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

[Jan Kara <jack@suse.cz>]

On Mon 19-04-21 15:20:20, Christian Brauner 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?

I'm not a huge fan of overloading fields either but in this particular case
I'm fine with that because:

a) storage size & type matches
b) it describes exactly the same information, just in a different way

It is not possible to store the pidfd elsewhere in fanotify_event_metadata.
But it is certainly possible to use extended event info to return pidfd
instead - similarly to how we return e.g. handle + fsid for some
notification groups. It just means somewhat longer events and more
complicated parsing of structured events in userspace. But as I write
above, in this case I don't think it is worth it - only if we think that
returning both pid and pidfd could ever be useful.

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