Re: [PATCH 5/7] fanotify: record new parent and name in MOVED_FROM event

[Amir Goldstein <amir73il@gmail.com>]

On Fri, Nov 12, 2021 at 6:48 PM Jan Kara <jack@suse.cz> wrote:
>
> On Fri 29-10-21 14:40:26, Amir Goldstein wrote:
> > In the special case of MOVED_FROM event, if we are recording the child
> > fid due to FAN_REPORT_TARGET_FID init flag, we also record the new
> > parent and name.
> >
> > Signed-off-by: Amir Goldstein <amir73il@gmail.com>
> ...
> > diff --git a/fs/notify/fanotify/fanotify.c b/fs/notify/fanotify/fanotify.c
> > index 795bedcb6f9b..d1adcb3437c5 100644
> > --- a/fs/notify/fanotify/fanotify.c
> > +++ b/fs/notify/fanotify/fanotify.c
> > @@ -592,21 +592,30 @@ static struct fanotify_event *fanotify_alloc_name_event(struct inode *id,
> >                                                       __kernel_fsid_t *fsid,
> >                                                       const struct qstr *name,
> >                                                       struct inode *child,
> > +                                                     struct dentry *moved,
> >                                                       unsigned int *hash,
> >                                                       gfp_t gfp)
> >  {
> >       struct fanotify_name_event *fne;
> >       struct fanotify_info *info;
> >       struct fanotify_fh *dfh, *ffh;
> > +     struct inode *dir2 = moved ? d_inode(moved->d_parent) : NULL;
>
> I think we need to be more careful here (like dget_parent()?). Fsnotify is
> called after everything is unlocked after rename so dentry can be changing
> under us, cannot it? Also does that mean that we could actually get a wrong
> parent here if the dentry is renamed immediately after we unlock things and
> before we report fsnotify event?

fsnotify_move() is called inside lock_rename() (both old_dir and new_dir locks),
which are held by the caller of vfs_rename(), and prevent d_parent/d_name
changes to child dentries, so moved->d_parent is stable here.
You are probably confusing with inode_unlock(target), which is the
child inode lock.

d_parent/d_name are also stable for fsnotify_create/link/unlink/mkdir/rmdir
per the vfs locking rules for those operations. In all those cases, the parent
dir lock is held for vfs helper callers.
This is why we can use dentry->d_name (and moved->d_name) in all those
fsnotify hooks.

Thanks,
Amir.