Re: [RFC] Check if file_data is initialized

[Pavel Begunkov <asml.silence@gmail.com>]

On 1/9/2020 5:51 PM, Jens Axboe wrote:
> On 1/9/20 7:26 AM, Pavel Begunkov wrote:
>> On 1/9/2020 4:17 PM, Dmitrii Dolgov wrote:
>>> With combination of --fixedbufs and an old version of fio I've managed
>>> to get a strange situation, when doing io_iopoll_complete NULL pointer
>>> dereference on file_data was caused in io_free_req_many. Interesting
>>> enough, the very same configuration doesn't fail on a newest version of
>>> fio (the old one is fc220349e4514, the new one is 2198a6b5a9f4), but I
>>> guess it still makes sense to have this check if it's possible to craft
>>> such request to io_uring.
>>
>> I didn't looked up why it could become NULL in the first place, but the
>> problem is probably deeper.
>>
>> 1. I don't see why it puts @rb->to_free @file_data->refs, even though
>> there could be non-fixed reqs. It needs to count REQ_F_FIXED_FILE reqs
>> and put only as much.
> 
> Agree on the fixed file refs, there's a bug there where it assumes they
> are all still fixed. See below - Dmitrii, use this patch for testing
> instead of the other one!
> 
>> 2. Jens, there is another line bothering me, could you take a look?
>>
>> io_free_req_many()
>> {
>> ...
>> 	if (req->flags & REQ_F_INFLIGHT) ...;
>> 	else
>> 		rb->reqs[i] = NULL;
>> ...
>> }
>>
>> It zeroes rb->reqs[i], calls __io_req_aux_free(), but did not free
>> memory for the request itself. Is it as intended?
> 
> We free them at the end of that function, in bulk. But we can't do that
> with the aux data.

Right, we can't do that with the aux data. But we NULL a req in the
array, which then passed to kmem_cache_free_bulk(). So, it won't be
visible to the *_free_bulk(). Am I missing something?

e.g.
1. initial reqs [req1 with files, ->io, etc]
2. set to NULL, so [NULL]
3. __io_req_aux_free(req)
4. bulk_free([NULL]);

> 
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 32aee149f652..b5dcf6c800ef 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -1218,6 +1218,8 @@ struct req_batch {
>  
>  static void io_free_req_many(struct io_ring_ctx *ctx, struct req_batch *rb)
>  {
> +	int fixed_refs = 0;
> +

If all are fixed, then @rb->need_iter == false (see
io_req_multi_free()), and @fixed_refs will be left 0. How about to set
it to rb->to_free, and zero+count for rb->need_iter == true?

>  	if (!rb->to_free)
>  		return;
>  	if (rb->need_iter) {
> @@ -1227,8 +1229,10 @@ static void io_free_req_many(struct io_ring_ctx *ctx, struct req_batch *rb)
>  		for (i = 0; i < rb->to_free; i++) {
>  			struct io_kiocb *req = rb->reqs[i];
>  
> -			if (req->flags & REQ_F_FIXED_FILE)
> +			if (req->flags & REQ_F_FIXED_FILE) {
>  				req->file = NULL;
> +				fixed_refs++;
> +			}
>  			if (req->flags & REQ_F_INFLIGHT)
>  				inflight++;
>  			else
> @@ -1255,8 +1259,9 @@ static void io_free_req_many(struct io_ring_ctx *ctx, struct req_batch *rb)
>  	}
>  do_free:
>  	kmem_cache_free_bulk(req_cachep, rb->to_free, rb->reqs);
> +	if (fixed_refs)
> +		percpu_ref_put_many(&ctx->file_data->refs, fixed_refs);
>  	percpu_ref_put_many(&ctx->refs, rb->to_free);
> -	percpu_ref_put_many(&ctx->file_data->refs, rb->to_free);
>  	rb->to_free = rb->need_iter = 0;
>  }
> 

-- 
Pavel Begunkov