Message-ID: <aUC0Op2trtt3z405@devvm11784.nha0.facebook.com>
Date: Mon, 15 Dec 2025 17:22:02 -0800
From: Bobby Eshleman <bobbyeshleman@...il.com>
To: Stefano Garzarella <sgarzare@...hat.com>
Cc: Paolo Abeni <pabeni@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Simon Horman <horms@...nel.org>,
	Stefan Hajnoczi <stefanha@...hat.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Jason Wang <jasowang@...hat.com>,
	Eugenio Pérez <eperezma@...hat.com>,
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
	"K. Y. Srinivasan" <kys@...rosoft.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>,
	Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
	Bryan Tan <bryan-bt.tan@...adcom.com>,
	Vishnu Dasa <vishnu.dasa@...adcom.com>,
	Broadcom internal kernel review list <bcm-kernel-feedback-list@...adcom.com>,
	Shuah Khan <shuah@...nel.org>, linux-kernel@...r.kernel.org,
	virtualization@...ts.linux.dev, netdev@...r.kernel.org,
	kvm@...r.kernel.org, linux-hyperv@...r.kernel.org,
	linux-kselftest@...r.kernel.org, berrange@...hat.com,
	Sargun Dhillon <sargun@...gun.me>,
	Bobby Eshleman <bobbyeshleman@...a.com>
Subject: Re: [PATCH net-next v12 04/12] vsock: add netns support to virtio
 transports

On Mon, Dec 15, 2025 at 03:11:22PM +0100, Stefano Garzarella wrote:
> On Fri, Dec 12, 2025 at 07:26:15AM -0800, Bobby Eshleman wrote:
> > On Tue, Dec 02, 2025 at 02:01:04PM -0800, Bobby Eshleman wrote:
> > > On Tue, Dec 02, 2025 at 09:47:19PM +0100, Paolo Abeni wrote:
> > > > On 12/2/25 6:56 PM, Bobby Eshleman wrote:
> > > > > On Tue, Dec 02, 2025 at 11:18:14AM +0100, Paolo Abeni wrote:
> > > > >> On 11/27/25 8:47 AM, Bobby Eshleman wrote:
> > > > >>> @@ -674,6 +689,17 @@ static int vhost_vsock_dev_open(struct inode *inode, struct file *file)
> > > > >>>  		goto out;
> > > > >>>  	}
> > > > >>>
> > > > >>> +	net = current->nsproxy->net_ns;
> > > > >>> +	vsock->net = get_net_track(net, &vsock->ns_tracker, GFP_KERNEL);
> > > > >>> +
> > > > >>> +	/* Store the mode of the namespace at the time of creation. If this
> > > > >>> +	 * namespace later changes from "global" to "local", we want this vsock
> > > > >>> +	 * to continue operating normally and not suddenly break. For that
> > > > >>> +	 * reason, we save the mode here and later use it when performing
> > > > >>> +	 * socket lookups with vsock_net_check_mode() (see vhost_vsock_get()).
> > > > >>> +	 */
> > > > >>> +	vsock->net_mode = vsock_net_mode(net);
> > > > >>
> > > > >> I'm sorry for the very late feedback. I think that at very least the
> > > > >> user-space needs a way to query if the given transport is in local or
> > > > >> global mode, as AFAICS there is no way to tell that when socket creation
> > > > >> races with mode change.
> > > > >
> > > > > Are you thinking something along the lines of sockopt?
> > > >
> > > > I'd like to see a way for the user-space to query the socket 'namespace
> > > > mode'.
> > > >
> > > > sockopt could be an option; a possibly better one could be sock_diag. Or
> > > > you could do both using dumping the info with a shared helper invoked by
> > > > both code paths, alike what TCP is doing.
> > > > >> Also I'm a bit uneasy with the model implemented here, as 'local' socket
> > > > >> may cross netns boundaris and connect to 'local' socket in other netns
> > > > >> (if I read correctly patch 2/12). That in turns AFAICS break the netns
> > > > >> isolation.
> > > > >
> > > > > Local mode sockets are unable to communicate with local mode (and global
> > > > > mode too) sockets that are in other namespaces. The key piece of code
> > > > > for that is vsock_net_check_mode(), where if either modes is local the
> > > > > namespaces must be the same.
> > > >
> > > > Sorry, I likely misread the large comment in patch 2:
> > > >
> > > > https://lore.kernel.org/netdev/20251126-vsock-vmtest-v12-2-257ee21cd5de@meta.com/
> > > >
> > > > >> Have you considered instead a slightly different model, where the
> > > > >> local/global model is set in stone at netns creation time - alike what
> > > > >> /proc/sys/net/ipv4/tcp_child_ehash_entries is doing[1] - and
> > > > >> inter-netns connectivity is explicitly granted by the admin (I guess
> > > > >> you will need new transport operations for that)?
> > > > >>
> > > > >> /P
> > > > >>
> > > > >> [1] tcp allows using per-netns established socket lookup tables - as
> > > > >> opposed to the default global lookup table (even if match always takes
> > > > >> in account the netns obviously). The mentioned sysctl specify such
> > > > >> configuration for the children namespaces, if any.
> > > > >
> > > > > I'll save this discussion if the above doesn't resolve your concerns.
> > > > I still have some concern WRT the dynamic mode change after netns
> > > > creation. I fear some 'unsolvable' (or very hard to solve) race I can't
> > > > see now. A tcp_child_ehash_entries-like model will avoid completely the
> > > > issue, but I understand it would be a significant change over the
> > > > current status.
> > > >
> > > > "Luckily" the merge window is on us and we have some time to discuss. Do
> > > > you have a specific use-case for the ability to change the netns >
> > > mode
> > > > after creation?
> > > >
> > > > /P
> > > 
> > > I don't think there is a hard requirement that the mode be change-able
> > > after creation. Though I'd love to avoid such a big change... or at
> > > least leave unchanged as much of what we've already reviewed as
> > > possible.
> > > 
> > > In the scheme of defining the mode at creation and following the
> > > tcp_child_ehash_entries-ish model, what I'm imagining is:
> > > - /proc/sys/net/vsock/child_ns_mode can be set to "local" or "global"
> > > - /proc/sys/net/vsock/child_ns_mode is not immutable, can change any
> > >   number of times
> > > 
> > > - when a netns is created, the new netns mode is inherited from
> > >   child_ns_mode, being assigned using something like:
> > > 
> > > 	  net->vsock.ns_mode =
> > > 		get_net_ns_by_pid(current->pid)->child_ns_mode
> > > 
> > > - /proc/sys/net/vsock/ns_mode queries the current mode, returning
> > >   "local" or "global", returning value of net->vsock.ns_mode
> > > - /proc/sys/net/vsock/ns_mode and net->vsock.ns_mode are immutable and
> > >   reject writes
> > > 
> > > Does that align with what you have in mind?
> > 
> > Hey Paolo, I just wanted to sync up on this one. Does the above align
> > with what you envision?
> 
> Hi Bobby, AFAIK Paolo was at LPC, so there could be some delay.
> 
> FYI I'll be off from Dec 25 to Jan 6, so if we want to do an RFC in the
> middle, I'll do my best to take a look before my time off.
> 
> Thanks,
> Stefano
> 

Sounds like a plan, thanks!

Best,
Bobby

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