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

[Stefan Metzmacher <metze@samba.org>]

Am 29.01.20 um 18:42 schrieb Jens Axboe:
> On 1/29/20 10:34 AM, 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.
>>
>>> It could be I'm wrong with the statement above, need to recheck all this
>>> code to be sure.
>>
>> I think you are :-)
>>
>>> 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.
> 
> In particular, would love a patch that only assigns req->work.creds if
> we do go async, so we can leave the put_cred() in io_put_work()
> instead of needing it in __io_req_aux_free().

I made some improvements here:

https://git.samba.org/?p=metze/linux/wip.git;a=shortlog;h=refs/heads/for-5.6/io_uring-vfs

Feel free to squash
https://git.samba.org/?p=metze/linux/wip.git;a=commitdiff;h=ce8812c9b935467bb08ed4d528dd92b9f67e221c
into
https://git.samba.org/?p=metze/linux/wip.git;a=commitdiff;h=22021e95e73d4658a6c834a3276886e127ab8425
and add my review to it.

If you're confident that it's safe you can
call io_req_work_drop_env() also in io_put_work(),

metze