Re: [PATCH 13/18] io_uring: add file set registration

[Jens Axboe <axboe@kernel.dk>]

On 2/12/19 10:21 AM, Alan Jenkins wrote:
> On 12/02/2019 15:17, Jens Axboe wrote:
>> On 2/12/19 5:29 AM, Alan Jenkins wrote:
>>> On 08/02/2019 15:13, Jens Axboe wrote:
>>>> On 2/8/19 7:02 AM, Alan Jenkins wrote:
>>>>> On 08/02/2019 12:57, Jens Axboe wrote:
>>>>>> On 2/8/19 5:17 AM, Alan Jenkins wrote:
>>>>>>>> +static int io_sqe_files_scm(struct io_ring_ctx *ctx)
>>>>>>>> +{
>>>>>>>> +#if defined(CONFIG_NET)
>>>>>>>> +	struct scm_fp_list *fpl = ctx->user_files;
>>>>>>>> +	struct sk_buff *skb;
>>>>>>>> +	int i;
>>>>>>>> +
>>>>>>>> +	skb =  __alloc_skb(0, GFP_KERNEL, 0, NUMA_NO_NODE);
>>>>>>>> +	if (!skb)
>>>>>>>> +		return -ENOMEM;
>>>>>>>> +
>>>>>>>> +	skb->sk = ctx->ring_sock->sk;
>>>>>>>> +	skb->destructor = unix_destruct_scm;
>>>>>>>> +
>>>>>>>> +	fpl->user = get_uid(ctx->user);
>>>>>>>> +	for (i = 0; i < fpl->count; i++) {
>>>>>>>> +		get_file(fpl->fp[i]);
>>>>>>>> +		unix_inflight(fpl->user, fpl->fp[i]);
>>>>>>>> +		fput(fpl->fp[i]);
>>>>>>>> +	}
>>>>>>>> +
>>>>>>>> +	UNIXCB(skb).fp = fpl;
>>>>>>>> +	skb_queue_head(&ctx->ring_sock->sk->sk_receive_queue, skb);
>>>>>>> This code sounds elegant if you know about the existence of unix_gc(),
>>>>>>> but quite mysterious if you don't.  (E.g. why "inflight"?)  Could we
>>>>>>> have a brief comment, to comfort mortal readers on their journey?
>>>>>>>
>>>>>>> /* A message on a unix socket can hold a reference to a file. This can
>>>>>>> cause a reference cycle. So there is a garbage collector for unix
>>>>>>> sockets, which we hook into here. */
>>>>>> Yes that's a good idea, I've added a comment as to why we go through the
>>>>>> trouble of doing this socket + skb dance.
>>>>> Great, thanks.
>>>>>
>>>>>>> I think this is bypassing too_many_unix_fds() though?  I understood that
>>>>>>> was intended to bound kernel memory allocation, at least in principle.
>>>>>> As the code stands above, it'll cap it at 253. I'm just now reworking it
>>>>>> to NOT be limited to the SCM max fd count, but still impose a limit of
>>>>>> 1024 on the number of registered files. This is important to cap the
>>>>>> memory allocation attempt as well.
>>>>> I saw you were limiting to SCM_MAX_FD per io_uring.  On the other hand,
>>>>> there's no specific limit on the number of io_urings you can open (only
>>>>> the standard limits on fds).  So this would let you allocate hundreds of
>>>>> times more files than the previous limit RLIMIT_NOFILE...
>>>> But there is, the io_uring itself is under the memlock rlimit.
>>>>
>>>>> static inline bool too_many_unix_fds(struct task_struct *p)
>>>>> {
>>>>> 	struct user_struct *user = current_user();
>>>>>
>>>>> 	if (unlikely(user->unix_inflight > task_rlimit(p, RLIMIT_NOFILE)))
>>>>> 		return !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN);
>>>>> 	return false;
>>>>> }
>>>>>
>>>>> RLIMIT_NOFILE is technically per-task, but here it is capping
>>>>> unix_inflight per-user.  So the way I look at this, the number of file
>>>>> descriptors per user is bounded by NOFILE * NPROC.  Then
>>>>> user->unix_inflight can have one additional process' worth (NOFILE) of
>>>>> "inflight" files.  (Plus SCM_MAX_FD slop, because too_many_fds() is only
>>>>> called once per SCM_RIGHTS).
>>>>>
