Also, make it slightly more powerful, by accepting a flags argument, and
make it safe for handling if more than one cmsg attribute happens to be
attached.
When forking of a child process for connecting to a container, pass
the preicse connection error to the calling process.
We already did this correctly for kdbus busses, let's do so for dbus1
busses, too.
We rely on the correct error used when opening the kdbus device node,
hence let's make sure we pass it up from the namespaced child process to
the process which actually wants to connect.
To be able to use `systemd-run` or `machinectl login` on a container
that is in a private user namespace, the sub-process must have entered
the user namespace before connecting to the container's D-Bus, otherwise
the UID and GID in the peer credentials are garbage.
So we extend namespace_open and namespace_enter to support UID namespaces,
and we enter the UID namespace in bus_container_connect_{socket,kernel}.
namespace_open will degrade to a no-op if user namespaces are not enabled
in the kernel.
Special handling is required for the setns call in namespace_enter with
a user namespace, since transitioning to your own namespace is forbidden,
as it would result in re-entering your user namespace as root.
Arguably it may be valid to check this at the call site, rather than
inside namespace_enter, but it is less code to do it inside, and if the
intention of calling namespace_enter is to *be* in the target namespace,
rather than to transition to the target namespace, it is a reasonable
approach.
The check for whether the user namespace is the same must happen before
entering namespaces, as we may not be able to access /proc during the
intermediate transition stage.
We can't instead attempt to enter the user namespace and then ignore
the failure from it being the same namespace, since the error code is
not distinct, and we can't compare namespaces while mid-transition.
This patch removes includes that are not used. The removals were found with
include-what-you-use which checks if any of the symbols from a header is
in use.
safe_close_pair() is more like safe_close(), except that it handles
pairs of fds, and doesn't make and misleading allusion, as it works
similarly well for socketpairs() as for pipe()s...
safe_close() automatically becomes a NOP when a negative fd is passed,
and returns -1 unconditionally. This makes it easy to write lines like
this:
fd = safe_close(fd);
Which will close an fd if it is open, and reset the fd variable
correctly.
By making use of this new scheme we can drop a > 200 lines of code that
was required to test for non-negative fds or to reset the closed fd
variable afterwards.
