DBUS: "Could not get password database information for UID of current process"

Recently I've been cross compiling DBUS and HAL to make them work on an embedded device with buildroot. It's been a bit of a journey with only sparse documentation available. There are lots of dependencies on things that often aren't present on embedded devices and it's not always easy to work out what they are. So, I'm going to try and document a few of the things I've come across here in the hope that someone finds it useful.

There are various reports of people getting this message but never any responses:

Starting system message bus: Could not get password database information for UID of current process: User "???" unknown or no memory to allocate password entry

This message appears despite /etc/passwd and /etc/group existing and looking correct.

The clue is in the message. It can't read this information because it is using glibc to do it. Glibc provides this information from various sources of which the passwd and group files are only one. It is necessary to enable the glibc back-end that reads the files (libnss_files) along with the library containing the generic functions (libnsl). This can be done in buildroot by enabling the BR2_GLIBC_NSL and BR2_GLIBC_NSS_FILES configuration options. Even better they can automatically be enabled when DBUS support is selected in buildroot by modifying the start of packages/dbus/Config.in to look something like:

config BR2_PACKAGE_DBUS
        bool "dbus"
        default n
        select BR2_PACKAGE_EXPAT
        select BR2_GLIBC_NSL
        select BR2_GLIBC_NSS_FILES

I'm not sure why the lack of libnsl doesn't manifest itself as an error from ld.so - if it had then it would have been far more obvious what the cause was.