Re: [PATCH 3/6] io_uring: add support for IORING_OP_OPENAT

[Jens Axboe <axboe@kernel.dk>]

On 1/8/20 4:03 PM, Stefan Metzmacher wrote:
> Am 08.01.20 um 23:53 schrieb Jens Axboe:
>> On 1/8/20 10:04 AM, Stefan Metzmacher wrote:
>>> Am 08.01.20 um 17:40 schrieb Jens Axboe:
>>>> On 1/8/20 9:32 AM, Stefan Metzmacher wrote:
>>>>> Am 08.01.20 um 17:20 schrieb Jens Axboe:
>>>>>> On 1/8/20 6:05 AM, Stefan Metzmacher wrote:
>>>>>>> Hi Jens,
>>>>>>>
>>>>>>>> This works just like openat(2), except it can be performed async. For
>>>>>>>> the normal case of a non-blocking path lookup this will complete
>>>>>>>> inline. If we have to do IO to perform the open, it'll be done from
>>>>>>>> async context.
>>>>>>>
>>>>>>> Did you already thought about the credentials being used for the async
>>>>>>> open? The application could call setuid() and similar calls to change
>>>>>>> the credentials of the userspace process/threads. In order for
>>>>>>> applications like samba to use this async openat, it would be required
>>>>>>> to specify the credentials for each open, as we have to multiplex
>>>>>>> requests from multiple user sessions in one process.
>>>>>>>
>>>>>>> This applies to non-fd based syscall. Also for an async connect
>>>>>>> to a unix domain socket.
>>>>>>>
>>>>>>> Do you have comments on this?
>>>>>>
>>>>>> The open works like any of the other commands, it inherits the
>>>>>> credentials that the ring was setup with. Same with the memory context,
>>>>>> file table, etc. There's currently no way to have multiple personalities
>>>>>> within a single ring.
>>>>>
>>>>> Ah, it's user = get_uid(current_user()); and ctx->user = user in
>>>>> io_uring_create(), right?
>>>>
>>>> That's just for the accounting, it's the:
>>>>
>>>> ctx->creds = get_current_cred();
>>>
>>> Ok, I just looked at an old checkout.
>>>
>>> In kernel-dk-block/for-5.6/io_uring-vfs I see this only used in
>>> the async processing. Does a non-blocking openat also use ctx->creds?
>>
>> There's basically two sets here - one set is in the ring, and the other
>> is the identity that the async thread (briefly) assumes if we have to go
>> async. Right now they are the same thing, and hence we don't need to
>> play any tricks off the system call submitting SQEs to assume any other
>> identity than the one we have.
> 
> I see two cases using it io_sq_thread() and
> io_wq_create()->io_worker_handle_work() call override_creds().
> 
> But aren't non-blocking syscall executed in the context of the thread
> calling io_uring_enter()->io_submit_sqes()?
> In only see some magic around ctx->sqo_mm for that case, but ctx->creds
> doesn't seem to be used in that case. And my design would require that.

For now, the sq thread (which is used if you use IORING_SETUP_SQPOLL)
currently requires fixed files, so it can't be used with open at the
moment anyway. But if/when enabled, it'll assume the same credentials
as the async context and syscall path.

-- 
Jens Axboe