netdev-linux: Avoid RTM_GETQDISC bug workaround on new-enough kernels.

Otherwise we can't detect classless qdiscs.

This has no useful effect for the currently supported qdiscs, which all
have classes, but it makes it possible to add support for new classless
qdiscs.

This suddenly makes netdev-linux complain about qdiscs it doesn't know
about (e.g. pfifo_fast), which isn't too useful, so this commit also
demotes that INFO message to DBG level.

Reported-by: Jonathan Vestin <[email protected]>
Signed-off-by: Ben Pfaff <[email protected]>

AUTHORS

@@ -268,6 +268,7 @@ Joan Cirer              [email protected]
 John Darrington         [email protected]
 John Galgay             [email protected]
 John Hurley             [email protected]
+Jonathan Vestin         [email protected]
 K 華                    [email protected]
 Kevin Mancuso           [email protected]
 Kiran Shanbhog          [email protected]

lib/netdev-linux.c

@@ -36,6 +36,7 @@
 #include <sys/types.h>
 #include <sys/ioctl.h>
 #include <sys/socket.h>
+#include <sys/utsname.h>
 #include <netpacket/packet.h>
 #include <net/if.h>
 #include <net/if_arp.h>
@@ -4405,6 +4406,31 @@ tc_del_qdisc(struct netdev *netdev_)
     return error;
 }
 
+static bool
+getqdisc_is_safe(void)
+{
+    static struct ovsthread_once once = OVSTHREAD_ONCE_INITIALIZER;
+    static bool safe = false;
+
+    if (ovsthread_once_start(&once)) {
+        struct utsname utsname;
+        int major, minor;
+
+        if (uname(&utsname) == -1) {
+            VLOG_WARN("uname failed (%s)", ovs_strerror(errno));
+        } else if (!ovs_scan(utsname.release, "%d.%d", &major, &minor)) {
+            VLOG_WARN("uname reported bad OS release (%s)", utsname.release);
+        } else if (major < 2 || (major == 2 && minor < 35)) {
+            VLOG_INFO("disabling unsafe RTM_GETQDISC in Linux kernel %s",
+                      utsname.release);
+        } else {
+            safe = true;
+        }
+        ovsthread_once_done(&once);
+    }
+    return safe;
+}
+
 /* If 'netdev''s qdisc type and parameters are not yet known, queries the
  * kernel to determine what they are.  Returns 0 if successful, otherwise a
  * positive errno value. */
@@ -4434,18 +4460,21 @@ tc_query_qdisc(const struct netdev *netdev_)
      * create will have a class with handle 1:0.  The built-in qdiscs only have
      * a class with handle 0:0.
      *
-     * We could check for Linux 2.6.35+ and use a more straightforward method
-     * there. */
+     * On Linux 2.6.35+ we use the straightforward method because it allows us
+     * to handle non-builtin qdiscs without handle 1:0 (e.g. codel).  However,
+     * in such a case we get no response at all from the kernel (!) if a
+     * builtin qdisc is in use (which is later caught by "!error &&
+     * !qdisc->size"). */
     tcmsg = tc_make_request(netdev_, RTM_GETQDISC, NLM_F_ECHO, &request);
     if (!tcmsg) {
         return ENODEV;
     }
-    tcmsg->tcm_handle = tc_make_handle(1, 0);
-    tcmsg->tcm_parent = 0;
+    tcmsg->tcm_handle = tc_make_handle(getqdisc_is_safe() ? 0 : 1, 0);
+    tcmsg->tcm_parent = getqdisc_is_safe() ? TC_H_ROOT : 0;
 
     /* Figure out what tc class to instantiate. */
     error = tc_transact(&request, &qdisc);
-    if (!error) {
+    if (!error && qdisc->size) {
         const char *kind;
 
         error = tc_parse_qdisc(qdisc, &kind, NULL);
@@ -4455,15 +4484,15 @@ tc_query_qdisc(const struct netdev *netdev_)
             ops = tc_lookup_linux_name(kind);
             if (!ops) {
                 static struct vlog_rate_limit rl2 = VLOG_RATE_LIMIT_INIT(1, 1);
-                VLOG_INFO_RL(&rl2, "unknown qdisc \"%s\"", kind);
+                VLOG_DBG_RL(&rl2, "unknown qdisc \"%s\"", kind);
 
                 ops = &tc_ops_other;
             }
         }
-    } else if (error == ENOENT) {
-        /* Either it's a built-in qdisc, or it's a qdisc set up by some
-         * other entity that doesn't have a handle 1:0.  We will assume
-         * that it's the system default qdisc. */
+    } else if ((!error && !qdisc->size) || error == ENOENT) {
+        /* Either it's a built-in qdisc, or (on Linux pre-2.6.35) it's a qdisc
+         * set up by some other entity that doesn't have a handle 1:0.  We will
+         * assume that it's the system default qdisc. */
         ops = &tc_ops_default;
         error = 0;
     } else {