### Files

``````commit b00a4fa78779ff0f304fa6cb34d49622679c86d4
Author: Mark Wielaard <mjw@redhat.com>
Date:   Thu Sep 3 10:50:58 2015 +0200

readelf: handle_core_item large right shift triggers undefined behaviour.

The problem is this:

int n = ffs (w);
w >>= n;

The intent is to shift away up to (and including) the first least
significant bit in w. But w is an unsigned int, so 32 bits. And the
least significant bit could be bit 32 (ffs counts from 1). Unfortunately
a right shift equal to (or larger than) the length in bits of the left
hand operand is undefined behaviour. We expect w to be zero afterwards.
Which would terminate the while loop in the function. But since it is
undefined behaviour anything can happen. In this case, what will actually
happen is that w is unchanged, causing an infinite loop...

https://bugzilla.redhat.com/show_bug.cgi?id=1259259

Signed-off-by: Mark Wielaard <mjw@redhat.com>

index d3c2b6b..aab8b5c 100644
@@ -8474,8 +8474,16 @@ handle_core_item (Elf *core, const Ebl_Core_Item *item, const void *desc,
unsigned int w = negate ? ~*i : *i;
while (w != 0)
{
-		int n = ffs (w);
-		w >>= n;
+		/* Note that a right shift equal to (or greater than)
+		   the number of bits of w is undefined behaviour.  In
+		   particular when the least significant bit is bit 32
+		   (w = 0x8000000) then w >>= n is undefined.  So
+		   explicitly handle that case separately.  */
+		unsigned int n = ffs (w);
+		if (n < sizeof (w) * 8)
+		  w >>= n;
+		else
+		  w = 0;
bit += n;

if (lastbit != 0 && lastbit + 1 == bit)
``````