Re: avc_open() and netlink_loop()

[Eamon Walsh <ewalsh@tycho.nsa.gov>]

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On 02/03/2010 01:19 AM, KaiGai Kohei wrote:
> 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?
>   


Hi, my apologies for the delayed response. Yes it appears as though this
is a bug. I think the solution is to configure the file descriptor in
blocking mode at the start of avc_netlink_loop(). When
avc_netlink_loop() is called, we always want blocking behavior. See the
attached patch.


-- 

Eamon Walsh 
National Security Agency


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diff --git a/libselinux/src/avc.c b/libselinux/src/avc.c
index 881b915..5c8def0 100644
--- a/libselinux/src/avc.c
+++ b/libselinux/src/avc.c
@@ -222,7 +222,7 @@ int avc_init(const char *prefix,
 		avc_enforcing = rc;
 	}
 
-	rc = avc_netlink_open(avc_using_threads);
+	rc = avc_netlink_open(0);
 	if (rc < 0) {
 		avc_log(SELINUX_ERROR,
 			"%s:  can't open netlink socket: %d (%s)\n",
diff --git a/libselinux/src/avc_internal.c b/libselinux/src/avc_internal.c
index 8372f52..90dfa51 100644
--- a/libselinux/src/avc_internal.c
+++ b/libselinux/src/avc_internal.c
@@ -233,6 +233,8 @@ void avc_netlink_loop(void)
 	int rc;
 	char buf[1024] __attribute__ ((aligned));
 
+	fcntl(fd, F_SETFL, fcntl(fd, F_GETFL, 0) & ~O_NONBLOCK)
+
 	while (1) {
 		errno = 0;
 		rc = avc_netlink_receive(buf, sizeof(buf));