On Tue, Jan 14, 2020 at 6:22 PM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
>
> On Tue, Jan 14, 2020 at 05:40:34PM +0200, Amir Goldstein wrote:
> > Generalize the take_name_snapshot()/release_name_snapshot() interface
> > so it is possible to snapshot either a dentry d_name or its snapshot.
> >
> > The order of fields d_name and d_inode in struct dentry is swapped
> > so d_name is adjacent to d_iname. This does not change struct size
> > nor cache lines alignment.
> >
> > Currently, we snapshot the old name in vfs_rename() and we snapshot the
> > snapshot the dentry name in __fsnotify_parent() and then we pass qstr
> > to inotify which allocated a variable length event struct and copied the
> > name.
> >
> > This new interface allows us to snapshot the name directly into an
> > fanotify event struct instead of allocating a variable length struct
> > and copying the name to it.
>
> Ugh... That looks like being too damn cute for no good reason. That
> trick with union is _probably_ safe, but I wouldn't bet a dime on
> e.g. randomize_layout crap not screwing it over in random subset of
> gcc versions. You are relying upon fairly subtle reading of 6.2.7
> and it feels like just the place for layout-mangling plugins to fuck
> up.
>
> With -fplan9-extensions we could go for renaming struct name_snapshot
> fields and using an anon member in struct dentry -
> ...
> struct inode *d_inode;
> struct name_snapshot; // d_name and d_iname
> struct lockref d_lockref;
> ...
>
> but IMO it's much safer to have an explicit
>
> // NOTE: release_dentry_name_snapshot() will be needed for both copies.
> clone_name_snapshot(struct name_snapshot *to, const struct name_snapshot *from)
> {
> to->name = from->name;
> if (likely(to->name.name == from->inline_name)) {
> memcpy(to->inline_name, from->inline_name,
> to->name.len);
> to->name.name = to->inline_name;
> } else {
> struct external_name *p;
> p = container_of(to->name.name, struct external_name, name[0]);
> atomic_inc(&p->u.count);
> }
> }
>
> and be done with that. Avoids any extensions or tree-wide renamings, etc.
I started with something like this but than in one of the early
revisions I needed
to pass some abstract reference around before cloning the name into the event,
but with my current patches I can get away with a simple:
if (data_type == FANOTIFY_EVENT_NAME)
clone_name_snapshot(&event->name, data);
else if (dentry)
take_dentry_name_snapshot(&event->name, dentry);
So that simple interface should be good enough for my needs.
Thanks,
Amir.
