Re: [PATCH v2 00/30] acl: add vfs posix acl api

[Casey Schaufler <casey@schaufler-ca.com>]

On 9/27/2022 12:41 AM, Christoph Hellwig wrote:
> On Mon, Sep 26, 2022 at 05:22:45PM -0700, Casey Schaufler wrote:
>> I suggest that you might focus on the acl/evm interface rather than the entire
>> LSM interface. Unless there's a serious plan to make ima/evm into a proper LSM
>> I don't see how the breadth of this patch set is appropriate.
> Umm. The problem is the historically the Linux xattr interface was
> intended for unstructured data, while some of it is very much structured
> and requires interpretation by the VFS and associated entities.  So
> splitting these out and add proper interface is absolutely the right
> thing to do and long overdue (also for other thing like capabilities).
> It might make things a little more verbose for LSM, but it fixes a very
> real problem.

Here's the problem I see. All of the LSMs see xattrs, except for their own,
as opaque objects. Introducing LSM hooks to address the data interpretation
issues between VFS and EVM, which is not an LSM, adds to an already overlarge
and interface. And the "real" users of the interface don't need the new hook.
I'm not saying that the ACL doesn't have problems. I'm not saying that the
solution you've proposed isn't better than what's there now. I am saying that
using LSM as a conduit between VFS and EVM at the expense of the rest of the
modules is dubious. A lot of change to LSM for no value to LSM.

I am not adamant about this. A whole lot worse has been done for worse reasons.
But as Paul says, we're overdue to make an effort to keep the LSM interface sane.