Comment #4 on issue 269 by [email protected]: comparison and difference
on unrelated pointers
http://code.google.com/p/address-sanitizer/issues/detail?id=269
When I used that feature with Insure++ (a long time ago), I almost never
saw any false positives. The only false positive that I remember, what
a program that was comparing 2 unrelated pointers both in the stack, to
find out in which direction the stack was growing. But that's a fairly
odd and rare thing to do in the first place. I think it was in the
only false positive I ever saw was in this function in Vim source code
(in vim/src/os_unix.c):
696 static int stack_grows_downwards;
697
698 /*
699 * Find out if the stack grows upwards or downwards.
700 * "p" points to a variable on the stack of the caller.
701 */
702 static void
703 check_stack_growth(p)
704 char *p;
705 {
706 int i;
707
708 stack_grows_downwards = (p > (char *)&i);
709 }
...
3053 int i;
3054
3055 check_stack_growth((char *)&i);
The comparison at line 708 is undefined as it is true
if the stack is growing in one direction or false if
the stack is growing in the other direction. That's
precisely what that function is checking, so in that
case it was a false positive.
Thinking about it, it should not complain when checking in both
directions as the pointer comparison is then valid. Example:
int *p;
int a[10];
...
if (p >= &a[0] && p < &a[10])
{
/* This always OK */
...
}
if (p >= &a[0])
{
/* This has undefined behaviour if p and &a[0] are unrelated pointers */
}
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