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

Stefan Metzmacher
2020-01-29T13:56:18Z

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Am 29.01.20 um 14:41 schrieb Pavel Begunkov:
> 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.

I'll wait for comments from Jens first:-)
I guess we'll have things changed in his branch, when I wake up
tomorrow. Otherwise I can also create patches and submit them.

But I currently don't have an environment where I can do runtime tests
with it.

>>> 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.

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.

What do you mean exactly with one extra get_current_cred()?
Is that any worse than calling get_cred() and having an if check?

It also seems to avoid req->work.creds from being filled at all
for the non-blocking case.

metze

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