Dear maintainers, I'm looking into io_uring internals and find there might be a bug in io_do_iopoll, but I'm struggling to construct a POC due to my lack of knowledge about the kernel internals. So maybe it's better to put it here for discussion. After [issuing an SQE](https://elixir.bootlin.com/linux/v6.9/source/io_uring/io_uring.c#L1920) in iopoll-enabled io_uring, if the return value is `IOU_ISSUE_SKIP_COMPLETE` and the op supports `iopoll_queue`, the req will be added to `ctx->iopoll_list` and later retrieved in `io_do_iopoll`, where `iopoll` or `uring_cmd_iopoll` of the req file op is [called](https://elixir.bootlin.com/linux/v6.9/source/io_uring/rw.c#L1167). IMHO here we miss a check of whether `iopoll` or `uring_cmd_iopoll` is implemented. A more understandable case for me is, the custom ioctl function with [IORING_OP_URING_CMD](https://elixir.bootlin.com/linux/v6.11.7/source/io_uring/opdef.c#L416) satisfies all the constraints and will go to this path if `uring_cmd` returns `-EIOCBQUEUED` [here](https://elixir.bootlin.com/linux/v6.9/source/io_uring/uring_cmd.c#L192). So this requires that all the ops with `->uring_cmd` returning `-EIOCBQUEUED` should support `->uring_cmd_iopoll` as well, which is not the case for [ublk_ch_fops](https://elixir.bootlin.com/linux/v6.9/source/drivers/block/ublk_drv.c#L1967) resulting in a nullptr-deref in `io_do_poll`. I'm wondering if this is legit. Note that the related code changes a bit for [newer kernel](https://elixir.bootlin.com/linux/v6.11/source/io_uring/uring_cmd.c#L261). Best Regards
