACL(inode) and IS_RICHACL(inode) macros.

>> Right now, filesystems that e2fsck is perfectly happy with can still
>> cause errors when used. It would be nice to fix that.
>>
>> With POSIX ACLs, this problem is slightly less severe because the ACL
>> isn't looked at for the owner; it would even be possible to replace a
>> corrupted POSIX ACL. Richacls unfortunately don't allow this
>> optimization.
>
> Is there code we can use to verify a richacl, and if it's corrupted,
> what are the options about how we can fix it?  Or do we just remove
> it, and just use the inode's i_uid field for the owner instead of
> whatever might be in the richacl?

The on-disk format is relatively simple, it's the same on ext4 and on
xfs and hopefully will be the same on other filesystems as well. That
code could be shared.

If a richacl is found to be corrupt, a safe way of recovering would be
to remove the richacl and clear the S_IRWXUGO mode bits. I don't think
trying to fix ACLs would make sense. The i_uid field is unrelated.

> Ideally, if you can send the patch to add support to validate / fix
> Richacl's in e2fsck, that would be great.

I can send the validation code; not sure how removing corrupt xattrs
would fit into e2fsck though. Can e2fsck remove individual xattrs?

>> This really should be a feature flag and not a mount option, it just
>> doesn't make sense to switch at mount time.
>>
>> From this discussion, I'm even more convinced that we should use an
>> incompat feature rather than a ro-incompat feature.
>
> OK, let's go with that.

Okay, good.

Thanks,
Andreas
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