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

Pavel Begunkov
2020-01-29T13:41:29Z

On 1/29/2020 4:11 PM, Stefan Metzmacher wrote:
> Am 29.01.20 um 11:17 schrieb Pavel Begunkov:
>> On 29/01/2020 03:54, Jens Axboe wrote:
>>> On 1/28/20 5:24 PM, Jens Axboe wrote:
>>>> On 1/28/20 5:21 PM, Pavel Begunkov wrote:
>>>>> On 29/01/2020 03:20, Jens Axboe wrote:
>>>>>> On 1/28/20 5:10 PM, Pavel Begunkov wrote:
>>>>>>>>>> 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().
>>>>>>>
>>>>>>
>>>>>> We grab one there, and an extra one. Then we drop one of them inline,
>>>>>> and the other in __io_req_aux_free().
>>>>>>
>>>>> Yeah, with the last patch it should make it even
>>>>
>>>> OK good we agree on that. I should probably pull back that bit to the
>>>> original patch to avoid having a hole in there...
>>>
>>> Done
>>>
>>
>> ("io_uring/io-wq: don't use static creds/mm assignments") and ("io_uring:
>> support using a registered personality for commands") looks good now.
>>
>> Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
>
>
> I'm very happy with the design, thanks!
> That exactly what I had in mind:-)
>
> It would also work with IORING_SETUP_SQPOLL, correct?
>

Yep

> However I think there're a few things to improve/simplify.
>

Since 5.6 is already semi-open, it'd be great to have an incremental
patch for that. I'll retoss things as usual, if nobody do it before.

>> https://git.kernel.dk/cgit/linux-block/commit/?h=for-5.6/io_uring-vfs&id=a26d26412e1e1783473f9dc8f030c3af3d54b1a6
>
> In fs/io_uring.c mmgrab() and get_current_cred() are used together in
> two places, why is put_cred() called in __io_req_aux_free while
> mmdrop() is called from io_put_work(). I think both should be called
> in io_put_work(), that makes the code much easier to understand.
>
> My guess is that you choose __io_req_aux_free() for put_cred() because
> of the following patches, but I'll explain on the other commit
> why it's not needed.
>
>> https://git.kernel.dk/cgit/linux-block/commit/?h=for-5.6/io_uring-vfs&id=d9db233adf034bd7855ba06190525e10a05868be
>
> A minor one would be starting with 1 instead of 0 and using
> idr_alloc_cyclic() in order to avoid immediate reuse of ids.
> That way we could include the id in the tracing message and
> 0 would mean the current creds were used.
>
>> +static int io_remove_personalities(int id, void *p, void *data)
>> +{
>> + struct io_ring_ctx *ctx = data;
>> +
>> + idr_remove(&ctx->personality_idr, id);
>
> Here we need something like:
> put_creds((const struct cred *)p);

Good catch

>
>> + return 0;
>> +}
>
>
> The io_uring_register() calles would look like this, correct?
>
> id = io_uring_register(ring_fd, IORING_REGISTER_PERSONALITY, NULL, 0);
> io_uring_register(ring_fd, IORING_UNREGISTER_PERSONALITY, NULL, id);
>
>> https://git.kernel.dk/cgit/linux-block/commit/?h=for-5.6/io_uring-vfs&id=eec9e69e0ad9ad364e1b6a5dfc52ad576afee235
>> +
>> + if (sqe_flags & IOSQE_PERSONALITY) {
>> + int id = READ_ONCE(sqe->personality);
>> +
>> + req->work.creds = idr_find(&ctx->personality_idr, id);
>> + if (unlikely(!req->work.creds)) {
>> + ret = -EINVAL;
>> + goto err_req;
>> + }
>> + get_cred(req->work.creds);> + old_creds = override_creds(req->work.creds);
>> + }
>> +
>
> Here we could use a helper variable
> const struct cred *personality_creds;
> and leave req->work.creds as NULL.
> It means we can avoid the explicit get_cred() call
> and can skip the following hunk too:
>
>> @@ -3977,7 +3977,8 @@ static int io_req_defer_prep(struct io_kiocb *req,
>> mmgrab(current->mm);
>> req->work.mm = current->mm;
>> }
>> - req->work.creds = get_current_cred();
>> + if (!req->work.creds)
>> + req->work.creds = get_current_cred();
>>
>> switch (req->opcode) {
>> case IORING_OP_NOP:
>
> 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.

--
Pavel Begunkov