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

[Jens Axboe <axboe@kernel.dk>]

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;


-- 
Jens Axboe