Re: [RFC] Check if file_data is initialized

[Jens Axboe <axboe@kernel.dk>]

On 1/9/20 8:17 AM, Pavel Begunkov wrote:
> 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]);

Yeah that looks wrong, I don't think you're missing something. We
should just use the flags check again. I'll double check this in
testing now.

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 49622a320317..d7a77830a2f2 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1235,8 +1235,6 @@ static void io_free_req_many(struct io_ring_ctx *ctx, struct req_batch *rb)
 			}
 			if (req->flags & REQ_F_INFLIGHT)
 				inflight++;
-			else
-				rb->reqs[i] = NULL;
 			__io_req_aux_free(req);
 		}
 		if (!inflight)
@@ -1246,7 +1244,7 @@ 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) {
+			if (req->flags & REQ_F_INFLIGHT)
 				list_del(&req->inflight_entry);
 				if (!--inflight)
 					break;

-- 
Jens Axboe