Re: [PATCH 7/9] io_uring: add per-task callback handler

[Jens Axboe <axboe@kernel.dk>]

On 2/23/20 4:02 AM, Pavel Begunkov wrote:
> On 23/02/2020 09:26, Jens Axboe wrote:
>> On 2/22/20 11:00 PM, Jens Axboe wrote:
>>> On 2/21/20 12:10 PM, Jens Axboe wrote:
>>>>> Got it. Then, it may happen in the future after returning from
>>>>> __io_arm_poll_handler() and io_uring_enter(). And by that time io_submit_sqes()
>>>>> should have already restored creds (i.e. personality stuff) on the way back.
>>>>> This might be a problem.
>>>>
>>>> Not sure I follow, can you elaborate? Just to be sure, the requests that
>>>> go through the poll handler will go through __io_queue_sqe() again. Oh I
>>>> guess your point is that that is one level below where we normally
>>>> assign the creds.
>>>
>>> Fixed this one.
> 
> Looking at
> 
> io_async_task_func() {
> 	...
> 	/* ensure req->work.creds is valid for __io_queue_sqe() */
> 	req->work.creds = apoll->work.creds;
> }
> 
> It copies creds, but doesn't touch the rest req->work fields. And if you have
> one, you most probably got all of them in *grab_env(). Are you sure it doesn't
> leak, e.g. mmgrab()'ed mm?

You're looking at a version that only existed for about 20 min, had to
check I pushed it out. But ce21471abe0fef is the current one, it does
a full memcpy() of it.

>>>>> BTW, Is it by design, that all requests of a link use personality creds
>>>>> specified in the head's sqe?
>>>>
>>>> No, I think that's more by accident. We should make sure they use the
>>>> specified creds, regardless of the issue time. Care to clean that up?
>>>> Would probably help get it right for the poll case, too.
>>>
>>> Took a look at this, and I think you're wrong. Every iteration of
>>> io_submit_sqe() will lookup the right creds, and assign them to the
>>> current task in case we're going to issue it. In the case of a link
>>> where we already have the head, then we grab the current work
>>> environment. This means assigning req->work.creds from
>>> get_current_cred(), if not set, and these are the credentials we looked
>>> up already.
> 
> Yeah, I've spotted that there something wrong, but never looked up properly.

And thanks for that!

>> What does look wrong is that we don't restore the right credentials for
>> queuing the head, so basically the opposite problem. Something like the
>> below should fix that.
>> index de650df9ac53..59024e4757d6 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -4705,11 +4705,18 @@ static void __io_queue_sqe(struct io_kiocb *req, const struct io_uring_sqe *sqe)
>>  {
>>  	struct io_kiocb *linked_timeout;
>>  	struct io_kiocb *nxt = NULL;
>> +	const struct cred *old_creds = NULL;
>>  	int ret;
>>  
>>  again:
>>  	linked_timeout = io_prep_linked_timeout(req);
>>  
>> +	if (req->work.creds && req->work.creds != get_current_cred()) {
> 
> get_current_cred() gets a ref.

Oops yes

> See my attempt below, it fixes miscount, and should work better for
> cases changing back to initial creds (i.e. personality 0)

Thanks, I'll fold this in, if you don't mind.

> Anyway, creds handling is too scattered across the code, and this do a
> lot of useless refcounting and bouncing. It's better to find it a
> better place in the near future.

I think a good cleanup on top of this would be to move the personality
lookup to io_req_defer_prep(), and kill it from io_submit_sqe(). Now
__io_issue_sqe() does the right thing, and it'll just fall out nicely
with that as far as I can tell.

Care to send a patch for that?

-- 
Jens Axboe