>>>>> Because io_uring doesn't check too_many_unix_fds(), I think it will let
>>>>> you have about 253 (or 1024) more process' worth of open files. That
>>>>> could be big proportionally when RLIMIT_NPROC is low.
>>>>>
>>>>> I don't know if it matters.  It maybe reads like an oversight though.
>>>>>
>>>>> (If it does matter, it might be cleanest to change too_many_unix_fds()
>>>>> to get rid of the "slop".  Since that may be different between af_unix
>>>>> and io_uring; 253 v.s. 1024 or whatever. E.g. add a parameter for the
>>>>> number of inflight files we want to add.)
>>>> I don't think it matters. The files in the fixed file set have already
>>>> been opened by the application, so it counts towards the number of open
>>>> files that is allowed to have. I don't think we should impose further
>>>> limits on top of that.
>>> A process can open one io_uring and 199 other files.  Register the 199
>>> files in the io_uring, then close their file descriptors.  The main
>>> NOFILE limit only counts file descriptors.  So then you can open one
>>> io_uring, 198 other files, and repeat.
>>>
>>> You're right, I had forgotten the memlock limit on io_uring.  That makes
>>> it much less of a practical problem.
>>>
>>> But it raises a second point.  It's not just that it lets users allocate
>>> more files.  You might not want to be limited by user->unix_inflight.
>>> But you are calling unix_inflight(), which increments it!  Then if
>>> unix->inflight exceeds the NOFILE limit, you will avoid seeing any
>>> errors with io_uring, but the user will not be able to send files over
>>> unix sockets.
>>>
>>> So I think this is confusing to read, and confusing to troubleshoot if
>>> the limit is ever hit.
>>>
>>> I would be happy if io_uring didn't increment user->unix_inflight.  I'm
>>> not sure what the best way is to arrange that.
>> How about we just do something like the below? I think that's the saner
>> approach, rather than bypass user->unix_inflight. It's literally the
>> same thing.
>>
>>
>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>> index a4973af1c272..5196b3aa935e 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -2041,6 +2041,13 @@ static int __io_sqe_files_scm(struct io_ring_ctx *ctx, int nr, int offset)
>>   	struct sk_buff *skb;
>>   	int i;
>>   
>> +	if (!capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN)) {
>> +		struct user_struct *user = ctx->user;
>> +
>> +		if (user->unix_inflight > task_rlimit(current, RLIMIT_NOFILE))
>> +			return -EMFILE;
>> +	}
>> +
>>   	fpl = kzalloc(sizeof(*fpl), GFP_KERNEL);
>>   	if (!fpl)
>>   		return -ENOMEM;
>>
>>
> 
> Welp, you gave me exactly what I asked for.  So now I'd better be 
> positive about it :-D.

;-)

> I hope this will be documented accurately, at least where the EMFILE 
> result is explained for this syscall.

How's this:

http://git.kernel.dk/cgit/liburing/commit/?id=37e48698a09aa1e37690f8fa6dfd8da69a48ee60

> Because EMFILE is different from the errno in af_unix.c, I will add a 
> wish for the existing documentation of ETOOMANYREFS in unix(7) to 
> reference this.
> 
> I'll stop bikeshedding there.  EMFILE sounds ok.  strerror() calls 
> ETOOMANYREFS "Too many references: cannot splice"; it doesn't seem to be 
> particularly helpful or well-known.

Agree

-- 
Jens Axboe