When using a GDBus client in a non-trivial user namespace, the result of
geteuid() can differ from the uid in the namespace where the server is
running. This would result in connection attempts being rejected, because
the identity that the client claims to have does not match the identity
that the server derives from its credentials.

RFC 4422 allows us to send an empty authorization identity, which means we
want to authenticate as whatever identity the server can derive from our
out-of-band credentials. In particular, this resolves the authentication
failure when crossing between different Linux user namespaces.

Because D-Bus does not have a way to represent an empty initial response
as distinct from the absence of an initial response, we cannot use the
initial-response optimization (RFC 4422 §4.3.a) in this case, and must
fall back to waiting for the server to send a challenge.

Unfortunately, GDBus versions older than !2826 did not implement
the server side of this protocol correctly, and would respond to the
missing initial response in a way that breaks the SASL state machine
(expecting a response without sending a challenge), causing client and
server to deadlock with each waiting for the other to respond. Until
fixed versions of GDBus are widespread, we can't rely on having a server
that can cope with this, so gate it behind a flag, which can be set for
connections that are known to cross non-trivial namespace boundaries.

Originally inspired by
<https://github.com/systemd/systemd/commit/1ed4723d38cd0d1423c8fe650f90fa86007ddf55

>,
and based on earlier work by Giuseppe Scrivano (in which the
cross-namespace behaviour was unconditional, rather than gated by a
flag).

Co-authored-by: Giuseppe Scrivano <giuseppe@scrivano.org>
Signed-off-by: Simon McVittie <smcv@collabora.com>