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

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

On 1/29/2020 8:34 PM, Jens Axboe wrote:
> On 1/29/20 7:23 AM, Pavel Begunkov wrote:
>>>>> The override_creds(personality_creds) has changed current->cred
>>>>> and get_current_cred() will just pick it up as in the default case.
>>>>>
>>>>> This would make the patch much simpler and allows put_cred() to be
>>>>> in io_put_work() instead of __io_req_aux_free() as explained above.
>>>>>
>>>>
>>>> It's one extra get_current_cred(). I'd prefer to find another way to
>>>> clean this up.
>>>
>>> As far as I can see it avoids a get_cred() in the IOSQE_PERSONALITY case
>>> and the if (!req->work.creds) for both cases.
>>
>> Great, that you turned attention to that! override_creds() is already
>> grabbing a ref, so it shouldn't call get_cred() there.
>> So, that's a bug.
> 
> It's not though - one is dropped in that function, the other when the
> request is freed. So we do need two references to it. With the proposed
> change to keep the override_creds() variable local for that spot we
> don't, and the get_cred() can then go.

You're right here. It seems, it was too much looking at the same code :)

> 
>> It could be I'm wrong with the statement above, need to recheck all this
>> code to be sure.
> 
> I think you are :-)

Considering above, there shouldn't be much difference indeed.
One extra rcu_dereference() in get_current_creds() instead of
get_cred(), but that's nothing.

Later we can hide one get by using submission state from the long
patchset, I sent a while ago.

> 
>> BTW, io_req_defer_prep() may be called twice for a req, so you will
>> reassign it without putting a ref. It's safer to leave NULL checks. At
>> least, until I've done reworking and fixing preparation paths.
> 
> Agree, the NULL checks are safer and we should keep them.
> 
> Going through the rest of this thread, I'm making the following changes:
> 
> - ID must be > 0. I like that change, as we don't need an sqe flag to
>   select personality then, and it also makes it obvious that id == 0 is
>   just using current creds.
> 
> - Fixed the missing put_cred() in the teardown
> 
> - Use a local variable in io_submit_sqe() instead of assigning the
>   creds to req->work.creds there
> 
> - Use cyclic idr allocation
> 
> I'm going to fold in as appropriate. If there are fixes needed on top of
> that, let's do them separately.
> 

-- 
Pavel Begunkov