[PATCH 2/3] fanotify: define bit map fields to hold response decision context

[Paul Moore]

On Thu, Oct 1, 2020 at 6:24 AM Jan Kara <jack at suse.cz> wrote:
> On Thu 01-10-20 12:12:19, Jan Kara wrote:
> > On Wed 30-09-20 12:12:28, Steve Grubb wrote:
> > > This patch defines 2 bit maps within the response variable returned from

Please use "bitmaps" instead of "bit maps".

> > So how likely do you find other context types are or that you'd need to
> > further expand the information passed from userspace? Because if there are
> > decent changes the expansion will be needed then I'd rather do something
> > like:
> >
> > struct fanotify_response {
> >       __s32 fd;
> >       __u16 response;
> >       __u16 extra_info_type;
> > };
> >
> > which is also backwards compatible and then userspace can set extra_info_type
> > and based on the type we'd know how much more bytes we need to copy from
> > buf to get additional information for that info type.
> >
> > This is much more flexible than what you propose and not that complex to
> > implement, you get type safety for extra information and don't need to use
> > macros to cram everything in u32 etc. Overall cleaner interface I'd say.

Yes, much cleaner.  Stealing bits from an integer is one of those
things you do as a last resort when you can't properly change an API.
Considering that APIs always tend to grow and hardly ever shrink, I
would much prefer Jan's suggestion.

> Now I realized we need to propagate the extra information through fanotify
> permission event to audit - that can be also done relatively easily. Just
> add '__u16 extra_info_type' to fanotify_perm_event and 'char
> extra_info_buf[FANOTIFY_MAX_RESPONSE_EXTRA_LEN]' for now for the data. If
> we ever grow larger extra info structures, we can always change this to
> dynamic allocation but that will be only internal fanotify change,
> userspace facing API can stay the same.

That fanotify/audit interface doesn't bother me as much since that is
internal and we can modify that as needed; the userspace/kernel
fanotify API and the audit record are the important things to focus
on.

Simply recording the "extra_info_type" integer and dumping the
"extra_info" as a hex encoded bitstring in the audit record is
probably the most future proof solution, but I'm not sure what Steve's
tooling would want from such a record.  It also seems to be in the
spirit of Steve's 3/3 patch.

-- 
paul moore
www.paul-moore.com