[ovs-dev] [PATCH] dpif-netlink: Fix feature negotiation for older kernels

[Mark Gray]

Older kernels do not reject unsupported features. This can lead
to a situation in which 'ovs-vswitchd' believes that a feature is
supported when, in fact, it is not.

This patch probes for this by attempting to set a known unsupported
feature.

Reported-by: Dumitru Ceara <dceara at redhat.com>
Reported-at: https://bugzilla.redhat.com/show_bug.cgi?id=2004083
Suggested-by: Ilya Maximets <i.maximets at ovn.org>
Signed-off-by: Mark Gray <mark.d.gray at redhat.com>
---
 lib/dpif-netlink.c | 66 ++++++++++++++++++++++++++++++----------------
 1 file changed, 43 insertions(+), 23 deletions(-)

diff --git a/lib/dpif-netlink.c b/lib/dpif-netlink.c
index 34fc04237333..c16323f7ee21 100644
--- a/lib/dpif-netlink.c
+++ b/lib/dpif-netlink.c
@@ -84,6 +84,8 @@ enum { MAX_PORTS = USHRT_MAX };
 #define EPOLLEXCLUSIVE (1u << 28)
 #endif
 
+#define OVS_DP_F_UNSUPPORTED (1 << 31);
+
 /* This PID is not used by the kernel datapath when using dispatch per CPU,
  * but it is required to be set (not zero). */
 #define DPIF_NETLINK_PER_CPU_PID UINT32_MAX
@@ -382,36 +384,54 @@ dpif_netlink_open(const struct dpif_class *class OVS_UNUSED, const char *name,
         dp_request.cmd = OVS_DP_CMD_SET;
     }
 
-    /* The Open vSwitch kernel module has two modes for dispatching upcalls:
-     * per-vport and per-cpu.
-     *
-     * When dispatching upcalls per-vport, the kernel will
-     * send the upcall via a Netlink socket that has been selected based on the
-     * vport that received the packet that is causing the upcall.
-     *
-     * When dispatching upcall per-cpu, the kernel will send the upcall via
-     * a Netlink socket that has been selected based on the cpu that received
-     * the packet that is causing the upcall.
-     *
-     * First we test to see if the kernel module supports per-cpu dispatching
-     * (the preferred method). If it does not support per-cpu dispatching, we
-     * fall back to the per-vport dispatch mode.
+    /* Some older kernels will not reject unknown features. This will cause
+     * 'ovs-vswitchd' to incorrectly assume a feature is supported. In order to
+     * test for that, we attempt to set a feature that we know is not supported
+     * by any kernel. If this feature is not rejected, we can assume we are
+     * running on one of these older kernels.
      */
     dp_request.user_features |= OVS_DP_F_UNALIGNED;
-    dp_request.user_features &= ~OVS_DP_F_VPORT_PIDS;
-    dp_request.user_features |= OVS_DP_F_DISPATCH_UPCALL_PER_CPU;
+    dp_request.user_features |= OVS_DP_F_VPORT_PIDS;
+    dp_request.user_features |= OVS_DP_F_UNSUPPORTED;
     error = dpif_netlink_dp_transact(&dp_request, &dp, &buf);
+    dp_request.user_features &= ~OVS_DP_F_UNSUPPORTED;
     if (error) {
-        dp_request.user_features &= ~OVS_DP_F_DISPATCH_UPCALL_PER_CPU;
-        dp_request.user_features |= OVS_DP_F_VPORT_PIDS;
+        /* The Open vSwitch kernel module has two modes for dispatching
+         * upcalls: per-vport and per-cpu.
+         *
+         * When dispatching upcalls per-vport, the kernel will
+         * send the upcall via a Netlink socket that has been selected based on
+         * the vport that received the packet that is causing the upcall.
+         *
+         * When dispatching upcall per-cpu, the kernel will send the upcall via
+         * a Netlink socket that has been selected based on the cpu that
+         * received the packet that is causing the upcall.
+         *
+         * First we test to see if the kernel module supports per-cpu
+         * dispatching (the preferred method). If it does not support per-cpu
+         * dispatching, we fall back to the per-vport dispatch mode.
+         */
+        dp_request.user_features |= OVS_DP_F_UNALIGNED;
+        dp_request.user_features &= ~OVS_DP_F_VPORT_PIDS;
+        dp_request.user_features |= OVS_DP_F_DISPATCH_UPCALL_PER_CPU;
         error = dpif_netlink_dp_transact(&dp_request, &dp, &buf);
-    }
-    if (error) {
-        return error;
+        if (error) {
+            dp_request.user_features &= ~OVS_DP_F_DISPATCH_UPCALL_PER_CPU;
+            dp_request.user_features |= OVS_DP_F_VPORT_PIDS;
+            error = dpif_netlink_dp_transact(&dp_request, &dp, &buf);
+        }
+        if (error) {
+            return error;
+        }
+
+        error = open_dpif(&dp, dpifp);
+        dpif_netlink_set_features(*dpifp, OVS_DP_F_TC_RECIRC_SHARING);
+    } else {
+        VLOG_INFO("Kernel does not correctly support feature negotiation. "
+                  "Using standard features");
+        error = open_dpif(&dp, dpifp);
     }
 
-    error = open_dpif(&dp, dpifp);
-    dpif_netlink_set_features(*dpifp, OVS_DP_F_TC_RECIRC_SHARING);
     ofpbuf_delete(buf);
 
     if (create) {
-- 
2.27.0