fanotify: allow to set errno in FAN_DENY permission response

fanotify: allow to set errno in FAN_DENY permission response

With FAN_DENY response, user trying to perform the filesystem operation
gets an error with errno set to EPERM.

It is useful for hierarchical storage management (HSM) service to be able
to deny access for reasons more diverse than EPERM, for example EAGAIN,
if HSM could retry the operation later.

Allow fanotify groups with priority FAN_CLASSS_PRE_CONTENT to responsd
to permission events with the response value FAN_DENY_ERRNO(errno),
instead of FAN_DENY to return a custom error.

Limit custom error values to errors expected on read(2)/write(2) and
open(2) of regular files. This list could be extended in the future.
Userspace can test for legitimate values of FAN_DENY_ERRNO(errno) by
writing a response to an fanotify group fd with a value of FAN_NOFD in
the fd field of the response.

The change in fanotify_response is backward compatible, because errno is
written in the high 8 bits of the 32bit response field and old kernels
reject respose value with high bits set.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/1e5fb6af84b69ca96b5c849fa5f10bdf4d1dc414.1731684329.git.josef@toxicpanda.com