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

[Jens Axboe <axboe@kernel.dk>]

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.

-- 
Jens Axboe