On 6/25/25 10:54, Stefano Garzarella wrote:

On Fri, Jun 20, 2025 at 09:52:45PM +0200, Michal Luczaj wrote:

quoted

Support returning VMADDR_CID_LOCAL in case no other vsock transport is
available.

Fixes: 0e12190578d0 ("vsock: add local transport support in the vsock core")
Suggested-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Michal Luczaj <redacted>
---
man vsock(7) mentions IOCTL_VM_SOCKETS_GET_LOCAL_CID vs. VMADDR_CID_LOCAL:

  Ioctls
      ...
      IOCTL_VM_SOCKETS_GET_LOCAL_CID
             ...
             Consider using VMADDR_CID_ANY when binding instead of
             getting the local CID with IOCTL_VM_SOCKETS_GET_LOCAL_CID.

  Local communication
      ....
      The local CID obtained with IOCTL_VM_SOCKETS_GET_LOCAL_CID can be
      used for the same purpose, but it is preferable to use
      VMADDR_CID_LOCAL.

I was wondering it that would need some rewriting, since we're adding
VMADDR_CID_LOCAL as a possible ioctl's return value.

IIRC the reason was, that if we have for example a G2H module loaded, 
the ioctl returns the CID of that module (e.g. 42). So, we can use both 
42 and VMADDR_CID_LOCAL to do the loopback communication, but we 
encourage to always use VMADDR_CID_LOCAL.  With this change we basically 
don't change that, but we change the fact that if there is only the 
loopback module loaded, before the ioctl returned VMADDR_CID_ANY, while 
now it returns LOCAL rightly.

So, IMO we are fine.

All right, got it.

quoted

---
net/vmw_vsock/af_vsock.c | 2 ++
1 file changed, 2 insertions(+)

diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
index a1b1073a2c89f865fcdb58b38d8e7feffcf1544f..4bdb4016bd14d790f3d217d5063be64a1553b194 100644
--- a/net/vmw_vsock/af_vsock.c
+++ b/net/vmw_vsock/af_vsock.c

@@ -2577,6 +2577,8 @@ static long vsock_dev_do_ioctl(struct file *filp,

		cid = vsock_transport_local_cid(&transport_g2h);
		if (cid == VMADDR_CID_ANY)
			cid = vsock_transport_local_cid(&transport_h2g);
+		if (cid == VMADDR_CID_ANY && transport_local)
+			cid = VMADDR_CID_LOCAL;

why not `cid = vsock_transport_local_cid(&transport_local)` like for 
H2G?

Sure, can do. I've assumed transport_local would always have a local CID of
VMADDR_CID_LOCAL. So taking mutex and going through a callback function to
get VMADDR_CID_LOCAL seemed superfluous. But I get it, if you want to have
it symmetrical with the other vsock_transport_local_cid()s.

Thanks,
Michal