Re: [PATCH v9 13/25] security: Introduce file_release hook

[Al Viro <viro@zeniv.linux.org.uk>]

On Tue, Jan 16, 2024 at 08:51:11AM -0800, Casey Schaufler wrote:
> On 1/16/2024 12:47 AM, Roberto Sassu wrote:
> > On Mon, 2024-01-15 at 19:15 +0000, Al Viro wrote:
> >> On Mon, Jan 15, 2024 at 07:17:57PM +0100, Roberto Sassu wrote:
> >>> From: Roberto Sassu <roberto.sassu@huawei.com>
> >>>
> >>> In preparation for moving IMA and EVM to the LSM infrastructure, introduce
> >>> the file_release hook.
> >>>
> >>> IMA calculates at file close the new digest of the file content and writes
> >>> it to security.ima, so that appraisal at next file access succeeds.
> >>>
> >>> An LSM could implement an exclusive access scheme for files, only allowing
> >>> access to files that have no references.
> >> Elaborate that last part, please.
> > Apologies, I didn't understand that either. Casey?
> 
> Just a hypothetical notion that if an LSM wanted to implement an
> exclusive access scheme it might find the proposed hook helpful.
> I don't have any plan to create such a scheme, nor do I think that
> a file_release hook would be the only thing you'd need.

Exclusive access to what?  "No more than one opened file with this
inode at a time"?  It won't serialize IO operations, obviously...
Details, please.