Re: [PATCH] io_uring/rsrc: Simplify buffer cloning by locking both rings

[Jann Horn <jannh@google.com>]

On Wed, Jan 15, 2025 at 6:18 PM Jens Axboe <axboe@kernel.dk> wrote:
> On 1/15/25 9:25 AM, Jann Horn wrote:
> > The locking in the buffer cloning code is somewhat complex because it goes
> > back and forth between locking the source ring and the destination ring.
> >
> > Make it easier to reason about by locking both rings at the same time.
> > To avoid ABBA deadlocks, lock the rings in ascending kernel address order,
> > just like in lock_two_nondirectories().
> >
> > Signed-off-by: Jann Horn <jannh@google.com>
> > ---
> > Just an idea for how I think io_clone_buffers() could be changed so it
> > becomes slightly easier to reason about.
> > I left the out_unlock jump label with its current name for now, though
> > I guess that should probably be adjusted.
>
> Looks pretty clean to me, and does make it easier to reason about. Only
> thing that stuck out to me was:
>
> > @@ -1067,7 +1060,18 @@ int io_register_clone_buffers(struct io_ring_ctx *ctx, void __user *arg)
> >       file = io_uring_register_get_file(buf.src_fd, registered_src);
> >       if (IS_ERR(file))
> >               return PTR_ERR(file);
> > -     ret = io_clone_buffers(ctx, file->private_data, &buf);
> > +     src_ctx = file->private_data;
> > +     if (src_ctx == ctx) {
> > +             ret = -ELOOP;
> > +             goto out_put;
> > +     }
>
> which is a change, as previously it would've been legal to do something ala:
>
> struct io_uring ring;
> struct iovec vecs[2];
>
> vecs[0] = real_buffer;
> vecs[1] = sparse_buffer;
>
> io_uring_register_buffers(&ring, vecs, 2);
>
> io_uring_clone_buffers_offset(&ring, &ring, 1, 0, 1, IORING_REGISTER_DST_REPLACE);
>
> and clone vecs[0] into slot 1. With the patch, that'll return -ELOOP instead.
>
> Maybe something like the below incremental, to just make the unlock +
> double lock depending on whether they are different or not? And also
> cleaning up the label naming at the same time.

Yeah, looks good to me. If nobody else has review feedback, do you
want to fold that in locally? If there's more feedback, I'll fold that
incremental into my v2.