[Issue 5176] Limit static object sizes

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=5176



--- Comment #32 from 9999 <sibaqexozequgaba at tempomail.fr> 2013-05-20 02:08:00 PDT ---
(In reply to comment #31)
> Yes sure. Let's consider the example below :
> 
> struct Foo {
>     ubyte[512] bar;
> }
> 
> struct Buzz {
>     ubyte[256] pad;
>     Foo[8] qux;
> }
> 
> Buzz* b;
> 
> auto deref1 = (b.qux[7]); // deref below 4kb.
> deref1.bar[300]; // offset is bellow 4kb, but teh address is above 4kb.
> 
> If we consider deref address only no check happens here. So it is needed to
> base the decision to check or not not based on the actual address, but
> according the the maximal address possibly reached.
> 
> In other words, a 4kb+ conglomerate of value types need null check on every
> single pointer operation, even the on bellow 4kb.

Your example is safe, as it will crash on the first dereference (value
semantics).

You probably meant something similar to:

Buzz* b;

auto deref1 = &(b.qux[7]); // take address below 4kb.
deref1.bar[300] = 0; // offset is bellow 4kb, but the address is above 4kb.

Maybe it's worth adding another case then, when both are true:
* Taking an address of an object whose size is above OS' guard page (Buzz in
the example).
* The referenced object's last available byte offset is above OS' guard page
(qux[7]'s last byte in the example).

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