Re: [PATCH v2 5/5] fanotify: add pidfd support to the fanotify API

On Sat, Jul 10, 2021 at 05:49:57PM +0300, Amir Goldstein wrote:

On Thu, Jun 10, 2021 at 3:22 AM Matthew Bobrowski [off-list ref] wrote:

quoted

Introduce a new flag FAN_REPORT_PIDFD for fanotify_init(2) which
allows userspace applications to control whether a pidfd info record
containing a pidfd is to be returned with each event.

If FAN_REPORT_PIDFD is enabled for a notification group, an additional
struct fanotify_event_info_pidfd object will be supplied alongside the
generic struct fanotify_event_metadata within a single event. This
functionality is analogous to that of FAN_REPORT_FID in terms of how
the event structure is supplied to the userspace application. Usage of
FAN_REPORT_PIDFD with FAN_REPORT_FID/FAN_REPORT_DFID_NAME is
permitted, and in this case a struct fanotify_event_info_pidfd object
will follow any struct fanotify_event_info_fid object.

Currently, the usage of FAN_REPORT_TID is not permitted along with
FAN_REPORT_PIDFD as the pidfd API only supports the creation of pidfds
for thread-group leaders. Additionally, the FAN_REPORT_PIDFD is
limited to privileged processes only i.e. listeners that are running
with the CAP_SYS_ADMIN capability. Attempting to supply either of
these initialisation flags with FAN_REPORT_PIDFD will result with
EINVAL being returned to the caller.

In the event of a pidfd creation error, there are two types of error
values that can be reported back to the listener. There is
FAN_NOPIDFD, which will be reported in cases where the process
responsible for generating the event has terminated prior to fanotify
being able to create pidfd for event->pid via pidfd_create(). The
there is FAN_EPIDFD, which will be reported if a more generic pidfd
creation error occurred when calling pidfd_create().

Signed-off-by: Matthew Bobrowski <redacted>

[...]

quoted

diff --git a/include/uapi/linux/fanotify.h b/include/uapi/linux/fanotify.h
index fbf9c5c7dd59..5cb3e2369b96 100644
--- a/include/uapi/linux/fanotify.h
+++ b/include/uapi/linux/fanotify.h

@@ -55,6 +55,7 @@
 #define FAN_REPORT_FID         0x00000200      /* Report unique file id */
 #define FAN_REPORT_DIR_FID     0x00000400      /* Report unique directory id */
 #define FAN_REPORT_NAME                0x00000800      /* Report events with name */
+#define FAN_REPORT_PIDFD       0x00001000      /* Report pidfd for event->pid */

Matthew,

One very minor comment.
I have a patch in progress to add FAN_REPORT_CHILD_FID (for reporting fid
of created inode) and it would be nice if we can reserve the flag space in the
same block with the rest of the FID flags.

If its not a problem, maybe we could move FAN_REPORT_PIDFD up to 0x80
right above FAN_REPORT_TID, which also happen to be related flags.

That's fine by me, no objections. Updated my patch series with the updated
flag value [0].

[0]
https://github.com/matthewbobrowski/linux/commit/02ba3581fee21c34bd986e093d9eb0b9897fa741#diff-c83e05fe10af36146658416e55756f304a099606f4a2b18d2fcb683b277c3c62R54

/M