util/hbitmap: strict hbitmap_reset

hbitmap_reset has an unobvious property: it rounds requested region up.
It may provoke bugs, like in recently fixed write-blocking mode of
mirror: user calls reset on unaligned region, not keeping in mind that
there are possible unrelated dirty bytes, covered by rounded-up region
and information of this unrelated "dirtiness" will be lost.

Make hbitmap_reset strict: assert that arguments are aligned, allowing
only one exception when @start + @count == hb->orig_size. It's needed
to comfort users of hbitmap_next_dirty_area, which cares about
hb->orig_size.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20190806152611.280389-1-vsementsov@virtuozzo.com>
[Maintainer edit: Max's suggestions from on-list. --js]
[Maintainer edit: Eric's suggestion for aligned macro. --js]
Signed-off-by: John Snow <jsnow@redhat.com>

3 files changed, 10 insertions, 1 deletions

diff --git a/include/qemu/hbitmap.h b/include/qemu/hbitmap.h
index 4afbe6292e..1bf944ca3d 100644
--- a/include/qemu/hbitmap.h
+++ b/include/qemu/hbitmap.h
@@ -132,6 +132,11 @@ void hbitmap_set(HBitmap *hb, uint64_t start, uint64_t count);
  * @count: Number of bits to reset.
  *
  * Reset a consecutive range of bits in an HBitmap.
+ * @start and @count must be aligned to bitmap granularity. The only exception
+ * is resetting the tail of the bitmap: @count may be equal to hb->orig_size -
+ * @start, in this case @count may be not aligned. The sum of @start + @count is
+ * allowed to be greater than hb->orig_size, but only if @start < hb->orig_size
+ * and @start + @count = ALIGN_UP(hb->orig_size, granularity).
  */
 void hbitmap_reset(HBitmap *hb, uint64_t start, uint64_t count);
 
diff --git a/tests/test-hbitmap.c b/tests/test-hbitmap.c
index eed5d288cb..e1f867085f 100644
--- a/tests/test-hbitmap.c
+++ b/tests/test-hbitmap.c
@@ -423,7 +423,7 @@ static void test_hbitmap_granularity(TestHBitmapData *data,
     hbitmap_test_check(data, 0);
     hbitmap_test_set(data, 0, 3);
     g_assert_cmpint(hbitmap_count(data->hb), ==, 4);
-    hbitmap_test_reset(data, 0, 1);
+    hbitmap_test_reset(data, 0, 2);
     g_assert_cmpint(hbitmap_count(data->hb), ==, 2);
 }
 
diff --git a/util/hbitmap.c b/util/hbitmap.c
index fd44c897ab..66db87c6ff 100644
--- a/util/hbitmap.c
+++ b/util/hbitmap.c
@@ -476,6 +476,10 @@ void hbitmap_reset(HBitmap *hb, uint64_t start, uint64_t count)
     /* Compute range in the last layer.  */
     uint64_t first;
     uint64_t last = start + count - 1;
+    uint64_t gran = 1ULL << hb->granularity;
+
+    assert(QEMU_IS_ALIGNED(start, gran));
+    assert(QEMU_IS_ALIGNED(count, gran) || (start + count == hb->orig_size));
 
     trace_hbitmap_reset(hb, start, count,
                         start >> hb->granularity, last >> hb->granularity);