Re: [Tinycc-devel] Fixes to bcheck and how it works correctly

[grischka]

Kirill Smelkov wrote:
> Let's imagine that bcheck checks pointers only on dereference. Then
> let's consider following:
>
>     int a[10], b[10];
>
> If we have p=&b[0], then do p--, how do we know whether there is no
> bounds error for p? p points to correct memory &a[9], but it is out of
> bounds - it started from b and crossed the limit.
>
> That's why in lib/bcheck pointers are checked not only on indir, but
> also on add. And that's why we have to pay the price. Btw - maybe "not
> ansi" comment was there for a reason...

Actually in bcheck.c:__bound_ptr_add there appears to be a special case
for code such as

        int a[10], *p;
        for (p = a; p < a + 10; ++p)
                *p = p - a;

to ignore the off by one pointer addition at end of region.  Now what
about the also not so uncommon reverse case

        int a[10], *p;
        for (p = a + 10; --p >= a;)
                *p = p - a;

I don't think I would like to change that just for bcheck.  Then again
if that were allowed, the vtop stuff in TCC could stay as it was, I
suppose?

--- grischka