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

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

On Fri, Apr 16, 2021 at 09:22:25AM +1000, Matthew Bobrowski wrote:

quoted

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 [off-list ref]
SUSE Labs, CR