linux-kernel - Re: [PATCH RFC net v2 3/3] vsock: Fix IOCTL_VM_SOCKETS_GET_LOCAL_CID to check also `transport

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Message-ID: <65da0d87-7b84-409f-9ffa-2ae4e0c9c745@rbox.co>
Date: Sun, 29 Jun 2025 23:26:29 +0200
From: Michal Luczaj <mhal@...x.co>
To: Stefano Garzarella <sgarzare@...hat.com>
Cc: "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>,
 Stefan Hajnoczi <stefanha@...hat.com>, virtualization@...ts.linux.dev,
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC net v2 3/3] vsock: Fix IOCTL_VM_SOCKETS_GET_LOCAL_CID
 to check also `transport_local`

On 6/27/25 10:10, Stefano Garzarella wrote:
> On Wed, Jun 25, 2025 at 11:23:54PM +0200, Michal Luczaj wrote:
>> On 6/25/25 10:54, Stefano Garzarella wrote:
>>> On Fri, Jun 20, 2025 at 09:52:45PM +0200, Michal Luczaj wrote:
>>>> 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@...hat.com>
>>>> Signed-off-by: Michal Luczaj <mhal@...x.co>
>>>> ---
>>>> 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.
>>
>>>> ---
>>>> 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.
> 
> Yeah, BTW for transport_h2g is the same, they always should return 
> VMADDR_CID_HOST, so I think we should be symmetrical.

Heh, I've missed that VMADDR_CID_HOST completely :)

Thanks,
Michal