Re: [RFC PATCH 0/4] net/io_uring: pass a kernel pointer via optlen_t to proto[_ops].getsockopt()

[Stefan Metzmacher <metze@samba.org>]

Am 01.04.25 um 17:45 schrieb Stanislav Fomichev:
> On 04/01, Breno Leitao wrote:
>> On Tue, Apr 01, 2025 at 03:48:58PM +0200, Stefan Metzmacher wrote:
>>> Am 01.04.25 um 15:37 schrieb Stefan Metzmacher:
>>>> Am 01.04.25 um 10:19 schrieb Stefan Metzmacher:
>>>>> Am 31.03.25 um 23:04 schrieb Stanislav Fomichev:
>>>>>> On 03/31, Stefan Metzmacher wrote:
>>>>>>> The motivation for this is to remove the SOL_SOCKET limitation
>>>>>>> from io_uring_cmd_getsockopt().
>>>>>>>
>>>>>>> The reason for this limitation is that io_uring_cmd_getsockopt()
>>>>>>> passes a kernel pointer as optlen to do_sock_getsockopt()
>>>>>>> and can't reach the ops->getsockopt() path.
>>>>>>>
>>>>>>> The first idea would be to change the optval and optlen arguments
>>>>>>> to the protocol specific hooks also to sockptr_t, as that
>>>>>>> is already used for setsockopt() and also by do_sock_getsockopt()
>>>>>>> sk_getsockopt() and BPF_CGROUP_RUN_PROG_GETSOCKOPT().
>>>>>>>
>>>>>>> But as Linus don't like 'sockptr_t' I used a different approach.
>>>>>>>
>>>>>>> @Linus, would that optlen_t approach fit better for you?
>>>>>>
>>>>>> [..]
>>>>>>
>>>>>>> Instead of passing the optlen as user or kernel pointer,
>>>>>>> we only ever pass a kernel pointer and do the
>>>>>>> translation from/to userspace in do_sock_getsockopt().
>>>>>>
>>>>>> At this point why not just fully embrace iov_iter? You have the size
>>>>>> now + the user (or kernel) pointer. Might as well do
>>>>>> s/sockptr_t/iov_iter/ conversion?
>>>>>
>>>>> I think that would only be possible if we introduce
>>>>> proto[_ops].getsockopt_iter() and then convert the implementations
>>>>> step by step. Doing it all in one go has a lot of potential to break
>>>>> the uapi. I could try to convert things like socket, ip and tcp myself, but
>>>>> the rest needs to be converted by the maintainer of the specific protocol,
>>>>> as it needs to be tested. As there are crazy things happening in the existing
>>>>> implementations, e.g. some getsockopt() implementations use optval as in and out
>>>>> buffer.
>>>>>
>>>>> I first tried to convert both optval and optlen of getsockopt to sockptr_t,
>>>>> and that showed that touching the optval part starts to get complex very soon,
>>>>> see https://git.samba.org/?p=metze/linux/wip.git;a=commitdiff;h=141912166473bf8843ec6ace76dc9c6945adafd1
>>>>> (note it didn't converted everything, I gave up after hitting
>>>>> sctp_getsockopt_peer_addrs and sctp_getsockopt_local_addrs.
>>>>> sctp_getsockopt_context, sctp_getsockopt_maxseg, sctp_getsockopt_associnfo and maybe
>>>>> more are the ones also doing both copy_from_user and copy_to_user on optval)
>>>>>
>>>>> I come also across one implementation that returned -ERANGE because *optlen was
>>>>> too short and put the required length into *optlen, which means the returned
>>>>> *optlen is larger than the optval buffer given from userspace.
>>>>>
>>>>> Because of all these strange things I tried to do a minimal change
>>>>> in order to get rid of the io_uring limitation and only converted
>>>>> optlen and leave optval as is.
>>>>>
>>>>> In order to have a patchset that has a low risk to cause regressions.
>>>>>
>>>>> But as alternative introducing a prototype like this:
>>>>>
>>>>>           int (*getsockopt_iter)(struct socket *sock, int level, int optname,
>>>>>                                  struct iov_iter *optval_iter);
>>>>>
>>>>> That returns a non-negative value which can be placed into *optlen
>>>>> or negative value as error and *optlen will not be changed on error.
>>>>> optval_iter will get direction ITER_DEST, so it can only be written to.
>>>>>
>>>>> Implementations could then opt in for the new interface and
>>>>> allow do_sock_getsockopt() work also for the io_uring case,
>>>>> while all others would still get -EOPNOTSUPP.
>>>>>
>>>>> So what should be the way to go?
>>>>
>>>> Ok, I've added the infrastructure for getsockopt_iter, see below,
>>>> but the first part I wanted to convert was
>>>> tcp_ao_copy_mkts_to_user() and that also reads from userspace before
>>>> writing.
>>>>
>>>> So we could go with the optlen_t approach, or we need
>>>> logic for ITER_BOTH or pass two iov_iters one with ITER_SRC and one
>>>> with ITER_DEST...
>>>>
>>>> So who wants to decide?
>>>
>>> I just noticed that it's even possible in same cases
>>> to pass in a short buffer to optval, but have a longer value in optlen,
>>> hci_sock_getsockopt() with SOL_BLUETOOTH completely ignores optlen.
>>>
>>> This makes it really hard to believe that trying to use iov_iter for this
>>> is a good idea :-(
>>
>> That was my finding as well a while ago, when I was planning to get the
>> __user pointers converted to iov_iter. There are some weird ways of
>> using optlen and optval, which makes them non-trivial to covert to
>> iov_iter.
> 
> Can we ignore all non-ip/tcp/udp cases for now? This should cover +90%
> of useful socket opts. See if there are any obvious problems with them
> and if not, try converting. The rest we can cover separately when/if
> needed.

That's what I tried, but it fails with
tcp_getsockopt ->
    do_tcp_getsockopt ->
      tcp_ao_get_mkts ->
         tcp_ao_copy_mkts_to_user ->
            copy_struct_from_sockptr
      tcp_ao_get_sock_info ->
         copy_struct_from_sockptr

That's not possible with a ITER_DEST iov_iter.

metze