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

[Matthew Bobrowski <repnop@google.com>]

On Mon, Apr 19, 2021 at 12:21:39PM +0200, Jan Kara wrote:
> On Fri 16-04-21 09:22:25, 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>

> Overall this looks OK to me. Just one style nit & one question below in
> addition to what Amir wrote.

Thanks for the quick review Jan!

> > ---
> >  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)) {
> 
> Please align the rest of the condition to the opening brace. I.e., like:
> 
> 	if (FAN_GROUP_FLAG(group, FAN_REPORT_PIDFD) &&
> 	    pid_has_task(event->pid, PIDTYPE_TGID)) {

ACK.

> BTW, why is the pid_has_task() check here?

My thought was that we add a means of ensuring that event->pid holds a
reference to a thread-group leader as pidfds aren't supported for
individual threads just yet. The same check is implemented in
pidfd_open(), so I thought to make the preliminary checks consistent.

Actually, now that I've writeten that, perhaps the pid_has_task()
check can be rolled up into pidfd_create()?

> And why is it OK to fall back to returning pid if pid_has_task() is
> false?

Ah, I see, it's not OK. Good catch Jan! I will need to fix this up in
the follow up series.

/M