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

[Stanislav Fomichev <stfomichev@gmail.com>]

On 04/01, Stefan Metzmacher wrote:
> 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

Can we create two iterators over the same memory? One for ITER_SOURCE and
another for ITER_DEST. And then make getsockopt_iter accept optval_in and
optval_out. We can also use optval_out position (iov_offset) as optlen output
value. Don't see why it won't work, but I agree that's gonna be a messy
conversion so let's see if someone else has better suggestions.