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From:   "Benjamin Coddington" <bcodding@redhat.com>
To:     "Hannes Reinecke" <hare@suse.de>
Cc:     "Jakub Kicinski" <kuba@kernel.org>,
        "Sagi Grimberg" <sagi@grimberg.me>,
        "Chuck Lever" <chuck.lever@oracle.com>, netdev@vger.kernel.org,
        linux-nfs@vger.kernel.org, linux-nvme@lists.infradead.org,
        linux-cifs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        ak@tempesta-tech.com, borisp@nvidia.com, simo@redhat.com
Subject: Re: [PATCH RFC 4/5] net/tls: Add support for PF_TLSH (a TLS handshake
 listener)
Date:   Thu, 28 Apr 2022 10:09:17 -0400
Message-ID: <E2BF9CFF-9361-400B-BDEE-CF5E0AFDCA63@redhat.com>
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On 28 Apr 2022, at 9:51, Hannes Reinecke wrote:

> On 4/28/22 15:30, Jakub Kicinski wrote:
>> On Thu, 28 Apr 2022 09:26:41 +0200 Hannes Reinecke wrote:
>>> The whole thing started off with the problem on _how_ sockets could be
>>> passed between kernel and userspace and vice versa.
>>> While there is fd passing between processes via AF_UNIX, there is no
>>> such mechanism between kernel and userspace.
>>
>> Noob question - the kernel <> user space FD sharing is just
>> not implemented yet, or somehow fundamentally hard because kernel
>> fds are "special"?
>
> Noob reply: wish I knew.  (I somewhat hoped _you_ would've been able to
> tell me.)
>
> Thing is, the only method I could think of for fd passing is the POSIX fd
> passing via unix_attach_fds()/unix_detach_fds().  But that's AF_UNIX,
> which really is designed for process-to-process communication, not
> process-to-kernel.  So you probably have to move a similar logic over to
> AF_NETLINK. And design a new interface on how fds should be passed over
> AF_NETLINK.
>
> But then you have to face the issue that AF_NELINK is essentially UDP, and
> you have _no_ idea if and how many processes do listen on the other end.
> Thing is, you (as the sender) have to copy the fd over to the receiving
> process, so you'd better _hope_ there is a receiving process.  Not to
> mention that there might be several processes listening in...
>
> And that's something I _definitely_ don't feel comfortable with without
> guidance from the networking folks, so I didn't pursue it further and we
> went with the 'accept()' mechanism Chuck implemented.
>
> I'm open to suggestions, though.

EXPORT_SYMBOL(receive_fd) would allow interesting implementations.

The kernel keyring facilities have a good API for creating various key_types
which are able to perform work such as this from userspace contexts.

I have a working prototype for a keyring key instantiation which allows a
userspace process to install a kernel fd on its file table.  The problem
here is how to match/route such fd passing to appropriate processes in
appropriate namespaces.  I think this problem is shared by all
kernel-to-userspace upcalls, which I hope we can discuss at LSF/MM.

I don't think kernel fds are very special as compared to userspace fds.

Ben