Re: [RFC PATCH v3 2/5] io_uring/cmd: zero-init pdu in io_uring_cmd_prep() to avoid UB

[Caleb Sander Mateos <csander@purestorage.com>]

On Fri, Aug 22, 2025 at 5:56 AM Sidong Yang <sidong.yang@furiosa.ai> wrote:
>
> The pdu field in io_uring_cmd may contain stale data when a request
> object is recycled from the slab cache. Accessing uninitialized or
> garbage memory can lead to undefined behavior in users of the pdu.
>
> Ensure the pdu buffer is cleared during io_uring_cmd_prep() so that
> each command starts from a well-defined state. This avoids exposing
> uninitialized memory and prevents potential misinterpretation of data
> from previous requests.
>
> No functional change is intended other than guaranteeing that pdu is
> always zero-initialized before use.
>
> Signed-off-by: Sidong Yang <sidong.yang@furiosa.ai>
> ---
>  io_uring/uring_cmd.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/io_uring/uring_cmd.c b/io_uring/uring_cmd.c
> index 053bac89b6c0..2492525d4e43 100644
> --- a/io_uring/uring_cmd.c
> +++ b/io_uring/uring_cmd.c
> @@ -203,6 +203,7 @@ int io_uring_cmd_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
>         if (!ac)
>                 return -ENOMEM;
>         ioucmd->sqe = sqe;
> +       memset(&ioucmd->pdu, 0, sizeof(ioucmd->pdu));

Adding this overhead to every existing uring_cmd() implementation is
unfortunate. Could we instead track the initialized/uninitialized
state by using different types on the Rust side? The io_uring_cmd
could start as an IoUringCmd, where the PDU field is MaybeUninit,
write_pdu<T>() could return a new IoUringCmdPdu<T> that guarantees the
PDU has been initialized.

Best,
Caleb

>         return 0;
>  }
>
> --
> 2.43.0
>