[libvirt] [PATCH 2/2] Fix race in finding available vnc port

[Jim Fehlig]

The qemu driver contains a subtle race in the logic to find next
available vnc port.  Currently it iterates through all available ports
and returns the first for which bind(2) succeeds.  However it is possible
that a previously issued port has not yet been bound by qemu, resulting
in the same port used for a subsequent domain.

This patch addresses the race by using a simple bitmap to "reserve" the
ports allocated by libvirt.
---
 src/qemu/qemu_driver.c |   22 ++++++++++++++++++++++
 1 files changed, 22 insertions(+), 0 deletions(-)

diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c
index 582fdee..329859e 100644
--- a/src/qemu/qemu_driver.c
+++ b/src/qemu/qemu_driver.c
@@ -2629,13 +2629,24 @@ qemuInitPCIAddresses(struct qemud_driver *driver,
     return ret;
 }
 
+static virBitmapPtr vnc_used_port_bitmap;
+
 static int qemudNextFreeVNCPort(struct qemud_driver *driver ATTRIBUTE_UNUSED) {
     int i;
 
+    if (vnc_used_port_bitmap == NULL) {
+        if ((vnc_used_port_bitmap = virBitmapAlloc(59635)) == NULL)
+            return -1;
+    }
+
     for (i = 5900 ; i < 65535 ; i++) {
         int fd;
         int reuse = 1;
         struct sockaddr_in addr;
+
+        if (virBitmapGetBit(vnc_used_port_bitmap, i - 5900))
+            continue;
+
         addr.sin_family = AF_INET;
         addr.sin_port = htons(i);
         addr.sin_addr.s_addr = htonl(INADDR_ANY);
@@ -2651,6 +2662,8 @@ static int qemudNextFreeVNCPort(struct qemud_driver *driver ATTRIBUTE_UNUSED) {
         if (bind(fd, (struct sockaddr*)&addr, sizeof(addr)) == 0) {
             /* Not in use, lets grab it */
             close(fd);
+            /* Add port to bitmap of reserved ports */
+            virBitmapSetBit(vnc_used_port_bitmap, i - 5900);
             return i;
         }
         close(fd);
@@ -3717,6 +3730,15 @@ retry:
 
     qemudRemoveDomainStatus(driver, vm);
 
+    /* Remove VNC port from vnc_used_port_bitmap, but only if it was reserved
+       by the driver (autoport=yes)
+    */
+    if ((vm->def->ngraphics == 1) &&
+        vm->def->graphics[0]->type == VIR_DOMAIN_GRAPHICS_TYPE_VNC &&
+        vm->def->graphics[0]->data.vnc.autoport)
+       virBitmapClearBit(vnc_used_port_bitmap,
+                         vm->def->graphics[0]->data.vnc.port - 5900);
+
     vm->pid = -1;
     vm->def->id = -1;
     vm->state = VIR_DOMAIN_SHUTOFF;
-- 
1.6.0.2