qemu_capabilities: Rework domain caps cache

Since v5.6.0-48-g270583ed we try to cache domain capabilities,
i.e. store filled virDomainCaps in a hash table in virQEMUCaps
for future use. However, there's a race condition in the way it's
implemented. We use virQEMUCapsGetDomainCapsCache() to obtain the
pointer to the hash table, then we search the hash table for
cached data and if none is found the domcaps is constructed and
put into the table. Problem is that this is all done without any
locking, so if there are two threads trying to do the same, one
will succeed and the other will fail inserting the data into the
table.

Also, the API looks a bit fishy - obtaining pointer to the hash
table is dangerous.

The solution is to use a mutex that guards the whole operation
with the hash table. Then, the API can be changes to return
virDomainCapsPtr directly.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1791790Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NPeter Krempa <pkrempa@redhat.com>

src/qemu/qemu_capabilities.c

@@ -598,6 +598,26 @@ struct _virQEMUCapsAccel {
     qemuMonitorCPUDefsPtr cpuModels;
 };
 
+
+typedef struct _virQEMUDomainCapsCache virQEMUDomainCapsCache;
+typedef virQEMUDomainCapsCache *virQEMUDomainCapsCachePtr;
+struct _virQEMUDomainCapsCache {
+    virObjectLockable parent;
+
+    virHashTablePtr cache;
+};
+
+G_DEFINE_AUTOPTR_CLEANUP_FUNC(virQEMUDomainCapsCache, virObjectUnref);
+
+static virClassPtr virQEMUDomainCapsCacheClass;
+static void virQEMUDomainCapsCacheDispose(void *obj)
+{
+    virQEMUDomainCapsCachePtr cache = obj;
+
+    virHashFree(cache->cache);
+}
+
+
 /*
  * Update the XML parser/formatter when adding more
  * information to this struct so that it gets cached
@@ -629,7 +649,7 @@ struct _virQEMUCaps {
 
     virArch arch;
 
-    virHashTablePtr domCapsCache;
+    virQEMUDomainCapsCachePtr domCapsCache;
 
     size_t ngicCapabilities;
     virGICCapability *gicCapabilities;
@@ -655,6 +675,9 @@ static int virQEMUCapsOnceInit(void)
     if (!VIR_CLASS_NEW(virQEMUCaps, virClassForObject()))
         return -1;
 
+    if (!(VIR_CLASS_NEW(virQEMUDomainCapsCache, virClassForObjectLockable())))
+        return -1;
+
     return 0;
 }
 
@@ -1625,6 +1648,22 @@ int virQEMUCapsGetDefaultVersion(virCapsPtr caps,
 }
 
 
+static virQEMUDomainCapsCachePtr
+virQEMUDomainCapsCacheNew(void)
+{
+    g_autoptr(virQEMUDomainCapsCache) cache = NULL;
+
+    if (virQEMUCapsInitialize() < 0)
+        return NULL;
+
+    if (!(cache = virObjectLockableNew(virQEMUDomainCapsCacheClass)))
+        return NULL;
+
+    if (!(cache->cache = virHashCreate(5, virObjectFreeHashData)))
+        return NULL;
+
+    return g_steal_pointer(&cache);
+}
 
 
 virQEMUCapsPtr
@@ -1642,7 +1681,7 @@ virQEMUCapsNew(void)
     if (!(qemuCaps->flags = virBitmapNew(QEMU_CAPS_LAST)))
         goto error;
 
-    if (!(qemuCaps->domCapsCache = virHashCreate(5, virObjectFreeHashData)))
+    if (!(qemuCaps->domCapsCache = virQEMUDomainCapsCacheNew()))
         goto error;
 
     return qemuCaps;
@@ -1832,7 +1871,7 @@ void virQEMUCapsDispose(void *obj)
 {
     virQEMUCapsPtr qemuCaps = obj;
 
-    virHashFree(qemuCaps->domCapsCache);
+    virObjectUnref(qemuCaps->domCapsCache);
     virBitmapFree(qemuCaps->flags);
 
     VIR_FREE(qemuCaps->package);
@@ -1992,9 +2031,82 @@ const char *virQEMUCapsGetPackage(virQEMUCapsPtr qemuCaps)
 }
 
 
-virHashTablePtr virQEMUCapsGetDomainCapsCache(virQEMUCapsPtr qemuCaps)
+struct virQEMUCapsSearchDomcapsData {
+    const char *path;
+    const char *machine;
+    virArch arch;
+    virDomainVirtType virttype;
+};
+
+
+static int
+virQEMUCapsSearchDomcaps(const void *payload,
+                         const void *name G_GNUC_UNUSED,
+                         const void *opaque)
 {
-    return qemuCaps->domCapsCache;
+    virDomainCapsPtr domCaps = (virDomainCapsPtr) payload;
+    struct virQEMUCapsSearchDomcapsData *data = (struct virQEMUCapsSearchDomcapsData *) opaque;
+
+    if (STREQ_NULLABLE(data->path, domCaps->path) &&
+        STREQ_NULLABLE(data->machine, domCaps->machine) &&
+        data->arch == domCaps->arch &&
+        data->virttype == domCaps->virttype)
+        return 1;
+
+    return 0;
+}
+
+
+virDomainCapsPtr
+virQEMUCapsGetDomainCapsCache(virQEMUCapsPtr qemuCaps,
+                              const char *machine,
+                              virArch arch,
+                              virDomainVirtType virttype,
+                              virArch hostarch,
+                              bool privileged,
+                              virFirmwarePtr *firmwares,
+                              size_t nfirmwares)
+{
+    virQEMUDomainCapsCachePtr cache = qemuCaps->domCapsCache;
+    virDomainCapsPtr domCaps = NULL;
+    const char *path = virQEMUCapsGetBinary(qemuCaps);
+    struct virQEMUCapsSearchDomcapsData data = {
+        .path = path,
+        .machine = machine,
+        .arch = arch,
+        .virttype = virttype,
+    };
+
+    virObjectLock(cache);
+
+    domCaps = virHashSearch(cache->cache, virQEMUCapsSearchDomcaps, &data, NULL);
+
+    if (!domCaps) {
+        g_autoptr(virDomainCaps) tempDomCaps = NULL;
+        g_autofree char *key = NULL;
+
+        /* hash miss, build new domcaps */
+        if (!(tempDomCaps = virDomainCapsNew(path, machine,
+                                             arch, virttype)))
+            goto cleanup;
+
+        if (virQEMUCapsFillDomainCaps(qemuCaps, hostarch, tempDomCaps,
+                                      privileged, firmwares, nfirmwares) < 0)
+            goto cleanup;
+
+        key = g_strdup_printf("%d:%d:%s:%s", arch, virttype,
+                              NULLSTR(machine), path);
+
+        if (virHashAddEntry(cache->cache, key, tempDomCaps) < 0)
+            goto cleanup;
+
+        domCaps = g_steal_pointer(&tempDomCaps);
+    }
+
+    virObjectRef(domCaps);
+ cleanup:
+    virObjectUnlock(cache);
+    return domCaps;
 }
 
