Re: [PATCH v6 3/3] io_uring: allow disabling rings during the creation

[Stefano Garzarella <sgarzare@redhat.com>]

Hi Jens,

On Thu, Aug 27, 2020 at 04:58:31PM +0200, Stefano Garzarella wrote:
> This patch adds a new IORING_SETUP_R_DISABLED flag to start the
> rings disabled, allowing the user to register restrictions,
> buffers, files, before to start processing SQEs.
> 
> When IORING_SETUP_R_DISABLED is set, SQE are not processed and
> SQPOLL kthread is not started.
> 
> The restrictions registration are allowed only when the rings
> are disable to prevent concurrency issue while processing SQEs.
> 
> The rings can be enabled using IORING_REGISTER_ENABLE_RINGS
> opcode with io_uring_register(2).
> 
> Suggested-by: Jens Axboe <axboe@kernel.dk>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> ---
> v4:
>  - fixed io_uring_enter() exit path when ring is disabled
> 
> v3:
>  - enabled restrictions only when the rings start
> 
> RFC v2:
>  - removed return value of io_sq_offload_start()
> ---
>  fs/io_uring.c                 | 52 ++++++++++++++++++++++++++++++-----
>  include/uapi/linux/io_uring.h |  2 ++
>  2 files changed, 47 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 5f62997c147b..b036f3373fbe 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -226,6 +226,7 @@ struct io_restriction {
>  	DECLARE_BITMAP(sqe_op, IORING_OP_LAST);
>  	u8 sqe_flags_allowed;
>  	u8 sqe_flags_required;
> +	bool registered;
>  };
>  
>  struct io_ring_ctx {
> @@ -7497,8 +7498,8 @@ static int io_init_wq_offload(struct io_ring_ctx *ctx,
>  	return ret;
>  }
>  
> -static int io_sq_offload_start(struct io_ring_ctx *ctx,
> -			       struct io_uring_params *p)
> +static int io_sq_offload_create(struct io_ring_ctx *ctx,
> +				struct io_uring_params *p)
>  {
>  	int ret;
>  
> @@ -7532,7 +7533,6 @@ static int io_sq_offload_start(struct io_ring_ctx *ctx,
>  			ctx->sqo_thread = NULL;
>  			goto err;
>  		}
> -		wake_up_process(ctx->sqo_thread);
>  	} else if (p->flags & IORING_SETUP_SQ_AFF) {
>  		/* Can't have SQ_AFF without SQPOLL */
>  		ret = -EINVAL;
> @@ -7549,6 +7549,12 @@ static int io_sq_offload_start(struct io_ring_ctx *ctx,
>  	return ret;
>  }
>  
> +static void io_sq_offload_start(struct io_ring_ctx *ctx)
> +{
> +	if ((ctx->flags & IORING_SETUP_SQPOLL) && ctx->sqo_thread)
> +		wake_up_process(ctx->sqo_thread);
> +}
> +
>  static inline void __io_unaccount_mem(struct user_struct *user,
>  				      unsigned long nr_pages)
>  {
> @@ -8295,6 +8301,9 @@ SYSCALL_DEFINE6(io_uring_enter, unsigned int, fd, u32, to_submit,
>  	if (!percpu_ref_tryget(&ctx->refs))
>  		goto out_fput;
>  
> +	if (ctx->flags & IORING_SETUP_R_DISABLED)
> +		goto out_fput;
> +

While writing the man page paragraph, I discovered that if the rings are
disabled I returned ENXIO error in io_uring_enter(), coming from the previous
check.

I'm not sure it is the best one, maybe I can return EBADFD or another
error.

What do you suggest?

I'll add a test for this case.

Thanks,
Stefano