From: Patrick McHardy <redacted>
Date: Tue, 02 Feb 2010 15:55:25 +0100

Alexey Dobriyan wrote:

quoted

Netlink code does module autoload if protocol userspace is asking for is
not ready. However, module can dissapear right after it was autoloaded.
Example: modprobe/rmmod stress-testing and xfrm_user.ko providing NETLINK_XFRM.

netlink_create() in such situation _will_ create userspace socket and
_will_not_ pin module. Now if module was removed and we're going to call
->netlink_rcv into nothing:

BUG: unable to handle kernel paging request at ffffffffa02f842a
					       ^^^^^^^^^^^^^^^^
	modules are loaded near these addresses here

...

If module was quickly removed after autoloading, return -E.

Return -EPROTONOSUPPORT if module was quickly removed after autoloading.

Signed-off-by: Alexey Dobriyan <redacted>
---

	NOTE: due to commit 513c25000005257e5474f261bf27d4a3c1dff1e3
	"[NETLINK]: Don't prevent creating sockets when no kernel socket is registered"
	error should probably be -ENOENT, but I have no idea what this
	pam-audit thing is.

Quoting from the thread back then (couldn't find a public reference):

quoted

FC4 includes a pam-0.77-audit.patch which adds a pam module that
uses libaudit to probe for audit availability. It calls audit_open
and ignores connection refused but returns an pam error for all
other errors.

The current version of libaudit properly checks for EPROTONOSUPPORT,
but I don't know when this was fixed and whether we can assume the
broken code is not used anymore.

In any event, the EPROTONOSUPPORT is being returned now in a case
that has been OOPS'ing.  So I think it's safe to apply Alexey's
fix as-is, and that is what I have just done in net-2.6 :-)

Thanks!