Re: qemu-sockets: account for trailing \0 byte in unix socket pathname

["Daniel P. Berrangé" <berrange@redhat.com>]

On Tue, Aug 31, 2021 at 09:22:01PM +0400, Marc-André Lureau wrote:
> Hi
> 
> On Tue, Aug 31, 2021 at 9:17 PM Michael Tokarev <mjt@tls.msk.ru> wrote:
> 
> > 31.08.2021 01:54, Michael Tokarev wrote:
> > > Linux kernel can return size of af_unix socket to be
> > > one byte larger than sockaddr_un structure - adding
> > > the trailing zero byte.
> > >
> > > Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
> > > Fixes: 4cfd970ec188558daa6214f26203fe553fb1e01f (first in 6.1.0)
> > > Cc: qemu-stable@nongnu.org
> > >
> > > diff --git a/util/qemu-sockets.c b/util/qemu-sockets.c
> > > index f2f3676d1f..83926dc2bc 100644
> > > --- a/util/qemu-sockets.c
> > > +++ b/util/qemu-sockets.c
> > > @@ -1345,8 +1345,9 @@ socket_sockaddr_to_address_unix(struct
> > sockaddr_storage *sa,
> > >       SocketAddress *addr;
> > >       struct sockaddr_un *su = (struct sockaddr_un *)sa;
> > >
> > > +    /* kernel might have added \0 terminator to non-abstract socket */
> > >       assert(salen >= sizeof(su->sun_family) + 1 &&
> > > -           salen <= sizeof(struct sockaddr_un));
> > > +           salen <= sizeof(struct sockaddr_un) + su->sun_path[0] ? 1 :
> > 0);
> > >
> > >       addr = g_new0(SocketAddress, 1);
> > >       addr->type = SOCKET_ADDRESS_TYPE_UNIX;
> >
> > Actually, this is not sufficient.
> >
> > While this change fixes one issue (the famous trailing null byte \0),
> > the actual assertion failure occurs because salen = 2, ie, too SMALL,
> > not too large.
> >
> > So it looks like libvirt provides an unnamed socket there, --
> > maybe from a socketpair(2)?
> >
> 
> Yes

No, libvirt binds to a named socket path and passes in a pre-opened
FD for the listener socket. There shouldn't be any socketpair involved.


Regards,
Daniel
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