They both are both short and contain similar parts and various helper will be
shared between both parts of the code so it's easier to use a single file.
Previously we'd used the existance of a specific AF_UNIX socket in the
abstract namespace as lock for disabling lookup recursions. (for
breaking out of the loop: userdb synthesized from nss → nss synthesized
from userdb → userdb synthesized from nss → …)
I did it like that because it promised to work the same both in static
and in dynmically linked environments and is accessible easily from any
programming language.
However, it has a weakness regarding reuse attacks: the socket is
securely hashed (siphash) from the thread ID in combination with the
AT_RANDOM secret. Thus it should not be guessable from an attacker in
advance. That's only true if a thread takes the lock only once and
keeps it forever. However, if a thread takes and releases it multiple
times an attacker might monitor that and quickly take the lock
after the first iteration for follow-up iterations.
It's not a big issue given that userdb (as the primary user for this)
never released the lock and we never made the concept a public
interface, and it was only included in one release so far, but it's
something that deserves fixing. (moreover it's a local DoS only, only
permitting to disable native userdb lookups)
With this rework the libnss_systemd.so.2 module will now export two
additional symbols. These symbols are not used by glibc, but can be used
by arbitrary programs: one can be used to disable nss-systemd, the other
to check if it is currently disabled.
The lock is per-thread. It's slightly less pretty, since it requires
people to manually link against C code via dlopen()/dlsym(), but it
should work safely without the aforementioned weakness.
Prompted by the discussion on #16110, let's migrate more code to
fd_wait_for_event().
This only leaves 7 places where we call into poll()/poll() directly in
our entire codebase. (one of which is fd_wait_for_event() itself)
poll() sets POLLNVAL inside of the poll structures if an invalid fd is
passed. So far we generally didn't check for that, thus not taking
notice of the error. Given that this specific kind of error is generally
indication of a programming error, and given that our code is embedded
into our projects via NSS or because people link against our library,
let's explicitly check for this and convert it to EBADF.
(I ran into a busy loop because of this missing check when some of my
test code accidentally closed an fd it shouldn't close, so this is a
real thing)