 

src/qemu/qemu_capabilities.h

@@ -575,7 +575,17 @@ const char *virQEMUCapsGetBinary(virQEMUCapsPtr qemuCaps);
 virArch virQEMUCapsGetArch(virQEMUCapsPtr qemuCaps);
 unsigned int virQEMUCapsGetVersion(virQEMUCapsPtr qemuCaps);
 const char *virQEMUCapsGetPackage(virQEMUCapsPtr qemuCaps);
-virHashTablePtr virQEMUCapsGetDomainCapsCache(virQEMUCapsPtr qemuCaps);
+
+virDomainCapsPtr
+virQEMUCapsGetDomainCapsCache(virQEMUCapsPtr qemuCaps,
+                              const char *machine,
+                              virArch arch,
+                              virDomainVirtType virttype,
+                              virArch hostarch,
+                              bool privileged,
+                              virFirmwarePtr *firmwares,
+                              size_t nfirmwares);
+
 unsigned int virQEMUCapsGetKVMVersion(virQEMUCapsPtr qemuCaps);
 int virQEMUCapsAddCPUDefinitions(virQEMUCapsPtr qemuCaps,
                                  virDomainVirtType type,

src/qemu/qemu_conf.c

@@ -1338,31 +1338,6 @@ virCapsPtr virQEMUDriverGetCapabilities(virQEMUDriverPtr driver,
 }
 
 
-struct virQEMUDriverSearchDomcapsData {
-    const char *path;
-    const char *machine;
-    virArch arch;
-    virDomainVirtType virttype;
-};
-
-
-static int
-virQEMUDriverSearchDomcaps(const void *payload,
-                           const void *name G_GNUC_UNUSED,
-                           const void *opaque)
-{
-    virDomainCapsPtr domCaps = (virDomainCapsPtr) payload;
-    struct virQEMUDriverSearchDomcapsData *data = (struct virQEMUDriverSearchDomcapsData *) opaque;
-
-    if (STREQ_NULLABLE(data->path, domCaps->path) &&
-        STREQ_NULLABLE(data->machine, domCaps->machine) &&
-        data->arch == domCaps->arch &&
-        data->virttype == domCaps->virttype)
-        return 1;
-
-    return 0;
-}
-
 /**
  * virQEMUDriverGetDomainCapabilities:
  *
@@ -1381,40 +1356,16 @@ virQEMUDriverGetDomainCapabilities(virQEMUDriverPtr driver,
                                    virArch arch,
                                    virDomainVirtType virttype)
 {
-    g_autoptr(virDomainCaps) domCaps = NULL;
     g_autoptr(virQEMUDriverConfig) cfg = virQEMUDriverGetConfig(driver);
-    virHashTablePtr domCapsCache = virQEMUCapsGetDomainCapsCache(qemuCaps);
-    struct virQEMUDriverSearchDomcapsData data = {
-        .path = virQEMUCapsGetBinary(qemuCaps),
-        .machine = machine,
-        .arch = arch,
-        .virttype = virttype,
-    };
-
-    domCaps = virHashSearch(domCapsCache,
-                            virQEMUDriverSearchDomcaps, &data, NULL);
-    if (!domCaps) {
-        g_autofree char *key = NULL;
-
-        /* hash miss, build new domcaps */
-        if (!(domCaps = virDomainCapsNew(data.path, data.machine,
-                                         data.arch, data.virttype)))
-            return NULL;
-
-        if (virQEMUCapsFillDomainCaps(qemuCaps, driver->hostarch, domCaps,
-                                      driver->privileged,
-                                      cfg->firmwares, cfg->nfirmwares) < 0)
-            return NULL;
-
-        key = g_strdup_printf("%d:%d:%s:%s", data.arch, data.virttype,
-                              NULLSTR(data.machine), NULLSTR(data.path));
-
-        if (virHashAddEntry(domCapsCache, key, domCaps) < 0)
-            return NULL;
-    }
 
-    virObjectRef(domCaps);
-    return g_steal_pointer(&domCaps);
+    return virQEMUCapsGetDomainCapsCache(qemuCaps,
+                                         machine,
+                                         arch,
+                                         virttype,
+                                         driver->hostarch,
+                                         driver->privileged,
+                                         cfg->firmwares,
+                                         cfg->nfirmwares);
 }