Re: [RFC] Check if file_data is initialized

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

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.

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?

> 
> More details about configuration:
> 
> [global]
> filename=/dev/vda
> rw=randread
> bs=256k
> direct=1
> time_based=1
> randrepeat=1
> gtod_reduce=1
> 
> [fiotest]
> 
> fio test.fio \
>     --readonly \
>     --ioengine=io_uring \
>     --iodepth 1024 \
>     --fixedbufs \
>     --hipri \
>     --numjobs=1 \
>     --runtime=10
> 
> Signed-off-by: Dmitrii Dolgov <9erthalion6@gmail.com>
> ---
>  fs/io_uring.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> I'm not entirely sure if my analysis is correct, but since this change
> fixes the issue for me, I've decided to post it.
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index c770c2c0eb52..c5e69dfc0221 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -1232,7 +1232,8 @@ 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);
>  	percpu_ref_put_many(&ctx->refs, rb->to_free);
> -	percpu_ref_put_many(&ctx->file_data->refs, rb->to_free);
> +	if (ctx->file_data)
> +		percpu_ref_put_many(&ctx->file_data->refs, rb->to_free);
>  	rb->to_free = rb->need_iter = 0;
>  }
>  
> 

-- 
Pavel Begunkov