I dug around with GDB, and dbus_bus_request_name is failing with the following error:

{error_ = {name = 0x5555555f59c0 "org.freedesktop.DBus.Error.AccessDenied", 
    message = 0x555555605640 "Connection \":1.66\" is not allowed to own the service \"org.chromium.Diagnosticsd\" due to security policies in the configuration file", dummy1 = 0, dummy2 = 0, dummy3 = 0, dummy4 = 0, dummy5 = 0, padding1 = 0x7fffffffd540}}

I'm wondering if the error in #1 is real, or if it's a byproduct of the way I'm starting the daemon with GDB - I might not be running the daemon as the diagnostics user. /etc/dbus-1/system.d/org.chromium.Diagnosticsd.conf looks okay, and the diagnostics user should be allowed to own org.chromium.Diagnosticsd.

Maksim did point out that the upstart script expects the directory /sys/devices/virtual/hwmon to exist, and it doesn't, so that could be the issue as well.

There are two issues here:

1.) The missing directory /sys/devices/virtual/hwmon - if I edit the upstart script to not try to mount this, I can work around this issue.

2.) the seccomp filters for socket need to be more permissive - it looks like emaxx@chromium.org already found this as well, and has a CL at https://chromium-review.googlesource.com/c/chromiumos/platform2/+/1310174

With these two changes, I can run diagnosticsd on Wilco.

Thanks for verifying that, Paul.
For the reference, attaching the corresponding CLs: https://crrev.com/c/1310174, https://crrev.com/c/1315287.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform2/+/8bd273f6ba0cef973b74065352c07ea4c1b0a474

commit 8bd273f6ba0cef973b74065352c07ea4c1b0a474
Author: Maksim Ivanov <emaxx@chromium.org>
Date: Sun Nov 04 00:16:54 2018

diagnostics: Allow socket AF_INET in seccomp filters

Whitelist calling socket with domain==AF_INET for both diagnosticsd
and diagnostics_processor. It's needed despite that no direct internet
requests are allowed or expected from these daemons. The reason why
it's required is that the gRPC library makes such call during its
initialization.

gRPC does this in order to check whether the SO_REUSEPORT option is
available. There doesn't seem to be a way to suppress this check.

BUG=chromium:869377, chromium:899766 
TEST=manual - check that the daemons don't crash

Change-Id: Id9f1596f8f2cec856654b82bf744b732d8beb584
Reviewed-on: https://chromium-review.googlesource.com/1310174
Commit-Ready: Maksim Ivanov <emaxx@chromium.org>
Tested-by: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>

[modify] https://crrev.com/8bd273f6ba0cef973b74065352c07ea4c1b0a474/diagnostics/init/diagnosticsd-seccomp-arm.policy
[modify] https://crrev.com/8bd273f6ba0cef973b74065352c07ea4c1b0a474/diagnostics/init/diagnosticsd-seccomp-amd64.policy
[modify] https://crrev.com/8bd273f6ba0cef973b74065352c07ea4c1b0a474/diagnostics/init/diagnostics_processor-seccomp-arm.policy
[modify] https://crrev.com/8bd273f6ba0cef973b74065352c07ea4c1b0a474/diagnostics/init/diagnostics_processor-seccomp-amd64.policy

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform2/+/37bd432e1462fef0756acce35489788013793a1f

commit 37bd432e1462fef0756acce35489788013793a1f
Author: Maksim Ivanov <emaxx@chromium.org>
Date: Wed Nov 07 14:34:43 2018

diagnostics: Skip binding non-existing directories

Fix the upstart script for the diagnosticsd daemon to not
request minijail to bind non-existing directories.

This refers to directories that the daemon wishes to read but
aren't crucial to them, and whose presence is outside of the
daemon's control. For example, the number of hwmon virtual
devices and their paths under /sys/devices/ vary depending on
the kernel and drivers versions.

Without this fix, the diagnosticsd daemon couldn't even start
when some of such directories are missing.

BUG=chromium:869377, chromium:899766 
TEST=check that the daemon starts successfully

Change-Id: Iee1247da1c1f548f903a67400b1f2bbdad181412
Reviewed-on: https://chromium-review.googlesource.com/1315287
Commit-Ready: Maksim Ivanov <emaxx@chromium.org>
Tested-by: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>

[modify] https://crrev.com/37bd432e1462fef0756acce35489788013793a1f/diagnostics/init/diagnosticsd.conf

pmoy@: I think we can close this one. Unless you were still observing the crashes?

You're right - sorry I've been slow on this!