avc_open() and netlink_loop()

[KaiGai Kohei <kaigai@ak.jp.nec.com>]

When we initialize userspace avc using avc_open(3), it internally calls
avc_init(3) without any callback functions. The avc_init() is introduced
as a deprecated interface from application code, so it is recommended to
use avc_open() instead for new applications.

The avc_init() internally calls avc_netlink_open(). If no thread callback
is not given, the 'blocking' argument shall be 0, then avc_netlink_open()
set O_NONBLOCK flag on the socket file descriptor.

Next, application will create a thread to receive messages via netlink
socket to invalidate userspace avc, using avc_netlink_loop().
However, if userspace avc of libselinux is already initialized,
the avc_netlink_loop() immediately returns with EWOULDBLOCK, because the
netlink socket is not blocked and avc_netlink_receive() does not expect
recvfrom() returns error.

It seems to me O_NONBLOCK is a wrong strategy in this case, and select(2)
should be checked in avc_netlink_check_nb() instead.

Eamon, what is your opinion?
-- 
OSS Platform Development Division, NEC
KaiGai Kohei <kaigai@ak.jp.nec.com>

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