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

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(a)linux.vnet.ibm.com&gt; Reviewed-by: Sascha Silbe <silbe(a)linux.vnet.ibm.com&gt; Reviewed-by: Bjoern Walk <bwalk(a)linux.vnet.ibm.com&gt; Signed-off-by: Marc Hartmayer <mhartmay(a)linux.vnet.ibm.com&gt; --- 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