[PATCH v8 4/6] security: Allow all LSMs to provide xattrs for inode_init_security hook

[Roberto Sassu]

On Tue, 2023-03-28 at 16:19 -0400, Paul Moore wrote:
> On Tue, Mar 28, 2023 at 3:47 AM Roberto Sassu
> <roberto.sassu at huaweicloud.com> wrote:
> > On Mon, 2023-03-27 at 17:02 -0400, Paul Moore wrote:
> > > On Mon, Mar 27, 2023 at 3:30 AM Roberto Sassu
> > > <roberto.sassu at huaweicloud.com> wrote:
> > > > On Fri, 2023-03-24 at 17:39 -0400, Paul Moore wrote:
> > > > > On Fri, Mar 24, 2023 at 9:26 AM Roberto Sassu
> > > > > <roberto.sassu at huaweicloud.com> wrote:
> > > > > > On Fri, 2023-03-24 at 11:18 +0100, Roberto Sassu wrote:
> > > > > > > On Thu, 2023-03-23 at 20:09 -0400, Paul Moore wrote:
> > > > > > > > On Tue, Mar 14, 2023 at 4:19 AM Roberto Sassu
> > > > > > > > <roberto.sassu at huaweicloud.com> wrote:
> > > > > > > > > From: Roberto Sassu <roberto.sassu at huawei.com>
> 
> ...
> 
> > > Okay, that's fair, but we could still pass the full xattrs array and a
> > > reference to the current count which could be both read and updated by
> > > the individual LSMs, right?
> > 
> > Yes, we could do.
> > 
> > > The issue is that the separate compaction stage is not something we
> > > want to have to do if we can avoid it.  Maybe we're stuck with it, but
> > > I'm not yet convinced that we can't make some minor changes to the
> > > LSMs to avoid the compaction step.
> > 
> > I liked more the idea that LSMs do what they are most familiar with,
> > get an offset in a security blob or, in this case, a starting slot in
> > the new_xattrs array, and write there.
> > 
> > v3 had the lsm_find_xattr_slot() helper, to get the starting slot, but
> > somehow I find it less intuitive.
> > 
> > Ok, if you prefer to avoid the compaction stage, I will rewrite this
> > patch.
> 
> My concern is having to look through the xattr array after each LSM
> has been run and in at least one case having to then do a memcpy() to
> keep the array packed.  There are some cases where there is no way to
> avoid all that extra work, but here I think we have the LSMs do the
> Right Thing with respect to packing the xattr array without overly
> burdening the individual LSMs.
> 
> Does that make sense?  It basically comes down to being smart about
> our abstractions and both selectively, and carefully, breaking them
> when there is a reasonable performance gain to be had.

Yes, ok, it is a good approach.

Thanks

Roberto