Re: [RFC PATCH 0/2] Add AF_VSOCK support to getnameinfo(3) and getaddrinfo(3)

[Stefan Hajnoczi <stefanha at redhat dot com>]

On Fri, Oct 07, 2016 at 04:02:19PM +0200, Florian Weimer wrote:
> On 10/06/2016 01:19 PM, Florian Weimer wrote:
> 
> > > > How do you expect that applications will know that they have to pass
> > > > AF_VSOCK to getaddrinfo instead of AF_UNSPEC?
> > > 
> > > For example ncat(1) has --unixsock and --udp command-line options.  A
> > > --vsock option can be added.  At that point the program knows to use
> > > AF_VSOCK and the same getaddrinfo(3) code path can be used by TCP, UDP,
> > > UNIX, and vsock.
> > 
> > ncat doesn't use getaddrinfo AFAICS, so this isn't going to help it. The
> > larger nmap codebase has a call to getaddrinfo, but the code leading to
> > it assumes 16-bit port numbers, so it won't be able to use getaddrinfo
> > either.
> > 
> > Do you have a better example?
> 
> I've looked briefly at nfs-utils.  I don't see how you can patch in AF_VSOCK
> support using the proposed getaddrinfo support, either.  It seems to do the
> address and service lookup separately.

There are quite a few code paths in mount.nfs(8), maybe you are looking
at an unrelated one?  The nfs-utils patch series I've proposed does not
use service lookup:

https://www.spinics.net/lists/linux-nfs/msg60291.html

  # mount.nfs 2:/export /mnt -v -o clientaddr=3,proto=vsock

(Most code paths are not relevant to NFS over AF_VSOCK, which only
supports NFSv4+ and therefore no UDP or no rpcbind/portmap.)

The port=NUM option is passed to mount(2) as a string.  See
nfs_parse_mount_options() in the kernel for parsing code.


One of the factors that makes me think glibc should support AF_VSOCK in
getaddrinfo(3) is that programs that pass around struct addrinfo are
hard to modify.  Only glibc can allocate struct addrinfo because only
glibc knows how to free addrinfo in freeaddrinfo().

It's not possible to skip getaddrinfo(3) for AF_VSOCK and allocate
struct addrinfo manually since the details of the struct addrinfo memory
allocation are private to glibc.

Stefan

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