util: Clear unused part of the map in virBitmapShrink

Some of the other functions depend on the fact that unused bits and longs are
always zero and it's less error-prone to clear it than fix the other functions.
It's enough to zero out one piece of the map since we're calling realloc() to
get rid of the rest (and updating map_len).

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1540817

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>

src/conf/domain_conf.c

@@ -18453,7 +18453,9 @@ virDomainCachetuneDefParse(virDomainDefPtr def,
 
     /* We need to limit the bitmap to number of vCPUs.  If there's nothing left,
      * then we can just clean up and return 0 immediately */
-    virBitmapShrink(vcpus, def->maxvcpus);
+    if (virBitmapShrink(vcpus, def->maxvcpus) < 0)
+        goto cleanup;
+
     if (virBitmapIsAllClear(vcpus)) {
         ret = 0;
         goto cleanup;

src/util/virbitmap.c

@@ -1201,21 +1201,35 @@ virBitmapSubtract(virBitmapPtr a,
 /**
  * virBitmapShrink:
  * @map: Pointer to bitmap
- * @b: last bit position to be excluded from bitmap
+ * @b: Size to reduce the bitmap to
  *
- * Resizes the bitmap so that no more than @b bits will fit into it.  Nothing
- * will change if the size is already smaller than @b.
- *
- * NB: Does not adjust the map->map_len so that a subsequent virBitmapExpand
- * doesn't necessarily need to reallocate.
+ * Reduces the bitmap to size @b.  Nothing will change if the size is already
+ * smaller than or equal to @b.
  */
-void
+int
 virBitmapShrink(virBitmapPtr map,
                 size_t b)
 {
+    size_t nl = 0;
+    size_t nb = 0;
+
     if (!map)
-        return;
+        return 0;
 
     if (map->max_bit >= b)
         map->max_bit = b;
+
+    nl = map->max_bit / VIR_BITMAP_BITS_PER_UNIT;
+    nb = map->max_bit % VIR_BITMAP_BITS_PER_UNIT;
+    map->map[nl] &= ((1UL << nb) - 1);
+
+    nl++;
+    if (nl == map->map_len)
+        return 0;
+
+    if (VIR_REALLOC_N(map->map, nl) < 0)
+        return -1;
+
+    map->map_len = nl;
+    return 0;
 }

src/util/virbitmap.h

@@ -153,6 +153,6 @@ void virBitmapIntersect(virBitmapPtr a, virBitmapPtr b)
 void virBitmapSubtract(virBitmapPtr a, virBitmapPtr b)
     ATTRIBUTE_NONNULL(1) ATTRIBUTE_NONNULL(2);
 
-void virBitmapShrink(virBitmapPtr map, size_t b);
+int virBitmapShrink(virBitmapPtr map, size_t b);
 
 #endif

src/util/virresctrl.c

@@ -941,7 +941,8 @@ virResctrlAllocParseProcessCache(virResctrlInfoPtr resctrl,
     if (!mask)
         return -1;
 
-    virBitmapShrink(mask, resctrl->levels[level]->types[type]->bits);
+    if (virBitmapShrink(mask, resctrl->levels[level]->types[type]->bits) < 0)
+        goto cleanup;
 
     if (virResctrlAllocUpdateMask(alloc, level, type, cache_id, mask) < 0)
         goto cleanup;

tests/virbitmaptest.c

@@ -656,6 +656,14 @@ test12(const void *opaque ATTRIBUTE_UNUSED)
 
     TEST_MAP(1024, "34,1023");
 
+    if (virBitmapShrink(map, 35) < 0)
+        goto cleanup;
+    TEST_MAP(35, "34");
+
+    if (virBitmapShrink(map, 34) < 0)
+        goto cleanup;
+    TEST_MAP(34, "");
+
     ret = 0;
 
  cleanup: