On Thu, Jun 07, 2018 at 03:15:15PM -0700, Cong Wang wrote:
> > You do realize that the same ->setattr() can be called by way of
> > chown() on /proc/self/fd/<n>, right? What would you do there -
> > bump refcount on that struct file when traversing that symlink and
> > hold it past the end of pathname resolution, until... what exactly?
>
> I was thinking about this:
>
> error = user_path_at(dfd,....); // hold dfd when needed
>
> if (!error) {
> error = chown_common(&path, mode);
> path_put(&path); // release dfd if held
>
> With this, we can guarantee ->release() is only possibly called
> after chown_common() which is after ->setattr() too.
No, we can't. You are assuming that there *is* dfd and that it points
to the opened socket we are going to operate upon. That is not guaranteed.
At all. If e.g. 42 is a file descriptor of an opened socket, plain chown(2)
on /proc/self/fd/42 will trigger that ->setattr(). No dfd in sight.
We do run across an opened file at some point, all right - when we traverse
the symlink in procfs. You can't bump ->f_count there. Even leaving aside
the "where would I stash the reference to that file?" and "how long would I
hold it?", you can't bump ->f_count on other process' files. That would
bugger the expectations of close(2) callers.
