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

Jens Axboe
2020-01-29T00:15:59Z

On 1/28/20 5:10 PM, Pavel Begunkov wrote:
> On 29/01/2020 02:51, Jens Axboe wrote:
>> On 1/28/20 4:40 PM, Jens Axboe wrote:
>>> On 1/28/20 4:36 PM, Pavel Begunkov wrote:
>>>> On 28/01/2020 22:42, Jens Axboe wrote:
>>>>> I didn't like it becoming a bit too complicated, both in terms of
>>>>> implementation and use. And the fact that we'd have to jump through
>>>>> hoops to make this work for a full chain.
>>>>>
>>>>> So I punted and just added sqe->personality and IOSQE_PERSONALITY.
>>>>> This makes it way easier to use. Same branch:
>>>>>
>>>>> https://git.kernel.dk/cgit/linux-block/log/?h=for-5.6/io_uring-vfs-creds
>>>>>
>>>>> I'd feel much better with this variant for 5.6.
>>>>>
>>>>
>>>> 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().
>
>> I think this may have gotten fixed with the later addition posted today?
>> I'll double check. But for the newer stuff, we put it for both cases
>> when the request is freed.
>
> Yeah, maybe. I got tangled trying to verify both at once and decided to start
> with the old one.
>
>
>>>> 2. shouldn't worker->creds be named {old,saved,etc}_creds? It's set as
>>>>
>>>> worker->creds = override_creds(work->creds);
>>>>
>>>> Where override_creds() returns previous creds. And if so, then the following
>>>> fast check looks strange:
>>>>
>>>> worker->creds != work->creds
>>>
>>> Don't care too much about the naming, but the logic does appear off.
>>> I'll take a look at both of these tonight, unless you beat me to it.
>
> Apparently, you're faster :)
>
>>
>> Testing this now, what a braino.
>>
>> diff --git a/fs/io-wq.c b/fs/io-wq.c
>> index ee49e8852d39..8fbbadf04cc3 100644
>> --- a/fs/io-wq.c
>> +++ b/fs/io-wq.c
>> @@ -56,7 +56,8 @@ struct io_worker {
>>
>> struct rcu_head rcu;
>> struct mm_struct *mm;
>> - const struct cred *creds;
>> + const struct cred *cur_creds;
>> + const struct cred *saved_creds;
>> struct files_struct *restore_files;
>> };
>>
>> @@ -135,9 +136,9 @@ static bool __io_worker_unuse(struct io_wqe *wqe, struct io_worker *worker)
>> {
>> bool dropped_lock = false;
>>
>> - if (worker->creds) {
>> - revert_creds(worker->creds);
>> - worker->creds = NULL;
>> + if (worker->saved_creds) {
>> + revert_creds(worker->saved_creds);
>> + worker->cur_creds = worker->saved_creds = NULL;
>> }
>>
>> if (current->files != worker->restore_files) {
>> @@ -424,10 +425,11 @@ static void io_wq_switch_mm(struct io_worker *worker, struct io_wq_work *work)
>> static void io_wq_switch_creds(struct io_worker *worker,
>> struct io_wq_work *work)
>> {
>> - if (worker->creds)
>> - revert_creds(worker->creds);
>> + if (worker->saved_creds)
>> + revert_creds(worker->saved_creds);
>>
>> - worker->creds = override_creds(work->creds);
>> + worker->saved_creds = override_creds(work->creds);
>> + worker->cur_creds = work->creds;
>> }
>
> How about as follows? rever_creds() is a bit heavier than put_creds().
>
> static void io_wq_switch_creds(struct io_worker *worker,
> struct io_wq_work *work)
> {
> const struct cred *old_creds = override_creds(work->creds);
>
> if (worker->saved_creds)
> put_cred(old_creds);
> else
> worker->saved_creds = old;
> worker->cur_creds = work->creds;
> }

Looks good to me, I'll fold.

--
Jens Axboe