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EINVAL
It's not our business to validate invalid user/group names or UID/GID.
Ideally, libc would filter these out, but they don't, hence we have to
filter, but let's not propagate this as error, but simply as "not found"
to the caller.
User name rules are pretty vaguely defined, and the rules defined by
POSIX clash with reality quite heavily (for example, utmp doesn't offer
enough room for user name length, and /usr/bin/chown permits separating
user/group names by a single dot, even though POSIX allows dots being
used in user/group names themselves.) We enforce stricter rules than
POSIX for good reason, and hence in doing so we should not categorically
return EINVAL on stuff we don't consider valid, but other components
might.
Fixes: #4983
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We always define those two in configure, so no need to provide a fallback.
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dbus-daemon does NSS name look-ups in order to enforce its bus policy. This
might dead-lock if an NSS module use wants to use D-Bus for the look-up itself,
like our nss-systemd does. Let's work around this by bypassing bus
communication in the NSS module if we run inside of dbus-daemon. To make this
work we keep a bit of extra state in /run/systemd/dynamic-uid/ so that we don't
have to consult the bus, but can still resolve the names.
Note that the normal codepath continues to be via the bus, so that resolving
works from all mount namespaces and is subject to authentication, as before.
This is a bit dirty, but not too dirty, as dbus daemon is kinda special anyway
for PID 1.
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Let's extend nss-systemd to also synthesize user/group entries for the
UIDs/GIDs 0 and 65534 which have special kernel meaning. Given that nss-systemd
is listed in /etc/nsswitch.conf only very late any explicit listing in
/etc/passwd or /etc/group takes precedence.
This functionality is useful in minimal container-like setups that lack
/etc/passwd files (or only have incompletely populated ones).
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With this NSS module all dynamic service users will be resolvable via NSS like
any real user.
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