RFC: [PATCH] SCM_CREDS support 1(2)

[Svante Signell]

Hi,

Patch 1(2) on SCM_CREDS support for GNU/Hurd.

Time to publish the SCM_CREDS proposal to the list. From the patch there
are some FIXMEs that need to be resolved for a final patch to be
created. I've been running two kvm-images with eglibc built with this
patch for a few weeks now, and haven't seen any problems so far. On the
list of things running properly with SCM_CREDS support are:
- All 11 gamin tests OK
- All 15+2 dbus tests OK (with upstream patches)
- 171+ of 182 glib2.0 tests OK (with upstream patches)
- Many more window managers, web browsers and X applications work.
- emacs works in X (when built with patched eglibc)
- gdm3 builds (to be tested)
- ...

This patch implements the first cases in the test code sent to the list
in September 2013, options <no flag> and <-z> see
http://lists.debian.org/debian-hurd/2013/09/msg00034.html 


Options:
no flag: 
-z: send a zero credentials byte as data (used by dbus)

1) Sent credentials and PID are correct:
./scm_cred_recv &
./scm_cred_send
Linux: OK, only sent IDs
kFreeBSD: OK, sent IDs + groups (irrespective of sent IDs)
Hurd: OK, sent IDs + groups

./scm_cred_recv -z&
./scm_cred_send -z
Linux: OK, same as above
kFreeBSD: OK, same as above
Hurd: OK, sent IDs + groups (irrespective of sent IDs)

2) When wrong credentials are sent the behaviour is different:
Linux:
no flag: ERROR [EPERM Operation not permitted] sendmsg
-z: ERROR [EPERM Operation not permitted] sendmsg

kFreeBSD: 
no flag: sent credentials are not honoured, received ones are created
-z: sent credentials are not honoured, received ones are created

Hurd:
no flag: ERROR [EPERM Operation not permitted] sendmsg
-z: sent credentials are not honoured, received ones are created

scm_creds-sendmsg.c.diff
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