[libvirt] [PATCH 1/3] util: bitmap: clarify virBitmapLastSetBit() behavior for empty bitmaps

[Marc Hartmayer]

Before the variable 'bits' was initialized with 0 (commit
3470cd860d517760b13e26d97b6a842ff72687a1), the following bug was
possible.

A function call with an empty bitmap leads to undefined
behavior. Because if 'bitmap->map_len == 0' 'unusedBits' will be <= 0
and 'sz == 1'. So the non global and non static variable 'bits' would
have never been set. Consequently the check 'bits == 0' results in
undefined behavior.

This patch clarifies the current version of the function by handling the
empty bitmap explicitly. Also, for an empty bitmap there is obviously no
bit set so we can just return -1 (indicating no bit set) right away. The
explicit check for 'bits == 0' after the loop is unnecessary because we
only get to this point if no set bit was found.

Reviewed-by: Boris Fiuczynski <fiuczy at linux.vnet.ibm.com>
Reviewed-by: Sascha Silbe <silbe at linux.vnet.ibm.com>
Reviewed-by: Bjoern Walk <bwalk at linux.vnet.ibm.com>
Signed-off-by: Marc Hartmayer <mhartmay at linux.vnet.ibm.com>
---
 src/util/virbitmap.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/src/util/virbitmap.c b/src/util/virbitmap.c
index 3276763..2029a73 100644
--- a/src/util/virbitmap.c
+++ b/src/util/virbitmap.c
@@ -952,7 +952,11 @@ virBitmapLastSetBit(virBitmapPtr bitmap)
     ssize_t i;
     int unusedBits;
     ssize_t sz;
-    unsigned long bits = 0;
+    unsigned long bits;
+
+    /* If bitmap is empty then there is no set bit */
+    if (bitmap->map_len == 0)
+        return -1;
 
     unusedBits = bitmap->map_len * VIR_BITMAP_BITS_PER_UNIT - bitmap->max_bit;
 
@@ -971,8 +975,8 @@ virBitmapLastSetBit(virBitmapPtr bitmap)
             goto found;
     }
 
-    if (bits == 0)
-        return -1;
+    /* Only reached if no set bit was found */
+    return -1;
 
  found:
     for (i = VIR_BITMAP_BITS_PER_UNIT - 1; i >= 0; i--) {
-- 
2.5.5