Re: [PATCH net-next v3 0/4] vsock: Add flags field in the vsock address

[Stefano Garzarella]

On Sat, Dec 12, 2020 at 09:16:08AM -0800, Jakub Kicinski wrote:
> On Fri, 11 Dec 2020 16:24:13 +0100 Stefano Garzarella wrote:
> > On Fri, Dec 11, 2020 at 12:32:37PM +0200, Andra Paraschiv wrote:
> > >vsock enables communication between virtual machines and the host they are
> > >running on. Nested VMs can be setup to use vsock channels, as the multi
> > >transport support has been available in the mainline since the v5.5 Linux kernel
> > >has been released.
> > >
> > >Implicitly, if no host->guest vsock transport is loaded, all the vsock packets
> > >are forwarded to the host. This behavior can be used to setup communication
> > >channels between sibling VMs that are running on the same host. One example can
> > >be the vsock channels that can be established within AWS Nitro Enclaves
> > >(see Documentation/virt/ne_overview.rst).
> > >
> > >To be able to explicitly mark a connection as being used for a certain use case,
> > >add a flags field in the vsock address data structure. The value of the flags
> > >field is taken into consideration when the vsock transport is assigned. This way
> > >can distinguish between different use cases, such as nested VMs / local
> > >communication and sibling VMs.
> > >
> > >The flags field can be set in the user space application connect logic. On the
> > >listen path, the field can be set in the kernel space logic.
> > >
> >
> > I reviewed all the patches and they are in a good shape!
> >
> > Maybe the last thing to add is a flags check in the
> > vsock_addr_validate(), to avoid that flags that we don't know how to
> > handle are specified.
> > For example if in the future we add new flags that this version of the
> > kernel is not able to satisfy, we should return an error to the
> > application.
> >
> > I mean something like this:
> >
> >      diff --git a/net/vmw_vsock/vsock_addr.c b/net/vmw_vsock/vsock_addr.c
> >      index 909de26cb0e7..73bb1d2fa526 100644
> >      --- a/net/vmw_vsock/vsock_addr.c
> >      +++ b/net/vmw_vsock/vsock_addr.c
> >      @@ -22,6 +22,8 @@ EXPORT_SYMBOL_GPL(vsock_addr_init);
> >
> >       int vsock_addr_validate(const struct sockaddr_vm *addr)
> >       {
> >      +       unsigned short svm_valid_flags = VMADDR_FLAG_TO_HOST;
> >      +
> >              if (!addr)
> >                      return -EFAULT;
> >
> >      @@ -31,6 +33,9 @@ int vsock_addr_validate(const struct sockaddr_vm *addr)
> >              if (addr->svm_zero[0] != 0)
> >                      return -EINVAL;
>
> Strictly speaking this check should be superseded by the check below
> (AKA removed). We used to check svm_zero[0], with the new field added
> this now checks svm_zero[2]. Old applications may have not initialized
> svm_zero[2] (we're talking about binary compatibility here, apps built
> with old headers).
>
> >      +       if (addr->svm_flags & ~svm_valid_flags)
> >      +               return -EINVAL;
>
> The flags should also probably be one byte (we can define a "more
> flags" flag to unlock further bytes) - otherwise on big endian the
> new flag will fall into svm_zero[1] so the v3 improvements are moot
> for big endian, right?

Right, I assumed the entire svm_zero[] was zeroed out, but we can't be 
sure.

So, I agree to change the svm_flags to 1 byte (__u8), and remove the 
superseded check that you pointed out.
With these changes we should be fully binary compatibility.

Thanks,
Stefano

> >              return 0;
> >       }
> >       EXPORT_SYMBOL_GPL(vsock_addr_validate);