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

[Jan Kara <jack@suse.cz>]

On Wed 21-04-21 12:29:14, Amir Goldstein wrote:
> On Wed, Apr 21, 2021 at 11:04 AM Jan Kara <jack@suse.cz> 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.
> >
> 
> But there is a middle path option.
> The event metadata can be self described without extending it:
> 
>  struct fanotify_event_metadata {
>         __u32 event_len;
>         __u8 vers;
> -       __u8 reserved;
> +#define FANOTIFY_METADATA_FLAG_PIDFD   1
> +       __u8 flags;
>         __u16 metadata_len;
>         __aligned_u64 mask;
>         __s32 fd;

Well, yes, but do we want another way to describe what fanotify_event_metadata
actually contains? I don't think parsing extended event information is that
bad to make changes like this worth it...

								Honza

-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR