Re: Fanotify API - Tracking File Movement

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On Fri, May 13, 2022 at 4:18 PM Matthew Bobrowski <repnop@xxxxxxxxxx> wrote:
>
> On Sat, May 07, 2022 at 07:03:13PM +0300, Amir Goldstein wrote:
> > Sorry Matthew, I was looking at the code to give you pointers, but there were
> > so many subtle details (as Jan has expected) that I could only communicate
> > them with a patch.
> > I tested that this patch does not break anything, but did not implement the
> > UAPI changes, so the functionality that it adds is not tested - I leave that
> > to you.
>
> No, that's totally fine. I had to familiarize myself with the
> FS/FAN_RENAME implementation as I hadn't gone over that series. So
> appreciate you whipping this together quickly as it would've taken a
> fair bit of time.
>
> Before the UAPI related modifications, we need to first figure out how
> we are to handle the CREATE/DELETE/MOVE cases.
>
> ...
>
> > My 0.02$ - while FAN_RENAME is a snowflake, this is not because
> > of our design, this is because rename(2) is a snowflake vfs operation.
> > The event information simply reflects the operation complexity and when
> > looking at non-Linux filesystem event APIs, the event information for rename
> > looks very similar to FAN_RENAME. In some cases (lustre IIRC) the protocol
> > was enhanced at some point exactly as we did with FAN_RENAME to
> > have all the info in one event vs. having to join two events.
> >
> > Hopefully, the attached patch simplifies the specialized implementation
> > a little bit.
> >
> > But... (there is always a but when it comes to UAPI),
> > When looking at my patch, one cannot help wondering -
> > what about FAN_CREATE/FAN_DELETE/FAN_MOVE?
> > If those can report child fid, why should they be treated differently
> > than FAN_RENAME w.r.t marking the child inode?
>
> This is something that crossed my mind while looking over the patch
> and is a very good thing to call-out indeed. I am of the opinion that
> we shouldn't be placing FAN_RENAME in the special egg basket and also
> consider how this is to operate for events
> FAN_CREATE/FAN_DELETE/FAN_MOVE.
>
> > For example, when watching a non-dir for FAN_CREATE, it could
> > be VERY helpful to get the dirfid+name of where the inode was
> > hard linked.
>
> Oh right, here you're referring to this specific scenario:
>
> - FAN_CREATE mark exclusively placed on /dir1/old_file
> - Create link(/dir1/old_file, /dir2/new_file)
> - Expect to receive single event including two information records
>   FID(/dir1/old_file) + DFID_NAME(/dir2/new_file)
>
> Is that correct?

Correct.
Exactly the same event as you would get from watching dir2 with
FAN_CREATE|FAN_EVENT_ON_CHILD in a group with flag
FAN_REPORT_TARGET_FID.

Thanks,
Amir.



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