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

[Amir Goldstein <amir73il@gmail.com>]

On Fri, Apr 23, 2021 at 11:02 AM Matthew Bobrowski <repnop@google.com> wrote:
>
> On Fri, Apr 23, 2021 at 10:39:46AM +0300, Amir Goldstein wrote:
> > On Fri, Apr 23, 2021 at 2:06 AM Matthew Bobrowski <repnop@google.com> wrote:
> > >
> > > 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
> >
> > struct fanotify_event_info_pidfd?
>
> Well, yeah? I mean, my line of thought was that we'd also need to
> include struct fanotify_event_info_header alongside the event to
> provide more meta-information about the additional event you'd expect
> to receive when FAN_REPORT_PIDFD is provided, so we'd end up with
> something like:
>
> struct fanotify_event_info_pidfd {
>        struct fanotify_event_info_header hdr;
>        __s32 pidfd;
> }
>
> Unless this of course is overbaking it and there's no need to do this?
>

We need this. I was just pointing out that you wrote fanotify_event_info_pid
must have been a typo.

Thanks,
Amir.