Re: [PATCH][RFC] fanotify: deprecate uapi FAN_ALL_* constants

[Amir Goldstein <amir73il@gmail.com>]

On Tue, Oct 2, 2018 at 6:56 PM Jan Kara <jack@suse.cz> wrote:
>
> On Fri 28-09-18 01:01:12, Amir Goldstein wrote:
> > We do not want to add new bits to the FAN_ALL_* uapi constants
> > because they have been exposed to userspace.  If there are programs
> > out there using these constants, those programs could break if
> > re-compiled with modified FAN_ALL_* constants and run on an old kernel.
> >
> > We deprecate the uapi constants FAN_ALL_* and define new FAN_USER_*
> > constants for internal use to replace them. New feature bits will be
> > added only to the new constants.
> >
> > Use high bits for kernel internal flag FAN_MARK_ONDIR and add
> > BUILD_BUG_ON to avoid collision between uapi and kernel internal
> > mark flags.
> >
> > Cc: <linux-api@vger.kernel.org>
> > Signed-off-by: Amir Goldstein <amir73il@gmail.com>
> > ---
> >
> > Jan,
> >
> > I have rebased the API changes (FAN_MARK_FILESYSTEM and
> > FAN_EVENT_INFO_TID) on top of commit 60f7ed8c7c4d ("fsnotify: send path
> > type events to group with super block marks") from your 'fsnotify'
> > branch starting with this change. The work is available on my branch
> > fanotify_api-v3 [1].
> >
> > The end result is that no existing uapi constant are modified and
> > new bit group definitions (FAN_MARK_TYPE_MASK, FAN_EVENT_INFO_FLAGS)
> > are not repeating past mistake and not exposed in uapi.
> >
> > If you agree with this approach and I will post the rest of the series.
> >
> > Thanks,
> > Amir.
> >
> > [1] https://github.com/amir73il/linux/commits/fanotify_api-v3
>
> WRT all stuff in your tree I'd prefer if we had to rebase less stuff. What
> about following:
>
> 1) AFAIU "fanotify: store fanotify_init() flags in group's fanotify_data"
> needs no change so I keep it.
>
> 2) I'll drop "fanotify: support reporting thread id instead of process id"
> from fsnotify for now and merge your new version once you post it together
> with your cleanup of mask constants.
>
> 3) I will keep "fanotify: add API to attach/detach super block mark" as is,
> just please write a separate small patch that resolves the clash between
> FAN_MARK_ONDIR and FAN_MARK_FILESYSTEM.
>

OK.

> Regarding to this patch I have just two nits:
>
> > diff --git a/fs/notify/fanotify/fanotify.c b/fs/notify/fanotify/fanotify.c
> > index 94b52157bf8d..e5a3c69848e4 100644
> > --- a/fs/notify/fanotify/fanotify.c
> > +++ b/fs/notify/fanotify/fanotify.c
> > @@ -131,8 +131,8 @@ static bool fanotify_should_send_event(struct fsnotify_iter_info *iter_info,
> >           !(marks_mask & FS_ISDIR & ~marks_ignored_mask))
> >               return false;
> >
> > -     if (event_mask & FAN_ALL_OUTGOING_EVENTS & marks_mask &
> > -                              ~marks_ignored_mask)
> > +     if (event_mask & FAN_USER_OUTGOING_EVENTS &
> > +         marks_mask & ~marks_ignored_mask)
> >               return true;
>
> I don't like the _USER_ part of the constant name. How about _KNOWN_?
> I.e., FAN_KNOWN_OUTGOING_EVENTS sounds about like what it should?
>

Do you mean just for *this* constant? I would rather have some uniformity
across constants if possible.

Errr... Maybe FAN_VALID_EVENTS/FAN_VALID_OUTGOING_EVENTS
Although _VALID_ is more appropriate for user inputs, so maybe just
FAN_KNOWN_OUTGOING_EVENTS as an exception to the convention
is fine.

Or simply FAN_EVENTS_MASK/FAN_OUTGOING_EVENTS_MASK, like
the recent new constant FAN_MARK_TYPE_MASK?

> ...
> > +     BUILD_BUG_ON(FAN_USER_MARK_FLAGS & FAN_KERN_MARK_FLAGS);
> > +
>
> We have in fsnotify_init():
>
> BUG_ON(hweight32(ALL_FSNOTIFY_EVENTS) != 23)
>
> Maybe we should have in fanotify_user_setup() something similar for
> fanotify flags including the internal ones?
>

OK.

> > diff --git a/include/linux/fanotify.h b/include/linux/fanotify.h
> > index 096c96f4f16a..a67430811006 100644
> > --- a/include/linux/fanotify.h
> > +++ b/include/linux/fanotify.h
> > @@ -4,6 +4,56 @@
> >
> >  #include <uapi/linux/fanotify.h>
> >
> > -/* not valid from userspace, only kernel internal */
> > -#define FAN_MARK_ONDIR               0x00000100
> > +/*
> > + * Flags not valid from userspace, only kernel internal.
> > + * Use high bits so we won't collide with userspace flags.
> > + */
> > +#define FAN_MARK_ONDIR               0x80000000
>
> This ought to be a separate change as I wrote above.
>

Sure and maybe I'll name it FAN_KERN_MARK_ONDIR.

Thanks,
Amir.