linux-ext4 - Re: [PATCH 00/12] acl: remove remaining posix acl handlers

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Date:   Mon, 30 Jan 2023 11:23:22 +0100
From:   Christian Brauner <brauner@...nel.org>
To:     Christoph Hellwig <hch@....de>
Cc:     linux-fsdevel@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>,
        Seth Forshee <sforshee@...nel.org>,
        linux-erofs@...ts.ozlabs.org, Jan Kara <jack@...e.com>,
        linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
        linux-mtd@...ts.infradead.org, ocfs2-devel@....oracle.com,
        reiserfs-devel@...r.kernel.org
Subject: Re: [PATCH 00/12] acl: remove remaining posix acl handlers

On Mon, Jan 30, 2023 at 10:16:15AM +0100, Christoph Hellwig wrote:
> On Mon, Jan 30, 2023 at 10:10:52AM +0100, Christian Brauner wrote:
> > However, a few filesystems still rely on the ->list() method of the
> > generix POSIX ACL xattr handlers in their ->listxattr() inode operation.
> > This is a very limited set of filesystems. For most of them there is no
> > dependence on the generic POSIX ACL xattr handler in any way.
> > 
> > In addition, during inode initalization in inode_init_always() the
> > registered xattr handlers in sb->s_xattr are used to raise IOP_XATTR in
> > inode->i_opflags.
> > 
> > With the incoming removal of the legacy POSIX ACL handlers it is at
> > least possible for a filesystem to only implement POSIX ACLs but no
> > other xattrs. If that were to happen we would miss to raise IOP_XATTR
> > because sb->s_xattr would be NULL. While there currently is no such
> > filesystem we should still make sure that this just works should it ever
> > happen in the future.
> 
> Now the real questions is: do we care?  Once Posix ACLs use an
> entirely separate path, nothing should rely on IOP_XATTR for them.
> So instead I think we're better off auditing all users of IOP_XATTR
> and making sure that nothing relies on them for ACLs, as we've very
> much split the VFS concept of ACLs from that from xattrs otherwise.

I had a patch like that but some filesystems create inodes that
explicitly remove IOP_XATTR to prevent any xattrs from being set on
specific inodes. I remember this for at least reiserfs and btrfs.

So we would probably need IOP_NOACL that can be raised by a filesystem
to explicitly opt out of them for specific inodes. That's probably fine
and avoids having to introduce something like SB_I_XATTR.