On Thu, Aug 1, 2024 at 7:38 PM Jan Kara <jack@xxxxxxx> wrote:
>
> On Thu 25-07-24 14:19:45, Josef Bacik wrote:
> > From: Amir Goldstein <amir73il@xxxxxxxxx>
> >
> > With group class FAN_CLASS_PRE_CONTENT, report offset and length info
> > along with FAN_PRE_ACCESS and FAN_PRE_MODIFY permission events.
> >
> > This information is meant to be used by hierarchical storage managers
> > that want to fill partial content of files on first access to range.
> >
> > Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx>
> > ---
> > fs/notify/fanotify/fanotify.h | 8 +++++++
> > fs/notify/fanotify/fanotify_user.c | 38 ++++++++++++++++++++++++++++++
> > include/uapi/linux/fanotify.h | 7 ++++++
> > 3 files changed, 53 insertions(+)
> >
> > diff --git a/fs/notify/fanotify/fanotify.h b/fs/notify/fanotify/fanotify.h
> > index 93598b7d5952..7f06355afa1f 100644
> > --- a/fs/notify/fanotify/fanotify.h
> > +++ b/fs/notify/fanotify/fanotify.h
> > @@ -448,6 +448,14 @@ static inline bool fanotify_is_perm_event(u32 mask)
> > mask & FANOTIFY_PERM_EVENTS;
> > }
> >
> > +static inline bool fanotify_event_has_access_range(struct fanotify_event *event)
> > +{
> > + if (!(event->mask & FANOTIFY_PRE_CONTENT_EVENTS))
> > + return false;
> > +
> > + return FANOTIFY_PERM(event)->ppos;
> > +}
>
> Now I'm a bit confused. Can we have legally NULL ppos for an event from
> FANOTIFY_PRE_CONTENT_EVENTS?
>
Sorry for the very late reply...
The short answer is that NULL FANOTIFY_PERM(event)->ppos
simply means that fanotify_alloc_perm_event() was called with NULL
range, which is the very common case of legacy permission events.
The long answer is a bit convoluted, so bare with me.
The long answer is to the question whether fsnotify_file_range() can
be called with a NULL ppos.
This shouldn't be possible AFAIK for regular files and directories,
unless some fs that is marked with FS_ALLOW_HSM opens a regular
file with FMODE_STREAM, which should not be happening IMO,
but then the assertion belongs inside fsnotify_file_range().
However, there was another way to get NULL ppos before I added the patch
"fsnotify: generate pre-content permission event on open"
Which made this "half intentional" change:
static inline int fsnotify_file_perm(struct file *file, int perm_mask)
{
- return fsnotify_file_area_perm(file, perm_mask, NULL, 0);
+ return fsnotify_file_area_perm(file, perm_mask, &file->f_pos, 0);
}
In order to implement:
"The event will have a range info of (0..0) to provide an opportunity
to fill the entire file content on open."
The problem is that do_open() was not the only caller of fsnotify_file_perm().
There is another call from iterate_dir() and the change above causes
FS_PRE_ACCESS events on readdir to report the directory f_pos -
Do we want that? I think we do, but HSM should be able to tell the
difference between opendir() and readdir(), because my HSM only
wants to fill dir content on the latter.
I think that we need to decide if we want to allow pre-content events
with no range reported (e.g. for readdir()) or if pre-content events must
report a range, can report (0..-1) or something for "entire range".
Thanks,
Amir.
