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

[Christian Brauner <christian.brauner@ubuntu.com>]

On Mon, Apr 19, 2021 at 04:53:32PM +0300, Amir Goldstein wrote:
> On Mon, Apr 19, 2021 at 4:20 PM Christian Brauner
> <christian.brauner@ubuntu.com> wrote:
> >
> > On Fri, Apr 16, 2021 at 09:22:25AM +1000, Matthew Bobrowski wrote:
> > > Introduce a new flag FAN_REPORT_PIDFD for fanotify_init(2) which
> > > allows userspace applications to control whether a pidfd is to be
> > > returned instead of a pid for `struct fanotify_event_metadata.pid`.
> > >
> > > FAN_REPORT_PIDFD is mutually exclusive with FAN_REPORT_TID as the
> > > pidfd API is currently restricted to only support pidfd generation for
> > > thread-group leaders. Attempting to set them both when calling
> > > fanotify_init(2) will result in -EINVAL being returned to the
> > > caller. As the pidfd API evolves and support is added for tids, this
> > > is something that could be relaxed in the future.
> > >
> > > If pidfd creation fails, the pid in struct fanotify_event_metadata is
> > > set to FAN_NOPIDFD(-1). Falling back and providing a pid instead of a
> > > pidfd on pidfd creation failures was considered, although this could
> > > possibly lead to confusion and unpredictability within userspace
> > > applications as distinguishing between whether an actual pidfd or pid
> > > was returned could be difficult, so it's best to be explicit.
> > >
> > > Signed-off-by: Matthew Bobrowski <repnop@google.com>
> > > ---
> > >  fs/notify/fanotify/fanotify_user.c | 33 +++++++++++++++++++++++++++---
> > >  include/linux/fanotify.h           |  2 +-
> > >  include/uapi/linux/fanotify.h      |  2 ++
> > >  3 files changed, 33 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
> > > index 9e0c1afac8bd..fd8ae88796a8 100644
> > > --- a/fs/notify/fanotify/fanotify_user.c
> > > +++ b/fs/notify/fanotify/fanotify_user.c
> > > @@ -329,7 +329,7 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
> > >       struct fanotify_info *info = fanotify_event_info(event);
> > >       unsigned int fid_mode = FAN_GROUP_FLAG(group, FANOTIFY_FID_BITS);
> > >       struct file *f = NULL;
> > > -     int ret, fd = FAN_NOFD;
> > > +     int ret, pidfd, fd = FAN_NOFD;
> > >       int info_type = 0;
> > >
> > >       pr_debug("%s: group=%p event=%p\n", __func__, group, event);
> > > @@ -340,7 +340,25 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
> > >       metadata.vers = FANOTIFY_METADATA_VERSION;
> > >       metadata.reserved = 0;
> > >       metadata.mask = event->mask & FANOTIFY_OUTGOING_EVENTS;
> > > -     metadata.pid = pid_vnr(event->pid);
> > > +
> > > +     if (FAN_GROUP_FLAG(group, FAN_REPORT_PIDFD) &&
> > > +             pid_has_task(event->pid, PIDTYPE_TGID)) {
> > > +             /*
> > > +              * Given FAN_REPORT_PIDFD is to be mutually exclusive with
> > > +              * FAN_REPORT_TID, panic here if the mutual exclusion is ever
> > > +              * blindly lifted without pidfds for threads actually being
> > > +              * supported.
> > > +              */
> > > +             WARN_ON(FAN_GROUP_FLAG(group, FAN_REPORT_TID));
> > > +
> > > +             pidfd = pidfd_create(event->pid, 0);
> > > +             if (unlikely(pidfd < 0))
> > > +                     metadata.pid = FAN_NOPIDFD;
> > > +             else
> > > +                     metadata.pid = pidfd;
> >
> > I'm not a fan of overloading fields (Yes, we did this for the _legacy_
> > clone() syscall for CLONE_PIDFD/CLONE_PARENT_SETTID but in general it's
> > never a good idea if there are other options, imho.).
> > Could/should we consider the possibility of adding a new pidfd field to
> > struct fanotify_event_metadata?
> 
> struct fanotify_event_metadata is fully booked.
> We could use a variable length record, but IMO that's an overkill for pidfd.
> 
> If you are concerned about users wrongly interpreting pidfd as pid and
> getting the the wrong process we could use invalid negative values for
> pidfd, e.g. metadata.pid = ~pidfd.
> This has the quality that 0 can mean both FAN_NOPIDFD and no pid.
> UAPI to this would be abstracted with
> #define FAN_EVENT_PIDFD(event) (~((event)->pid))
> 
> I am not convinced that this helps more than it hurts users, but abstracting
> the accessor to pidfd could be a good idea anyway.

Abstracting the accessor may be a good idea but let's not do the
negative values thing that seems clunky to me.