Re: [PATCH v5 8/9] rust: file: add `DeferredFdCloser`

[Alice Ryhl]

On Wed, Mar 20, 2024 at 3:22 PM Christian Brauner <brauner@xxxxxxxxxx> wrote:
>
> On Fri, Feb 09, 2024 at 11:18:21AM +0000, Alice Ryhl wrote:
> >
> > +/// Helper used for closing file descriptors in a way that is safe even if the file is currently
> > +/// held using `fdget`.
> > +///
> > +/// Additional motivation can be found in commit 80cd795630d6 ("binder: fix use-after-free due to
> > +/// ksys_close() during fdget()") and in the comments on `binder_do_fd_close`.
> > +pub struct DeferredFdCloser {
> > +    inner: Box<DeferredFdCloserInner>,
> > +}
> > +
> > +/// SAFETY: This just holds an allocation with no real content, so there's no safety issue with
> > +/// moving it across threads.
> > +unsafe impl Send for DeferredFdCloser {}
> > +unsafe impl Sync for DeferredFdCloser {}
> > +
> > +/// # Invariants
> > +///
> > +/// If the `file` pointer is non-null, then it points at a `struct file` and owns a refcount to
> > +/// that file.
> > +#[repr(C)]
> > +struct DeferredFdCloserInner {
> > +    twork: mem::MaybeUninit<bindings::callback_head>,
> > +    file: *mut bindings::file,
> > +}
> > +
> > +impl DeferredFdCloser {
>
> So the explicitly deferred close is due to how binder works so it's not
> much of a general purpose interface as I don't recall having other
> codepaths with similar problems. So this should live in the binder
> specific rust code imo.

Hmm. Are there really no other ioctls that call ksys_close on a
user-provided fd?

As far as I can tell, this kind of deferred API is the only way for us
to provide a fully safe Rust api for closing an fd. Directly calling
ksys_close must unsafely assert that the fd does not have an active
fdget call. So it makes sense to me as an API that others might want
to use.

Still I can move it elsewhere if necessary.

Alice