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

[Stefan Metzmacher <metze@samba.org>]

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Am 09.01.20 um 00:05 schrieb Jens Axboe:
> 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.

I'm sorry, but I'm still unsure we're talking about the same thing
(or maybe I'm missing some basics here).

My understanding of the io_uring_enter() is that it will execute as much
non-blocking calls as it can without switching to any other kernel thread.

And my fear is that openat will use get_current_cred() instead of
ctx->creds.

I'm I missing something?

Thanks!
metze


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