Re: [PATCH 2/2] fanotify: Add pidfd support to the fanotify API

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On Wed, Apr 21, 2021 at 10:04:49AM +0200, Jan Kara wrote:
> On Tue 20-04-21 12:36:59, Matthew Bobrowski wrote:
> > On Mon, Apr 19, 2021 at 05:02:33PM +0200, Christian Brauner wrote:
> > > A general question about struct fanotify_event_metadata and its
> > > extensibility model:
> > > looking through the code it seems that this struct is read via
> > > fanotify_rad(). So the user is expected to supply a buffer with at least
> > > 
> > > #define FAN_EVENT_METADATA_LEN (sizeof(struct fanotify_event_metadata))
> > > 
> > > bytes. In addition you can return the info to the user about how many
> > > bytes the kernel has written from fanotify_read().
> > > 
> > > So afaict extending fanotify_event_metadata should be _fairly_
> > > straightforward, right? It would essentially the complement to
> > > copy_struct_from_user() which Aleksa and I added (1 or 2 years ago)
> > > which deals with user->kernel and you're dealing with kernel->user:
> > > - If the user supplied a buffer smaller than the minimum known struct
> > >   size -> reject.
> > > - If the user supplied a buffer < smaller than what the current kernel
> > >   supports -> copy only what userspace knows about, and return the size
> > >   userspace knows about.
> > > - If the user supplied a buffer that is larger than what the current
> > >   kernel knows about -> copy only what the kernel knows about, zero the
> > >   rest, and return the kernel size.
> > > 
> > > Extension should then be fairly straightforward (64bit aligned
> > > increments)?
> > 
> > You'd think that it's fairly straightforward, but I have a feeling
> > that the whole fanotify_event_metadata extensibility discussion and
> > the current limitation to do so revolves around whether it can be
> > achieved in a way which can guarantee that no userspace applications
> > would break. I think the answer to this is that there's no guarantee
> > because of <<reasons>>, so the decision to extend fanotify's feature
> > set was done via other means i.e. introduction of additional
> > structures.
> 
> There's no real problem extending fanotify_event_metadata. We already have
> multiple extended version of that structure in use (see e.g. FAN_REPORT_FID
> flag and its effect, extended versions of the structure in
> include/uapi/linux/fanotify.h). The key for backward compatibility is to
> create extended struct only when explicitely requested by a flag when
> creating notification group - and that would be the case here -
> FAN_REPORT_PIDFD or how you called it. It is just that extending the
> structure means adding 8 bytes to each event and parsing extended structure
> is more cumbersome than just fetching s32 from a well known location.
> 
> On the other hand extended structure is self-describing (i.e., you can tell
> the meaning of all the fields just from the event you receive) while
> reusing 'pid' field means that you have to know how the notification group
> was created (whether FAN_REPORT_PIDFD was used or not) to be able to
> interpret the contents of the event. Actually I think the self-describing
> feature of fanotify event stream is useful (e.g. when application manages
> multiple fanotify groups or when fanotify group descriptors are passed
> among processes) so now I'm more leaning towards using the extended
> structure instead of reusing 'pid' as Christian suggests. I'm sorry for the
> confusion.

This approach makes sense to me.

Jan/Amir, just to be clear, we've agreed to go ahead with the extended
struct approach whereby specifying the FAN_REPORT_PIDFD flag will
result in an event which includes an additional struct
(i.e. fanotify_event_info_pid) alongside the generic existing
fanotify_event_metadata (also ensuring that pid has been
provided). Events will be provided to userspace applications just like
when specifying FAN_REPORT_FID, correct?

/M



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