[Jamvm-general] The macro IS_OBJECT(ptr) is not reliable

[Lingyu Z. <lyn...@gm...>]

Macro IS_OBJECT(ptr) is used during scaning C or java stack. However its
implementation is not reliable. This macro only makes sure ptr is in range
of (heapbase, heaplimit). When there's a long value happends to be in that
range, jamvm will treat the value as a root reference and marks it.


In my Linux configuration, the gc heap range is
(0x7fffe7428000,  0x7fffe8428000), the following code verifies my point.

public class TestOStack {
        public long a;
        public void setA(long i) {
                System.gc();
                a  = i;
        }

        public static void main(String args[]) {
                TestOStack o = new TestOStack();
                o.setA(*0x7fffe7728000L*);
        }
}

During GC, 0x*7fffe7728000* will be marked.
Though the false mark does not effect the correctness of the heap
management, and only leads to only a 8-byte heap leak which is not
permanent.   I think IS_OBJECT should check the object's header, to ensure
a alloc bit is set. Though the extra check is still not 100% reliable, it
could reduce false mark posibility.