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

[Jens Axboe <axboe@kernel.dk>]

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.

> I'll add a test for this case.

Thanks!

-- 
Jens Axboe