netdev - Re: [PATCH net-next v2 1/4] vm_sockets: Include flags field in the vsock address data structure

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Message-ID: <20201209093019.1caae20e@kicinski-fedora-pc1c0hjn.DHCP.thefacebook.com>
Date:   Wed, 9 Dec 2020 09:30:19 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     "Paraschiv, Andra-Irina" <andraprs@...zon.com>
Cc:     Stefano Garzarella <sgarzare@...hat.com>,
        netdev <netdev@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "David S . Miller" <davem@...emloft.net>,
        David Duncan <davdunc@...zon.com>,
        Dexuan Cui <decui@...rosoft.com>,
        Alexander Graf <graf@...zon.de>,
        Jorgen Hansen <jhansen@...are.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH net-next v2 1/4] vm_sockets: Include flags field in the
 vsock address data structure

On Wed, 9 Dec 2020 17:17:56 +0200 Paraschiv, Andra-Irina wrote:
> > I agree that could be a problem, but here some considerations:
> > - I checked some applications (qemu-guest-agent, ncat, iperf-vsock) and
> >   all use the same pattern: allocate memory, initialize all the
> >   sockaddr_vm to zero (to be sure to initialize the svm_zero), set the
> >   cid and port fields.
> >   So we should be safe, but of course it may not always be true.
> >
> > - For now the issue could affect only nested VMs. We introduced this
> >   support one year ago, so it's something new and maybe we don't cause
> >   too many problems.
> >
> > As an alternative, what about using 1 or 2 bytes from svm_zero[]?
> > These must be set at zero, even if we only check the first byte in the
> > kernel.  
> 
> Thanks for the follow-up info.
> 
> We can also consider the "svm_zero" option and could use 2 bytes from 
> that field for "svm_flags", keeping the same "unsigned short" type.

Or use svm_zero as a gate for interpreting other fields?
If svm_zero[0]* == something start checking the value of reserved1?
* in practice the name can be unioned to something more palatable ;)