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

[Stefano Garzarella <sgarzare@redhat.com>]

On Tue, Sep 08, 2020 at 07:57:08AM -0600, Jens Axboe wrote:
> On 9/8/20 7:44 AM, Stefano Garzarella wrote:
> > 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?
> 
> EBADFD seems indeed the most appropriate - the fd is valid, but not in the
> right state to do this.

Yeah, the same interpretation as mine!

Also, in io_uring_register() I'm returning EINVAL if the rings are not
disabled and the user wants to register restrictions.
Maybe also in this case I can return EBADFD.

I'll send a patch with the fixes.

Thanks,
Stefano