Re: IORING_REGISTER_CREDS[_UPDATE]() and credfd_create()?

[Jens Axboe <axboe@kernel.dk>]

On 1/28/20 5:15 PM, Jens Axboe wrote:
> On 1/28/20 5:10 PM, Pavel Begunkov wrote:
>> On 29/01/2020 02:51, Jens Axboe wrote:
>>> On 1/28/20 4:40 PM, Jens Axboe wrote:
>>>> On 1/28/20 4:36 PM, Pavel Begunkov wrote:
>>>>> On 28/01/2020 22:42, Jens Axboe wrote:
>>>>>> I didn't like it becoming a bit too complicated, both in terms of
>>>>>> implementation and use. And the fact that we'd have to jump through
>>>>>> hoops to make this work for a full chain.
>>>>>>
>>>>>> So I punted and just added sqe->personality and IOSQE_PERSONALITY.
>>>>>> This makes it way easier to use. Same branch:
>>>>>>
>>>>>> https://git.kernel.dk/cgit/linux-block/log/?h=for-5.6/io_uring-vfs-creds
>>>>>>
>>>>>> I'd feel much better with this variant for 5.6.
>>>>>>
>>>>>
>>>>> Checked out ("don't use static creds/mm assignments")
>>>>>
>>>>> 1. do we miscount cred refs? We grab one in get_current_cred() for each async
>>>>> request, but if (worker->creds != work->creds) it will never be put.
>>>>
>>>> Yeah I think you're right, that needs a bit of fixing up.
>>>
>>
>> Hmm, it seems it leaks it unconditionally, as it grabs in a ref in
>> override_creds().
>>
>>> I think this may have gotten fixed with the later addition posted today?
>>> I'll double check. But for the newer stuff, we put it for both cases
>>> when the request is freed.
>>
>> Yeah, maybe. I got tangled trying to verify both at once and decided to start
>> with the old one.
>>
>>
>>>>> 2. shouldn't worker->creds be named {old,saved,etc}_creds? It's set as
>>>>>
>>>>>     worker->creds = override_creds(work->creds);
>>>>>
>>>>> Where override_creds() returns previous creds. And if so, then the following
>>>>> fast check looks strange:
>>>>>
>>>>>     worker->creds != work->creds
>>>>
>>>> Don't care too much about the naming, but the logic does appear off.
>>>> I'll take a look at both of these tonight, unless you beat me to it.
>>
>> Apparently, you're faster :)
>>
>>>
>>> Testing this now, what a braino.
>>>
>>> diff --git a/fs/io-wq.c b/fs/io-wq.c
>>> index ee49e8852d39..8fbbadf04cc3 100644
>>> --- a/fs/io-wq.c
>>> +++ b/fs/io-wq.c
>>> @@ -56,7 +56,8 @@ struct io_worker {
>>>  
>>>  	struct rcu_head rcu;
>>>  	struct mm_struct *mm;
>>> -	const struct cred *creds;
>>> +	const struct cred *cur_creds;
>>> +	const struct cred *saved_creds;
>>>  	struct files_struct *restore_files;
>>>  };
>>>  
>>> @@ -135,9 +136,9 @@ static bool __io_worker_unuse(struct io_wqe *wqe, struct io_worker *worker)
>>>  {
>>>  	bool dropped_lock = false;
>>>  
>>> -	if (worker->creds) {
>>> -		revert_creds(worker->creds);
>>> -		worker->creds = NULL;
>>> +	if (worker->saved_creds) {
>>> +		revert_creds(worker->saved_creds);
>>> +		worker->cur_creds = worker->saved_creds = NULL;
>>>  	}
>>>  
>>>  	if (current->files != worker->restore_files) {
>>> @@ -424,10 +425,11 @@ static void io_wq_switch_mm(struct io_worker *worker, struct io_wq_work *work)
>>>  static void io_wq_switch_creds(struct io_worker *worker,
>>>  			       struct io_wq_work *work)
>>>  {
>>> -	if (worker->creds)
>>> -		revert_creds(worker->creds);
>>> +	if (worker->saved_creds)
>>> +		revert_creds(worker->saved_creds);
>>>  
>>> -	worker->creds = override_creds(work->creds);
>>> +	worker->saved_creds = override_creds(work->creds);
>>> +	worker->cur_creds = work->creds;
>>>  }
>>
>> How about as follows? rever_creds() is a bit heavier than put_creds().
>>
>> static void io_wq_switch_creds(struct io_worker *worker,
>> 			       struct io_wq_work *work)
>> {
>> 	const struct cred *old_creds = override_creds(work->creds);
>>
>> 	if (worker->saved_creds)
>> 		put_cred(old_creds);
>> 	else
>> 		worker->saved_creds = old;
>> 	worker->cur_creds = work->creds;
>> }
> 
> Looks good to me, I'll fold.

Actually, it doesn't clear current->cred then, which seems a bit..
unfortunate. Could be a source of issues.

-- 
Jens Axboe