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In 68511cebe5 the ability to pass the
coredump's mount namespace fd from the coredump patter handler was added
to systemd-coredump. For this the protocol was augmented, in attempt to
provide both forward and backward compatibility.
The protocol as of v256: one or more datagrams with journal log fields
about the coredump are sent via an SOCK_SEQPACKET connection. It is
finished with a zero length datagram which carries the coredump fd (this
last datagram is called "sentinel" sometimes).
The protocol after 68511cebe5 is extended
so that after the sentinal a 2nd sentinel is sent, with a pair of fds:
the coredump fd *again* and a mount fd (acquired via open_tree()) of the
container's mount tree. It's a bit ugly to send the coredump fd a 2nd
time, but what's more important the implementation didn't work: since on
SOCK_SEQPACKET a zero sized datagram cannot be distinguished from EOF
(which is a Linux API design mistake), an early EOF would be
misunderstood as a zero size datagram lacking any fd, which resulted in
protocol termination.
Moreover, I think if we touch the protocol we should make the move to
pidfs at the same time.
All of the above is what this protocol rework addresses.
1. A pidfd is now sent as well
2. The protocol is now payload, followed by the coredump fd datagram (as
before). But now followed by a second empty datagram with a pidfd,
and a third empty datagram with the mount tree fd. Of this the latter
two or last are optional. Thus, it's now a stream of payload
datagrams with one, two or three fd-laden datagrams as sentinel. If
we read the 2nd or 3rd sentinel without an attached fd we assume this
is actually an EOF (whether it actually is one or not doesn't matter
here). This should provide nice up and down compatibility.
3. The mount_tree_fd is moved into the Context object. The pidfd is
placed there too, as a PidRef. Thus the data we pass around is now
the coredump fd plus the context, which is simpler and makes a lot
more semantical sense I think.
4. The "first" boolean is replaced by an explicit state engine enum
Fixes: #34130
313537da6f
